The Effects of a Perceived Predation Risk on Chickadee

Body Weight and Foraging

Amy Elizabeth Gordon

Neuroscience Graduate Program

Submitted in partial fultillment of the requirernents

for the degree of iMaster of Science

Faculty of Graduate Studies

The University of Western Ontario

London, Ontario, CANADA

O Amy Elizabeth Gordon 1999

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lncreased fat reserves in passerines protect against starvation, but a b decrease

flight ability, possibly increasing predation risk. Theoretically, a trade-off should exist

between the starvation cïsk and the predation risk of maintaining lowered fat reserves.

Experiment one tested how black-capped chickadees (Purus orricapillus) adjun both

intemal (fat) and extemai (cache) reserves under a predation threat. Birds were exposed to

a taxidermic mount of either a sharpshïnned hawk (Accipiler striafus) or a red-winged

blackbird (Agelaius phoeniceirs). Taxiderm ic mount days altemated with baseline days.

Evening weight and caching were significantly lower on hawk mount days than on the

following baseline days. Behaviour on hawk mount days minimized predation nsk and

die next day minimized starvation risk. Experiment two ex-ned the prolonged effect of

hawk exposure on feeding. No signitïcant di fferences in weight or daily intake were

found, although these results may have been confounded with the deteriorating condition

of the birds.

Kevwords: Chickadee; predation risk: starvation risk; body weight; foraging.

iii

Acknowledgment of Co-Authors hip

Al1 experimental work was c d e d out solely by Amy Gordon. Dr. Sherry

participated in the conception of the experiments presented here and in the writing of the

manuscnpt-

Acknow ledgments

First, I would like to thank my supenrisor, Dr. David Sherry, for his advice and

support throughout this project. I also want to thank my Iab-mate, Mike Boisvert, for

keeping things interesthg around the [ab (and for providing an exarnple of senous results-

oriented dedication). Thank you also to Shelley, Jen, Kathleen and Ioanne for your

support, encouragement and Eendship. Findly, 1 wodd iike to thank m y parents for

thrü incredible support and encouragement over the years.

................................................. 1.4 Predation Risk 9

......................... 1.4.1 Starvation Risk Versus Predation Risk 10

........................ 1.4.2 Predation Risk and Food-Ston'ng Birds 13

................. 1-43 Regulation OP Fat Reserves and Cache Reserves 14

................................................. 1.5CurrentStudy 15

................................................ 1 -6 Reference List 16

Chapter 2

Effect of a perceived predation threat on chickadee body weight and forrging . . - 2 3

2.1Introduction .................................................. 24

2.2Methods ..................................................... 27

................................................. 2.2.1 Subjects 27

................................................. 2.2.2 Design -27

............................................... 2.2.3 Procedure 28

................................................. 2.2.4 Analysis 30

2.3 Results ...................................................... 31

2.3.1 Weight ................................................. - 3 1

.................................................. 2.3.2 Eating 35

................................................. 2.3.3 Caching 35

2.3.4Activity ................................................. 35

2.3.5 Vigilance .....................................-......... - 3 7

................................................... 2.4 Discussion 39

.................................. 2.4.1 Body Weight and Foraging 39

........................................ 2.4.2 Change in Activity 40

2.4.3Vigilmce ................................................ 40

.............................................. 2.4.4 Cohclusions 4t

........................................ 2.4.5 Other Explanations 42

..................................... 2.4.6 Significance OP Results 44

................................................. 2.5 Reference List 47

Chapter 3

............................................ Conclusioas and Perspectives 50

.................................... . 3 L Conclusions and Perspectives 51

....................................... 3.1 -1 Summary of Results 51

.......................... 3.1.2 Alternate Anti-Predator Behaviours -51

............................... 3.1.3 Appiicability for Other Species 52

............................................. 3.1.4 Future Work 52

................................................ 3.2 Reference List 53

List of Figures

Figure 2.1 Mean afternoon body weight over the three experimental blocks in experiment

one .................................................................. 32

. .............. Figure 2.2 Afiemoon weight over the 18 trial days in experiment one ~3

....................... Figure 2 3 Mean weight gain per trial in experirnent one - 3 4

...................... Figure 2.4 Mean caching rate per trial in experiment one - 3 6

............... Figure 2.5 Long head-ups over the 18 trial days in expriment one - 3 8

Chapter 1

General Introduction

1, l The Blac k-camed C hickadee

The b lack-capped c hickadee (Purzrs rrrriccrpiZizcs) is a small common bird found

throughout most of North Amerka. C hickadees generally weigh between IO and 14 g

(Smith 199 1). in the winter. chickadees fotm large flocks. while in the spnng and

sumrner they segregate into temtonal breeding pairs (Smith l991). Chickadees feed

mainly on seeds d u ~ g the faIl and winter. and mainly on insects during the spring and

summer (Smith 199 1).

1 -1.2 Food-S torinp;

Chickadees are a food-storing species. meaning that they cache individual food

items in separate locations and retrieve them at a later the. Most food-storing is done in

the fall. when chickadees can store up to hundreds of food items a day (Odum 1942,

Sherry 1984).

Food-storing is generdly believed to be an adaptation which allows individuals to

survive penods of unpredictable or variable food supplies (McNamara et al. 1990).

Problems may aise from fluctuations in the food supply itself, changes in the cost of

foraging, changes in energy requirements or increased cornpetition for resources

(McNarnara et al. 1990). Hoarding more food than can be immediately consumed may

permit the bird survive periods of shortage.

3

Lucas and Walter ( 199 I ) summarize four main theories about the advantages of

food-storing. Fint, if caching a food item is faster than eating it, caching may dlow a

forager to get a disproportionate share of a short-lived resource (Clarkson et al, 2986).

Second, if birds cache most when food is abundant and retrieve most when food is scarce,

caching may reduce the rïsk of starvation. Third. if fat storage is costly, cachîng may act

as an alternative form of reserve (Lima 1 986). Fourth. cactiing may "decouple" the need

for food and the need to forage for it (S heny L 985). This would allow the bud to eat

when it is most profitable to do so, such as when predation risk is low.

t -1.3 Chickadees and Predation Risk

The main predators of chickadees are hawks ofthe genus Accipiter: the sharp-

shimed hawk (A. striarus) and the Cooper's hawk (A. cooperii) (Smith 199 1). Other

common predators include Amencan kestrals (Fuko spurverius), merlins (F.

colirmbarius), northem pygmy owls (Glaucidium gnoma), eastem screech owls (Otus

asio), boreal O wls (Aegolius jicnerr us) and northern shr i kes (Lanius exaibitor) (Bent

1946, Smith 1991). Healthy chickadees are generally agile enough to escape such

predators and usually only birds weakemd by age or disease are captured (Smith 199 1).

Non-avian predators, such as weasels (ibhsrela sp.), climbing snakes and squirrels. are

most likely to capture chickadees at the nest or roost site (Smith 199 1).

The sharp-shinned harvk. however. is probably the most common predator of

4

chickadees (Smith 199 1). Their diet consists primady of small birds and is known to

include chickadees (Bent 193 7)-

1.2 Anti-Predator Behaviours

Predation risk plays an important role in the behaviour of smdl birds. Even

though the predation of adult birds is relatively rare. the rnere presence of predatoa elicits

anti-predator behaviour (Lima 1993. Lima and Di1 1990).

SrnaIl birds exhibit a wide variety ofanti-predator behaviours. One common fom

of predator evasion is flock formation. As flock size increases. the arnount of vigilance

by individual flock members decreases, thus increasing the amount of foraging time

available for each individual (Waite L987). SimiIarlyt the unpredictable movements of

flocks rnay help to prevent hawks from leaming their daily routine (Gaddis 1980).

Staying close to protective cover is another common form of anti-predator

behaviour (Witter and Cuthill 1993). Birds may also alter when they forage, where diey

forage, what food items they eat and how they handle food items (Lima and Di11 1990).

For example. birds may select a less profitable food item if it allows the bird to be more

vigilant while eating it (Le. if the bird can eat the food item with its head up rather than its

head down),

I -2- 1 Escatie Tactics

When faced with an actual predator attack. birds may either freeze in an attempt to

avoid detection, or they may make a quick dash for shetter (Lima 2993). Different

species tend to have different escape tactics. Tactics may also Vary depending on the

surrounding habitat, the availabte shelter. the weather and the type of predator (Lima

1993)-

Chickadees and titrnice oFeastem North Amerka use a woody-cover dependent

tactic common to many passerines (Lima 1993). In other words, they make a quick dash

to cover when under attack. Dense woody cover is likely a fairly safe refuge, although

accipiter hawks have been known to pursue birds into such cover (Smith 199 1).

1 -3 Passerine Bodv Mass

1 -3.1 Variations in Bodv Mass

The body mass of small birds at high latitudes

the more northerly populations of a species tend to be

more southerly populations (Blem 1976. King 1972).

varies seasonally. Individuais in

heavier than individuals in the

In addition, the mass of individual

birds in a northem pcipulation tends to peak during midwinter (Helms and Dniry 1960).

Finally, birds show more pronounced daily fluctuations in body mass in the winter

(Metcalfe and Ure 1995). These daily variations in body mass occur because birds build

up fat reserves during the day and deplete them ovemight (Metcalfe and Ure 1995). In

6

the winter. birds may lose 7-1 5% of their body mass overnight (Huriy 1992)- Fat reserves

have been s h o m to account for most of these variations in mass (Hurly 1992)-

Little is known about the daily pattern of weight gain in small birds. Hurly (1992)

found that weight gain was delayed until the end of the day in ma& tits (Parus

puIzatrr's)). This tactic was maintained when the birds were on both a low variability and

a high variability food suppIy. He suggests that the marsh tits were using stored food to

keep them on a set weight trajectory each day. In other words, the stored food would

even out variability in the food suppIy.

1 - 3 2 Cause of Variations in Fat Reserves

A number of factors may influence how much fat reserves a bird carries. These

factors inchde (but are not limited to) temperature (Rogers 1995), photoperîod (Witter et

ai. 1995), corticosterone leveb (Witter et al. 1995), sociai status (Ekrnan and Lilliendahl

1993), the predictability of the food supply (Blem 1990. Ekman and Hake 1990,

Pravosudov and Gmbb 1997. Witter et al. 1995) and differences in energy expenditure .

It appears that changes in body mass are not sirnply a passive refîection of food

availability or energetic expenditure and that birds actively regulate the size of their

energy reserve according to their needs (Lima 1986, Witter and Cuthill 1993, Witter et al.

1995)-

1 -3 -3 Function of lricreased Fat Reserves

Excess fat reserves may provide body insulatioa mechanical suppoh protection,

buoyancy and both sexuai and social signals ( Witter and CuthiIl 1 993). More

imponantly, however. it is generally assumed that the more fat reserves an animal cm-es,

the lower the rïsk of starvation that animal faces. Fat reserves are the major energy

reserve of most birds (Grïminger 1986. Witter and Cuthill 1993). McNamara and

Houston (1990) suggested that starvation risk decreases almost exponentially as fat

reserves increase. These fat reserves proiect against cold temperatures, long winter nights

when birds are unable to forage. and unpredictable food supplies (Blem 1990, King

1972).

In theory, therefore. under conditions of limited food availability, birds should

always carry the maximum fat reserves that they are able to. in order to reduce the risk of

starvation. However, this almost never occurs in nature (Witter and Cuthill 1993). This

suggests that there are costs associated with carrying fat.

1 -3 -4 Costs of Carrving Fat Reserves

Carrying extra fat resen-es may require increased muscuIature. which rnay

increase metabolic costs, even during inactivity (Le. because of higher oxygen

consumption and increased energy demand) (Witter and Cuthill 1993). More heat may be

required to warm a larger body and excess fat may decrease thermal conductivity (Scott et

al. 1996). Flying with the extra mass also requires more energy (Scott et al. 1996)- On

the other hand, incfeased fat reserves could have insulatory benefits which would

8

decrease metabolic rate at low temperatures (Witter and Cuthill 1993). An indatory

benefit of fat has not, however. been demonstrated in birds (Scott et al. 1996).

More importantly. Blem (1975) suggested that carrying excess fat may impair

flight ability. Specifically. he suggested that excess fat may affect wing loading and lead

to inefficient flight energetics. An increase in m w without an accompanying change in

wing shape (Le. an increase in wing loading) is predicted to decrease take off angle.

manoeuverability, linear accelerat ion, and the maximum rate of ascent (Alemam and

Lindstrom 1990, Hedenstrom 1992. Marden 198% Norberg 1990). In fact, there is

considerable evidence that fat levels have a negative eKect on flight performance.

Wiîter et al. (1994) added weights to European Starlings (Sturnm vulgaris) and

found that birds with a higher "body" mass were less manoeuverable and had a Iower rate

of ascent than control birds. Adding weights to birds may not, however, mimic n a d

fat reserves. The weights may. for example. shift the bird's center of gravity (Metcalfe

and Ure 1995).

Metcalfe and Ure ( 1995) studied the take offs of captive zebra finches

(Taeniopygia guttata). They compared flight performance at dawn, when the birds were

lighter. to performance at dusk. when the birds were heavier. Individuds were

significantly slower, showed less manoeuverability and took longer to reach a particular

vertical height when they were heavy compared to when they were light. The birds were

only 7% heavier at dusk than at dawn and yet were over 30% slower at dusk- The birds in

this study showed Iess daily variation in body m a s than wild birds, suggesting that the

impact of body mass on flight pertbrmance may be even more pronounced in the wild.

They predicted that the daily mass gain of small birds would produce a 20 - 50% increase

in the time required to Ay a fixed vertical distance-

1-4 Predation Rkk

Lima (1986), however. pointed out that if fat reserves are required for swival.

then they should be maintained regardless of the eEect on flight eficiency. h other

words, if fat reserves are required for survival. then birds have no choice but to pay the

cost of decreased flight effkiency. There must be some other factor, then, that influences

how much fat a smali bird carries.

It has been suggested that one reason birds do not maintain maximum fat reserves

is that this would increase predation risk (Grubb and Pravosudov 1994, Lima 1986.

McNamara 1990, McNamara and Houston 1990). As discussed by Lima (1 986), this

could happen in two ways. Fint of ail. maintaining increased fat reserves requires the

bird to spend more time foraging. which increases the likelihood of encomtering a

predator. Secondly, the decrease in Ilight ability may increase the chances of k ing

caught by a predator. A small bird threatened by a hawk must be able to take off rapidly

because most avian predaton rely on surprise attacks and have a greater chance of

10

success before the prey is fully airbome (Metcalfe and Lice 1995). The bird must also be

manoeuverable in the air in order to avoid any subsequent attacks. As discussed earlier.

excess fat reserves will decrease take off angle. linear acceleration, clirnb rate and

manoeuverability and so even minor variations in the speed and agility of small birds rnay

have a major impact on their risk of predation.

Kullberg et al. (1996) studied the take offs of blackcaps (Sylvia africapiZla) under

a simulated hawk attack. They assumed that this would force birds to put maximal effort

into their escape speed and angle. They were concemed that on a normal take off, a fatter

bird might conserve energy by reducing take off speed and angle and that this would lead

to an overestimation of the effects of fat on tlight ability (Le- for Metcalfe and Ure 1995).

They found that birds with a Iarger fat load were slower and had a lower angle of ascent

than leaner birds. Interestingly. however. the effect of fat on speed and angle were

considerably Iess pronounced than those found by Metcalfe and Ure (1993). This rnay

indicate that fatter birds do indeed try to conserve energy on a normal take off, although

some of the difference rnay be due to speciss differences. Nevertheless, high fat reserves

do reduce flight ability even during a simulated predatory attack.

1.4- 1 Starvation Risk Versus Predation Risk

It appears, there fore. t hat there rnay be a trade-O K between starvat ion risk and

predation risk. As birds get heavier. their starvation risk declines, but their predation rïsk

rnay increase. Similady, if a bird [oses weiglit. it rnay lower its predation nsk, but

11

increase its stawation nsk- Theoretically, birds should regulate their weight according to

whether starvation or predation is the greater threat- Therefore. birds facing a predation

risk should lower their fat levels because predation is the larger threat (Lima 1986).

Experimental resdts, however. have not been clear cut-

Gosler et ai. (1995) found that a population of great tits ( P u m major) in England

maintained higher fat reserves dunng a penod of years when sparrowhawks (Acc@iter

nisus) were absent (due to pesticide poisoning). They then maintained lower fat reserves

when the sparrowhawks retumed. These changes were due to changes in the mass of

individual tits and not due to selection of heavier tits by the hawks. Wrens, a species

rarely taken by the sparrowhawks, showed no significant change in body m a s over this

time period.

uiterestingly, when sparrowhawks were present, the great tits were leaner in years

when the great tits' preferred food, beechmast, was abundant than in years when it was

not abundant (Gosler et al. L995). When sparrowhawks were absent. however, mass was

unaffécted by beechrnast abundance. This suggests that when food is plentiful, the

reduced starvation nsk aliows birds rnaintain Lower fat reserves. This, in tum, Iowers

predation nsk. When predatos are absent. however. there is no cost to maintaining

higher fat reserves. The birds can therefore aKord to protect more thoroughly against

starvation risk. This strongly supports the theory of a trade-off between starvation risk

and predation risk.

12

Only two experiments have directly tested the effect of a perceived predation

threat on body weight. Lilliendahl ( 1997) tound that greenfinches (Corduelis chlori")

maintained a lower evening body mass in response to a mounted hawk The rnount, or a

plastic bottle (the control). was rotated around the room in a circle five times a day. Body

m a s was lower &et a hawk trial than afier a control trial- Birds were also slower to

resume foraging afier seeing the hawk than the control. This study supports the

prediction that birds wili maintain a Iower body weight under a predation risk.

In contras& Pravosudov and Gmbb ( 19%) found that tufted titmice (Parus

bicolor) maintained a higher weight under a perceived predation risk. They exposed the

titmice to a taxidermie mount of either a mouming dove (Zenaida rnacroura) (no ttireat

control) or a sharp-shinned hawk (titmouse predator). Birds perched and flew during

dove exposure, but fioze during hawk exposure. Both vigilance and delay in ceturning to

foraging were significantly greater following hawk exposure. Birds therefore appeared to

be responding to an increased predation risk. Evening body mass and mean daily mass

gain were significantly greater during the hawk treatment. The increase in gain rate seen

during the treatment period. suggests that the birds fed more intensively throughout the

day, even though they intempted al1 activity during exposure to the hawk model.

Pravosudov and Grubb's findings do not support the prediction of a starvation - predation risk trade-off and differ from the results of Gosler et al- (1995) and Lilliendahl

(1997). It is possible that different species may have different tactics. Greenfinches feed

13

in open areas and fty to couer when under attack. This rnay make flight ability very

important (Lilliendahl 1997). Titrnice. on the other hand, tend to feed under cover. which

rnay mean that they can freeze when a predator is in the area rather than having to fly to

cover. Altematively, Pravosudov and Gmbb (1 998) point out that the random

interruptions of feeding caused by hawk exposure rnay have affiected starvation risk and

lead the birds to carry excess fat as a b a e r against fasùng periods spent under cover. In

nature. however, food suppIy rnay be Iimited and unpredictable and birds rnay not be able

to increase their body m a s quickly between predator appearances. Therefore, in the

wild, body weight rnay decrease as a consequence of reduced foraging time and not

because carrying fat is costly.

The results of Pravosudov and Gmbb ( 1 998) suggest that carrying more weight is

not necessarily associated with a higher risk of predation. More work is, however.

needed to determine why the results of this study differ from the theory of a predation -

starvation risk trade-off.

1 -4.2 Predation Risk and Food-Stonng Birds

Food-storing birds have an option which other birds do not have: they can store

food reserves as intemal fat reserves or in external cache reserves. Both snategies have

costs and benefits. Fat reserves wiil increase body mass, which rnay increase predation

Bsk and energy expenditure, as discussed above. On the other hanci, fat reserves can be

accessed and metabolized 21 hours a day. Making and retrieving caches consumes

14

energy and lowers vigilance for predators (Pravosudov and Gmbb 1997). Caches can

also be lost due to memory loss and pilferage (McNamara et al. 1990).

1 -4.3 Regulation of Fat Reserves and Cache Reserves

Food-storing birds appear to cegulate both their fat reserves and their cache

reserves. Both types of reserves play an important role in swivai - fat reserves are just

as important for food-stocing species as for non-storing species. For example,

Pravosudov and Gnibb (1997) found that the body mass and caching rate of tufted titrnice

bo th increased under an ;npredictable food supply. S imilarly. subordinate willow tits.

who have a less predictable food supply than do dominant birds. tend to carry greater fat

reserves than dominant birds (Clark and Ekman 1995. Ekman and Lilliendahl 1993.

Witter and Cuthill 1993).

It is possible that birds may regulate their fat and cache reserves differenrially and

may favour one mode of storage over the other depending on the situation. Many factors

affect the division of food reserves between fit and caches, such as predictability of the

food supply, dominance aatus, ambient temperature and the presence of predatos (Grubb

and Pravosudov 1 994, McNarnara et al. 1990. Pravosudov and Gmbb 1997). For

example, Hurly ( 1992) found that marsh tits increased caching, but not body weight.

under an unpredictable food supply.

How food-storing birds regulate caching and îàt reserves under a predation risk

has not been examined.

1-5 Current Studv

This thesis explores the putative trade-off between the starvation risk and the

predation risk of carrying increased fat reserves. The contradictory resultî of Pravosudov

and Grubb (1 998) and LiIiiendahl ( 2 997) indicate that there is much to be learned in this

area. We exarnined the effects of a perceived predation risk on the body mass and

foraging behaviour of black-capped chickadees. In particular. we were interested in how

chickadees regulate their fat and cache reserves.

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of Experïmental Biolow, 130,235-258.

McNamara, J.M. IWO. The starvat ion-predation trade-O ff and some behavioral and

ecological consequences. In: Behavioral Mechanisms of Food Selection (Ed. By

R.N. Hughes), pp.39-58. Berlin: S pringer-Verlag.

McNamara, J .M- & Houston- A L 1990- The value of fat reserves and the trade-off

between starvation and predation, Acta Biotheoretica, 38-3 7-6 1.

McNamara, LLM, Houston, A L & Krebs, LR- 2990- Why hoard? The economics of

food storing in tits, Parzrs spp. Behavioral Ecolooy, 1. 12-23.

Metcalfe, N.B. & Ure, S.E. 1995. Diumal variation in flight performance and hence

potentiai predation risk in small birds- Proceedinrrs of the Royd Society of

Series6 1,395400,

Norberg, U.M. 1990. VertebrutejZight. Berlin: Springer-Verlag.

Odum, E.P. 1942. Annual cycle of the black-capped chickadee 3. Auk, 59,499-53 1.

Pravosudov, V.V. & Gmbb, T C . Jr. 1997. management of fat reserves and food caches

in tufted titmice (Panîs bicolor) in relation to unpredictable food supply.

Behavioral Ecolow, 8.332439-

21

Pravosudov, V.V. & Gmbb. T C . Jr. 1998. Management of fat reserves in tufied titmice

Baelophus bicolor in relation to risk ~Epredation. Animal Behavior, 56,4944.

Rogers. C.M. 1987. Predation risk and fasting capacity: do wïntering birds maintain

optimal body mas? Ecolonv7 68. 1 OS 1 - 1 O6 1.

Rogers, C.M. 1995. Experimental evidence for temperature-dependent winter lipid

storage in the dark-eyed junco (Junco hyernalis oreganus) and song sparrow

(Melospua melodirr morphna). P hvsio loeical Zooloa: 68,277-289.

Scott, I., Mitchell, P.I. & Evans. PR, 1996. How does variation in body composition

affect the basal metabolic rate of birds? Functional Ecolow, 10,307-3 13.

Sherry, D.F. 1984. Food storage by black-capped chickadees: memory for the location

and contents of caches. Animai Behaviour, 32,45 1-464.

Sherry, D.F. 1985. Food storage by birds and mammals. Adv. Studv Behav., 15. 153-

188.

Sih. A. 1992. Prey uncertainty and the balancing of antipredator and feeding needs. The

American Naîuralist, 139, 1052- 1069-

37 -- Smith. S.M. 199 1. The Black-capped Chickadee. Comell Univ. Press, Ithaca, New

York.

Waite, T.A. L 987. Vigilance in the White-breasted Nuthatch: effects of dominance and

sociality. Auk, 1 04.429-434.

Witter, M.S. & Cuthill, LC. 1993. The ecological costs of avian fat storage.

Philosoohical Transactions of the Roval Societv of London. Senes B,340,73-92.

Witter, M.S., Cuthill, I.C. & Bonser. R.H.C. 1994. Expenmental investigations of mass-

dependent predation risk in the European Starling, Sfurms vulgaris. Animal

Behaviour, 48,20 1-222.

Witter, M.S.. Swaddle, J.P. & Cuthill. I.C. 1995. Periodic food availability and strategic

regdation of body mass in the European Starling, Sturnus vulgaris. Functional

E C O ~ O ~ ~ L 9,568-574.

Chapter 2

Effect of a perceived predation threat on chickadee body weight and foraging

A.E. Gordon and D.F. Sherry

Submitted to: h i m d Behmiozrr

2- 1 Introduction

The body mass of small passerines shows Iarge seasonal and daily changes. Birds

are heavier in the \inter, but they aiso build up their fat reserves each day and then

deplete them ovemight (Metcaife and Ure 1995)- [n the winter, birds may Lose 7-1 5% of

their body m a s overnight (Hurly 1992). Fat reserves have been shown to account for

most of these variations in mass (Huriy 1992)-

It is generdy assumed that the more fat reserves a bird carries, the lower the cïsk

of starvation the bird faces. These fat reserves protect against cold tempemes, long

overnight fmts and unpredictable food supplies @lem 1990, King 1972). In theocy,

therefore, birds should always carry the mavimum fat reserves that they are able to. but

this almost never occurs in nature (Witter and Cuthill L993). This suggests that there are

costs associated with carrying fat.

Several studies have shown that excess fat may impair flight ability by increasing

the energetic costs of flight and decreaçing acceleration (Metcalfe and Ure 1995, Witter et

al. 1994). Lima (1986) pointed ouf however. that if fat reserves are indeed required for

swival, then they should be maintained regardless of the decrease in flight efficiency.

That is, birds may have Little choice but to pay the cost of decreased flight eficiency if it

is necessary to maintain large fat reserves in order to survive. Given that birds do not

maintain maximum fat reserves, however. there must be some additional factor that

influences how much fat a srnail bird carries.

25

Therefore, it has been suggested that one reason birds do not maintain maximum

fat reserves is that this would increase predation risk (Gnibb and Pravosudov 1 994, Lima

1986. McNamara 1 W02 McNamara and Houston 1990)- As discussed by Lima (1986)

this could happen in two ways. First of dl, maintaining increased fat reserves requires

the bird to spend more t h e foraging. which increases the likelihood of encountering a

predator. Secondly, the decrease in flight ability may increase the chances of k ing

caught by a predator- A smail bird threatened by a hawk must be able to take off rapidly

because most avian predaton rely on surptise attacks and have a greater chance of

success before the prey is fulIy airborne (Metcal fie and LJre 1995). The bird must also be

manoeuvrable in the air in order to avoid any subsequent attacks. An increase in weight

wiI1 decrease take off angle, linear acceleration. climb rate and manoeuvrability and thus

even minor variations in the speed and agility of smail birds may have a major impact on

their risk of predation. Kullberg et al. ( 1996) showed that under conditions of a

perceived predation threat. fatter birds are indeed stower and have a lower angle of ascent

than ieaner birds.

There rnay be, thmefore. a trade-off between starvation risk and predation risk.

Birds facing a predation risk should lower their fat levels because predation is the larger

threat (Lima 1986). Experïmental results. however. have not been clear cut. Lilliendahl

(1997) found supponing results with greenfinches (Crirdueiis chloris). The greenfinches

maintained a lower body weight in the presence of a mounted spmwhawk. A recent

study by Pravosudov and Gmbb ( 1 998). however. found that tufied titrnice (Buelophus

26

bicolor) rnaintained a higher body wei& when faced with a perceived predation ïisk.

The titmice tended to remaîn fiozen while the stuffed hawk was in the aviary.

Pravosudov and Gmbb (1 998) proposed that the birds may carcy excess fat as a buffer

against potential hture interruptions of feeding caused by the hawk.

These d i f f e ~ g results rnay indicate that diffierent species have diEerent tactics. or

that the original theocy, of a predation-starvation risk trade-off, needs to be adjusted. We

examined the effects of a perceived predation risk on the body mass and foraging

behaviour of black-capped chic kadees (Purus utricapillus).

Chickadees are a food-ston'ng species. They cache (or hide) seeds in different

places and then corne back to retrieve them at a later time. This means that they can deal

with a perceived predation threat by adjusting both extemal food stores (caches) and

intemal food stores (fat reserves). Chickadees rnay regulate the two types of stores in a

similar or different manner. For example. by increasing their cache reserves, but not their

fat reserves, they could avoid gaining weight while still gathering food reserves. The

results of this experirnent will help clarify how chickadees manage both their extemal and

their intemal food stores under a perceived predation threat-

2 2 Methods

2.2.1 Subjects

Seven experimentally naive b lack-capped chic kadees were used in this

experiment. All birds were captured on the campus of the University of Westem Ontarîo.

London, Ontario, Canada (43O1 I ' N 8 1°1 8'W)- They were housed individuaily in wire

mesh cages (36 x 36 x 6 1 cm). The birds were kept in captivity for at least one week

before and afier the experiment and were then released.

Birds were maintained on a descending Iight cycle (in an attempt to promote food

storing). On Day 1, the lights were on for an approximate 10: 14 L:D cycle. The length of

the light phase decreased by 4 min a day for the remainder of the experiment (light onset

2 min later each moming and offset 2 min earlier each evening). During the day, birds

were provided with ad Libitum rnash. peanuts and striped sunflower seeds. Four birds

were tested in November 1998 and three were tested in January/February 1999.

2.22 Desim

A taxidermic mount of a juvenile sharp-shimed hawk was used to simdate a

predation threat (the mount was borrowed €rom the Zoology Department Museum at the

University of Westem Ontario). Sharp-shinned hawks are chickadee predators in the wild

and previous studies have successfiilly used a taxidermic mount in the lab to simulate a

predation threat (i.e. Pravosudov and Gmbb 1998)- The control mount was a juvenile

red-winged bIackbird (ilgelaius phoenicvrrr). which in nature poses no threat to

chickadees. The experiment was conducted as a within subjects design.

Two habituation days (in the aviary) preceded testing. The 18 day testing penod

was divided into three 6-day b tocks: redwing 1. hawk. redwing 2. Within each block, we

alternated mount presentation days and baseline days (when birds were released hto the

aviary, but no mount was shown). The birds saw each mount three times during each

block-

2.2.3 Procedure

Testing was done six days a week between 0800 and 1 ZOO h. Birds were food

deprived ovemight (approximately 17 h) in order to stimulate keding and caching

behaviours. The order in which individual birds were tested was balanced across days.

Each testing session lasted 45 min- Birds were released individually into an aviary (7m x

3m) and observed through a one-way mirror. Behaviour (see below) was recorded on a

computer event recorder.

A bowl of black oil sunflower seeds and a bowl of water were provided. Six

branches were distributed around the aviary. The branches were approximately 2 m high

and 5 cm in diameter, held venically in stands. Each branch contained 10 storage sites,

each 1 cm deep and 0.5 cm in diameter. A dowel perch, 5 cm long, was placed 3 cm

below each site. The positions of the branches were changed slightly each day, in order

29

to promote caching. The branches- however. did not move far and generdly maintained

their spatid relationship to each other.

On mount presentation days, the rnount was inserted into the aviary, through a

small window, after 10 min had elapsed in the trial and placed on a stand for

approximately 1 min- Birds were weighed irnrnediately d e r each trial and agah just

pnor to food deprivatlon at the end of the day (1600-1 700 h). Birds were caught by hand.

placed in a mesh bag and weighed on a digital scale (accurate to 0.01g). Weight gain was

calculated by subtracting moming weight from aftemoon weight and treating gain as a

percentage of the moming weight-

A number of behaviours were recorded during each triai. First, the number of

seeds eaten and cached dunng a trial were recorded. Caches were checked at the end of

each trial and removed- Second, al1 flights and perch locations were recorded. Each tree

was divided into three zones-the upper. middle and bottom. Only movements within a

zone of over 10 cm were recorded, while ail movements between zones or branches were

recorded. This data was used to estimate the distance each bird fiew during the trial. as

welI as the time spent in each branch and zone. Third, vigilance was estimated by the

average time between head-ups made while eating or handling a sunflower seed (as in

Pravosudov and Gmbb L 998)- A head-up was scored when a bird raised its head to at

least the level of its shoulders and looked either left or right. A long head-up was scored

when the head-up lasted longer than 2 sec or the bird "scanned" the room, looking both

30

right and lefi. Long head-ups w-ere scored as a percentage of regular head-ups. Founh.

we examined how long each bird spent inactive during each trial (inactivity was defined

as no flight or foraging activity for more than 2 min; however, the bird may have k e n

preening or lookuig around the room). FinaIly. the trial was divided into nine 5-min

segments and the distribution ofcaches. number ofseeds eaten. distance flown and head-

ups over these segments were calculated.

2-2.4 Analvsis

A three-way repeated measures ANOVA was used with one factor k i n g the block

(redwing 1, hawk and redwing 2). one being mount presentation (mount or baseline) and

one being days (the three successive mount or baseline days within a block). This

analysis was done for each dependent variabIe separately, with the exception of those

variables discussed below. Post hoc cornparisons were made using Tukey's HSD test.

Significance was set at the 0.05 level. Long head-ups and weight gain were percentage

variables, and so were arcsine transfomied prior to analysis (in order to nonnalize them).

Only hawk mount and baseline days were compared for the time spent in each

branch and zone and for the distribution ofbehaviours over t h e . Tukey's HSD test was

used to compare the time spent in branches and zones. The distribution of behaviours

over time were each analyzed using a two-way ANOVA, with one factor king mount

presentation (mount or baseline) and one being interval (the nine intervals per trial).

2-3 Results

The chickadees immediately responded to the presentation of the hawk mount

with vocalizations and a dramatic increase in erratic movementsl They continued to

forage normally durÏng the presentation of the redwing mount-

2-3-2 Wei&

There was a significant main effect of days on morning weight (ANOVA:

F(2,12)=L4.054, P=O.OO 1 ). with no interaction with any other factors. Weight on day 3

was significantly greater than either day 1 or day 2 (Pc 0.0 1). The days variable was

completely nested within blocks and mount presetations, so a heavier weight on day 3

indicates that the birds tended to t'rnish each part of the experiment heavier than they

began it. That implies that morning weight increased over the experiment.

Afiemoon weight was significantly lower on hawk mount days compared to hawk

baseline days (Tukey: Q(16)=3 -9 19. PcO.05) (Fig.2.1). Aftemoon weight dropped off

markedly on the first hawk mount day and showed a distinct altemation of high and low

weight on hawk mount and baseiine days respectively (Fig.2.2).

Birds gained significantly more weight on baseline days than on mount days

(ANOVA: F(1,6)=6.830. P4.04) (Fig.Z.3 ). This was particularly clear on hawk baseiine

days compared to hawk mount days (Tukey: Q(16)=1.174, P<O.OS). No difference was

found between mount and baseline days for redwing 1 or redwing 2.

Hawk

Bbck

Fieure 2.1 : Mean aftemoon body weight (2 SE) over the three expenmental blocks for experiment one. Dark bars are mount presentation days and light bars are baseline days.

Redwing 1

1 1-50

M B M

Hawk T

B M B M B

D ~ Y

Figure 2.2: Mernoon weight (+ SE) over the 18 trial days in experiment one. Each point is the mean afiemoon weight of al! seven birds on that day. Dark diamonds are mount presentation days and Light diamonds are baseline days.

Redwing 1

r

Hawk

BI&

Figure 2.3: Mean weight gain per trial (5 SE) in experiment one. Dark bars are mount presentation days and light bars are baselint days.

2.3.2 Eating

ïhere was a significant main effect of block on the number of seeds eaten per triai

(ANOVA: F(2,1 î)=S,673, P=O-O 1 8)- Post hoc tests showed that birds ate more seeds

during redwing 2 than during either redwing 1 or hawk (P<0.05). There was no

interaction with mount,

2.3-3 Cachinp

OnIy the birds in the group tested in November cached regularly and so data are

only presented for those four birds. Caching showed a significant interaction between

mount and block (ANOVA: F(2.6)=7.265. P=O.OZS) (Fig.2.4). Post hoc tests revealed

t h a ~ birds cached fewer seeds on hawk mount days than on hawk baseline days and also

cached fewer seeds on redwing I baseline days than on hawk baseline days (P<O.05 for

both).

There were no significant effects for the distance birds flew (ANOVA:

F(2,12)=0-302, NS) or for time spent inactive during each triai (ANOVA: F(2,12)=0-993,

NS).

Birds spent more time in the branch that was furchest fiom the hawk mount during

hawk mount days than during hawk baseline days (Tukey: Q(16)=3.704. P<O.OS). They

Fieure 2.4: Mean caching rate per trial (5 SE) in experiment one. Values shown include only the four birds observed in November. Dark bars are mount presentation days and light bars are baseline days.

37

also spent more time in the branch that was closest to the hawk on hawk mount days than

on hawk baseline days (Tukey: Q(17)=3.304 Pc0.05). This may be because the birds

were flying back and forth between the m-O trees. The birds also spent more time in the

tops of the branches during hawk mount days than durhg hawk baseline days (Tukey:

Q( l2)=3.17 1, P<0.05). There was no significant effect for the middle and Iower parts of

the branches-

There was no interaction beiween mount and interval for the distribution of

caches, number of seeds eaten. head-ups or distance flown over time. ui other words. the

number of caches, number of seeds eaten. head-ups or distance flown per each of the nine

5-min intervals did not differ between hawk mount and hawk baseline days. This

indicates bat there were no major differences in behavioural pattern during the 35 min

following hawk exposure on hawk mount and hawk baseline days.

2.3 -5 Vinilance

Long head-ups showed a main effect of block (ANOVA: F(2,12)=9.295,

P=0.004), with birds making a greater proportion of long head-ups during redwing 2 and

hawk than during redwing 1 (Pc0.0 1 for both). There was also a significant interaction

between block and days (ANOVA: F(4,24)=7.460, P=0.000) (Fig.2.5). Birds made more

long head-ups on Day 3 of hawk than day 2 or day 1 of hawk (P<O.O 1), more on day 2 of

hawk than day 1 of hawk (P<O.OS) and more on day 1 of redwing 2 than day 2 or day 3 of

redwing 2 (P~0.05). This indicates that the proportion of long head-ups increased during

Redwing 1 Redwing 2

B M B M B

Figure 2.5: Long head-ups (+ SE) over the 18 t d days in experiment one. Long head- ups are expressed as a percentage of regular head-ups. Each point is the mean for ail seven birds on that day.

the hawk block and decreased during the redwing 2 block.

There was a significant main effect of block for regular head-ups (ANOVA:

F(2. L Z)= 14.905, P=0.00 1 ). There was more time between head-ups during redwing 1

than during either hawk or redwing 2 (P<0.01). This suggests that vigilance increased

over the experiment.

2.4 Discussion

2.4.1 Body Weight and Foraeing

Morning weight showed no etrect of the treatments. which is expected because the

birds were weighed immediately following rach trial. It is unlikely that a chickadee could

significantly adjust its weight in the 35 min that immediately followed mount

presentation. The effect of days is restricted to the slight tendency to increase weight

over the course of the experiment-

Aftemoon weight and weight gain were lower on hawk mount days and higher on

the following baseline day. This suggests that the chickadees maintained a lower weight

while a hawk was in the area and then maintained a higher weight on the following day.

perhaps to compensate for the drop in weight on the previous day.

The number of seeds eaten per trial increased over the course of the experiment,

although birds showed a slight. but not significant. tendency to eat fewer seeds during the

trials on hawk mount days than on hawk b a d i n e days.

Birds also cached fewr seeds on haw-k mount days than on hawk baseline days.

Caching was highest on hawk baseline days and tended to drop off during redwing 2.

2-42 Channe in Activitv

Chickadees did not change the distribution oftheir activity over the vial or their

overall activity levels on hawk mount days. This suggests that the reduction in caching

was not merely a side effect of a change in activity.

There were some changes in where birds spent their t h e after seeing the hawk-

Birds spent more time in the two branches that were the Furthest and the closest. to the

hawk on hawk mount days. The chickadees may have been alternately avoiding the hawk

and inspecting it-

2-43 Vigilance

Both regular and long head-ups increased d e r the hawk exposure and remained

high for the rest of the experirnent. The proportion of long head-ups increased over the

three hawk days and declined slowly during redwing 2. These two measures of vigilance

indicate that not only did the birds look up more ofien while handling a seed, but they

also made a greater proportion of long looks.

41

Birds spent significantly more time in the top third of the branches on hawk

mount days than on hawk baseline days. This suggests that the birds were k i n g more

vigilant on hawk mount days-perching in the top of the branch may d l o w a more

unobsuucted view, although it would likely also make the bird more visibie to a predator.

3-44 Conciusions

Overail, on the day they saw the hawk. chickadees decreased cachuig. weight gain

and afternoon weight and increased vigilance. The drop in body weight would

presumably increase flight ability and make the bird more likely to escape attack. On the

following day. it appears that the chickadees increased caching, weight gain and

afiemoon weight The drop in body rveight on the previous day may have led to a

perceived increase in starvation risk. Body weight therefore retumed to (or above)

normal levels. The increase in caching may protect the bird against tiiture interruptions in

feeding caused by predator exposure. Consequently. it would appear that on hawk mount

days behaviour minimized predation risk and on hawk baseline days minimized starvation

risk.

The results aIso showed that chickadees regulate both extemal cache reserves and

intemal fat reserves in a similar manner. Both dropped on hawk rnount days and

increased on hawk baseline days.

It is important to note that these resuits are seen after the birds s aw the hawk for

42

only 1 min. Body weight changes are. presumably. the result ofa change in eating andor

rnetabolism over the remainder of the day when birds were in their home cages with ad

libitum food. This suggests that even a brief hawk erposure may have fairly long term

effects on chickadee behaviour-

in fact hawk exposure may have had carry over effects that lasted into the

redwing 2 block. Behaviour appeared to be altered during redwing 2 compared to

redwing 1. Vigilance remained high throughout redwing 2 and body weight. eating and

caching showed a tendency to be higher during redwing 2 than redwing 1. It suggests that

once the birds had encountered a hawk in the are* they made long-term adjustments to

their behaviour.

2.4.5 Other Explanations

The expenment lasted 18 days and consequently habituation to the aviary rnay

have been responsible for the differences between redwing 1 and 2. A general increase in

weight or caching cannot, however. account for the marked differences between hawk

mount and baseline days. Moreover. the chickadees do not appear to have habituated to

the hawk. Indeed, long head-ups increased over the three hawk exposures. indicating that

the birds were becoming more. not less. vigilant.

We decided to food deprive the chickadees ovemight in order to promote caching

and eating dunng the trial. Food deprivation is. however, a potential confound. The

43

birds Likely perceived a starvation nsk. which would have acted to increase their weight

(Pravosudov and Gnibb 1997). Weight tended to increase over the experiment.

Nevertheless. weight clearly decreased on hawk mount days.

Our daily handling of the birds may have been perceived as a predation threat in

and of itself (Lilliendahl 1997). The capturing and handling of wild birds is known to

increase corticosterone levels (Wingfield et ai. 1992); however. nothing is known about

how quickly captive birds habituate to being handled. It is therefore possible that the

hawk presentation was perceived as an additional predation threat.

We did not alter the temperature in the lab and so the birds were brought from a

colder environrnent into a warmer environment- There is some debate about whether

temperature is a proximate cue for weight changes (Witter and Cuthill 1993). Some

studies have found that birds maintained higher fat reserves when temperature was

decreased in the lab, while other studies found no effect of temperature on fat reserves

(Witter and Cuthill 1993). [t seems likely that if the birds had reacted to the change in

temperature. they would have maintained lower fat reserves. Weight tended to increase

over the experiment, which suggests that temperature was not a major influence.

This experiment found evidence for short-term adjustments in body weight

following hawk exposure. A ditTerent type ofexperiment would be required to determine

whether any long-term adjustments in body weight result from hawk exposure.

2-4.6 Sienificance of Results

This experïment clearly found an effect of hawk exposure on chickadee body

weight and caching Chickadees ap&r to keep their weight low on the day they see a

hawk and to increase their weight on the next day. Predation and starvation nsk,

therefore. appear to aiternate with each other as the major determinants of behaviour.

Body mass may influence predation risk by either decreasing flight ability or by

increasing foraging time. Either (or both) factor(s) may be at play in this experiment.

although the fact that caching was reduced does suggest that at least foraging time was

being reduced. [t is dificult to Say whether the observed drap in weight would be

suficient to dismpt flight ability. Kullberg et al. (1996) found that. under a predation

threat. only fairly large changes in the body weight of blackcaps (Sylvia atricapik) had

an impact on flight ability- They found a significant impairment of take-off ability when

fat load was greater than 40% of fat fiee body mass. It is dificuit to compare this value

to the chickadee data, but typically fat loads of over 50% of fat fiee body mass are only

seen in migratory birds crossing wide barries. such as the Sahara (Bairlein 1991).

The results of this study appear to support the prevailing theory that birds should

decrease their body weight in the presence oPa predator (Le. Lima 1986). Gosler et al.

( 1 995) found that great tits cany less fat during years of greater predation risk. Witter et

al. (1 994) found that birds cany more fat in the presence of protective cover and

45

Lilliendahl(l997) found that greenfinches decreased evening body weight after exposure

to a hawk mount.

These results, however. contradict the findings oFPravosudov and Gmbb (1998)

who found that tufied titmice increasrd their body weight f i e r seeing a hawk. This

discrepancy may be explained by differences in experimental design. Pravosudov and

Gmbb employed a more naturaiistic set-up, housing birds individuaiiy in an outdoor

aviary and presenting the mount at van-ous intervals throughout the day. They did not

food depnve their birds, atthough this should actuaily have Iowered the risk of starvation.

Moreover, the titrnice saw the hawk mount for a total of 16 min a day. with no

intervening baseline days. This may have Lead to a greater interruption of feeding and

there fore a greater starvation risk than the chickadees in our study,

Of course, the two species rnay use different escape tactics. although according to

Lima (1993) al1 parid species tvpically dive for cover- We did not provide cover in our

aviary. which may have forced the chickadees to employ a different escape mechanism.

The favoured parid strategy. illusrrated by

while the hawk is in the area- This would

the titmicé. may be to disrupt foraging and hide

make starvation nsk the major determinant of

their behaviour. On the other hand. if cover is unavailable, parids may remain active,

making predation risk and avoidinp an actual attack the major determinant of their

be haviour.

Altematively. it could be argued that the chickadees in the present experiment

were in fact increasing their body weight htfter seeing the hawk and that the increase just

did not occur until the following baseline day, The increased body weight on hawk

baseline days may have acted as a buffer which allowed birds to lower their weight on

hawk rnount days, when they actually saw the hawk. The mean of mount and baseline

days for aftemoon weight tends to be higher during hawk than during redwing 1. Ws

would suppoa the findings of Pravosudov and Gnibb (1998).

Both of these experiments were conducted in an aviary where food was readily

available, making it difficult to generalize the results to wild birds. Foraging in the wild

requires more effort than simply flying down to a food dish £311 of seeds, and perhaps a

greater sacrifice of vigilance. The reduction in caching and body weight observed in this

experiment may be even more pronounced in the wild. Alternatively. perhaps the birds

could affiord to reduce foraging precisely because food was so plentiful, while in nature

they rnay not have this Iuxury,

Obviously there is much more to learn about the effect of predation nsk on

passerines. Further studies using a more naturalistic design with unpredictabie food

access, available cover and longer hawk exposure would be helpful. Monitoring intake

will also cl&@ whether changes in body weight are mediated by changes in intake or in

metabolism. It would also be interestinp to compare food-storing species with non-storîng

species to see what effect the ability to store Food has on a bird's reaction to a predator.

2-5 Reference List

Bairlein, F. 1991. Body mass ot'garden warblers Svlvia borin on migration: a review of

field data, Vo~elwarte- 36.48-6 1.

B lem, C.R. 1990. Avian energy storage. Current Ornitholow. 7.59-1 13.

Gosler, A.G., Greenwood. J.J.D, & Pemins. C. 1995. Predation risk and the cost of king

fat. Nature, 377.621-623.

Grubb. TC. Jr. & Pravosudov. V.V. 1994. Toward a generai theory of energy

management in wintering birds. Journal of Avian Biolow, 25,255-160.

Hurly. T.A. 1992. Energetic reserves of marsh tits (Partrs pulustris): food and fat storage

in response to variable food supply. Behavioral Ecoloev. 3. 18 1-1 88.

King, J.R. 1972. Adaptive periodic fat storage by birds. Proceedines of the 1 Sh

International Ornithologv Conmess. pp. 200-2 17.

Kuliberg, C.. Fransson. T. & kkobsson. S. 1996. lmpaired predator evasion in fat

blackcaps (Sylvia atricapiZZn). Proceedings of the Roval Societv of London,

Lilliendahl. K. 1997. The effect of predator presence on body mass in captive

greenfinches- Animal Behaviour, 53,758 1,

Lima S.L. 1986. Predation risk and unpredictable feeding conditions: detemiinants of

body mass in birds. Ecolow. 67.3 77-3 85.

Lima, S.L. 1993. Ecological and evolutionq perspectives on escape fiom predatory

attack: a survey of north arnerican birds. The Wilson Bulletin, 105, 147.

McNamara LM. 1990. The starvation-predation trade-off and some behavioral and

ecological consequences. In: Behu~*ic)ral Mechanisms of Food Selection (Ed. By

R.N. Hughes), pp.39-58. Berlin: Springer-Verlag.

McNamara, J.M. & Houston, A.[. 1990, The value of fat reserves and the trade-off

be tween starvation and predation. -4 LW Biotheoretica. 3 8.3 7-6 1.

Metcalfe, N.B. & Ure. S.E. 1995. Diumal variation in Bight performance and hence

potential predation risk in srnaIl birds- Proc. R. Soc. Lond, B, 26 1.395400.

Pravosudov, V.V. & Grubb. T.C.. Sr. 1998. Management of fat reserves in tufied titmice

19

BueIophus bicoLor in relation to rÏsk of predation. Animai Behavior, 56.49-54.

Wingfield, J-C.. teck, C.M. & Moore. M.C. 1992. Seasonal changes of the

adrenocorticai response to stress in birds o f the Sonoran desert. Journal of

Exmimental Bioloev, 264.4 1 9-428-

Witter, M.S. & Cuthill, K. 19%. The ecological costs of avian fat storage- Phil. Trans.

R- Soc. London B, 340,73-92-

Witter, M.S., Cuthiil, 1-C. & Bonser, R.H.C. 1994- Experimentai investigations of mass-

dependent predation risk in the European Starling, Sts(rnus vdgaris. Animal

Behaviour, 48,20 1-32

Chapter 3

Conclusions and Perspectives

3.1 Conclusions and Pers~ectives

3.1 -1 Sumrnarv of Results

The data presented in this thesis extend our understanding of how predation risk

cari affect the body weight and behaviour o€smaLl birds. We found that black-capped

chickadees lowered their body weight and caching under a perceived predation nsk. This

supports the prediction that there is a trade-off between starvation risk and predation risk

(Lima 1986). It also suggests that chickadees reguiate fat and cache reserves in a similar

marner.

3- 1 -2 Altemate Anti-Predator Behaviours

Wild birds exhibit a wide variety of anti-predator behaviours. which means that

th& anti-predator strategy is likdy to be very plastic according to the situation.

Therefore, a bird with a high predation risk, may alter its behaviour to lower that risk. A

heavy bird, for example, rnay stay closer to cover than lighter birds and thereby equalize

its nsk of predation (Witter and Cuthill 1993).

Consequently, as Witter et al. ( 1994) point out, it is difficult to evaluate the

predation risk associated with a particular factor. ifone factor, such as weight, acts to

increase predation risk, the bird may adjust another factor, such as proximity to cover, to

compensate and lower predation risk. ThereFore, even though chickadees lowered their

body weight in this experiment. it does not necessarily mean that they will always do so

in the wild or that a heavy chickadee is necessdy at more risk than a Lighter chickadee.

3.1 -3 A~~hcabiIitv for Other S~ec ies

Different species typically have different fat levels depending on the niche they

occupy (Lilliendahl 1 997). Ground- feeding birds, for example. are typicdly fatter than

tree- feeding birds (Rogers 1 987). Predator avoidance tactics also differ between habitats

(Lima 1993) and so the cost of being fat is likely very different for different species

(LiIliendahl 1997)- ïherefore. different species would likely react very differently to this

experimental manipulation,

3.1.4 Future Work

Obviously, much more work is needed to completely understand the effect

predation nsk has on the body mass and behaviour of small birds. A series of

experiments comparing the presence and absence of cover and cornparhg species with

different natural histories would be very valuable. It would also be interesting to

compare a predictable food supply with an unpredictable one. Eventually, field studies

will be required to relate the laboratory findings to behaviour in the wild. This is a

promising area for future work-

3-2 Reference List

Lilliendahl. K. 1997. The effect of predator presence on body mass in captive

greenfuches. Animal Behaviour. 53.75-8 1 .

Lima, S.L. 1986. Predation risk and unpredictable feeding conditions: deteminants of

body mass in birds. Ecolow, 67.377-385-

Lima, S.L. 1993. Ecological and evolutionaiy perspectives on escape from predatory

attack: a survey of North Arnerican birds. The Wilson Bulletin, 105, 147.

Pravosudov, V.V. & Grubb, TC. Jr. 1998. management of fat reserves in tufied titrnice

Baelophu biocolor in relation to risk ofpredation. Animal Behavior, 56,49-54.

Rogers, C.M. 1987. Predation rïsk and fasting capacity: do wintering birds maintain

optimal bodymass? Ecolow, 68. 1051-1061-

Witter, M.S. & Cuthill. LC. 1993. The ecological costs of avian fat storage.

Philoso~hical Transactions of the Roval Societv of London, Series B, 340,73-92.

54

Witter, M.S., Cuthill, K. & Bonser. R.H.C. 1994. Experimental investigations of mass-

dependent predation risk in the European Starling, Stumus vutgaris- Animal

Behaviow. 48.20 1-222-

Research and Teaching Activities

Teaching Assistant Department o f P s y c h o l o ~ University of Western Ontario O 1/98 - 05/99

Research Assistant Laboratory of Dr. Cindy Staicer Department of Biology Daihousie University 05/96 - 08/96

Research Assistant Laboratory of Dr. [an Meinectzhagen Department of Psychology Dalhousie University 05/95 - 08/95 and 04/94 - 08/94