Jonathan Wright

Helpers-at-the-nest have the same provisioning rule as parents:experimental evidence from play-backs of chick begging

Received: 17 June 1997 /Accepted after revision: 28 February 1998

Abstract Despite the success of kin selection in ex-plaining helping-at-the-nest among communally breed-ing birds, we know almost nothing about how helpersregulate their chick-feeding e�ort. This is especially in-teresting given how much we now know about parentalprovisioning `rules-of-thumb' and the evolution of chickbegging as an honest signal of `need'. This study ex-plores the provisioning rules of helpers and parents inArabian babblers (Turdoides squamiceps), using tapeplay-backs to supplement chick-begging signals and in-crease apparent brood demand. In all eight groups tes-ted, both helpers and parents fed older, noisier broods athigher rates. Total provisioning rates to nests increasedduring begging play-back days compared to controldays. Absolute provisioning rates by helpers and thescale of their responses to play-backs were statisticallyindistinguishable from those of parents. In both helpersand parents, increases in nest visits during play-backswere associated with reductions in foraging distancefrom the nest and increases in size of prey delivered.Older birds of both sexes delivered slightly larger preyitems, possibly re¯ecting di�erences in foraging abilitydue to experience. These results are consistent with theidea that, like the parents, helpers-at-the-nest in Arabianbabblers provision nestlings as part of a strategy of in-vestment, irrespective of helper age, dominance status orsex. In this species, high relatedness within groups mayprovide parents and helpers with similar kin-selected®tness bene®ts, although the mutualistic advantages tohelpers from simply augmenting group sizes cannot beruled out.

Key words Helpers-at-the-nest á Parental care áChick feeding á Cooperative breeding áArabian babbler Turdoides squamiceps

Introduction

In over 200 species of birds, breeding pairs are assistedby `helpers-at-the-nest' which contribute towards feed-ing chicks that are not their own (Brown 1987). Anumber of alternative, but not mutually exclusive, hy-potheses have been proposed to explain why this ap-parently altruistic behaviour has evolved. These include:(a) direct or indirect ®tness bene®ts derived from feedingrelated young via kin selection (Hamilton 1964; May-nard Smith 1964); (b) future increases in survivorshipand/or reproduction of helpers through the mutualismof pseudo-reciprocity and augmenting group size(Woolfenden and Fitzpatrick 1978, 1984; Ligon 1981;Brown 1983, 1987; Connor 1986, 1995); (c) bene®ts viasignalling, due to improvements in social bonds either(1) to secure group membership and its associated ben-e®ts (`pay-to-stay', Gaston 1978; Reyer 1984), (2) toenhance status and dominance rank in the group andtherefore future breeding opportunities (`social prestige',Zahavi 1974, 1977, 1990, 1995), or (3) to form associa-tions with the young in the nest (`kinship deceit', Connorand Curry 1995); (d) useful experience for helpers interms of foraging and feeding young in the nest (Rowley1977; see also Brown 1987) and (e) unselected conse-quences of non-dispersal of grown young responding tothe stimulus of begging (Jamieson 1986, 1989; Jamiesonand Craig 1987).

Despite the general success of kin selection in ex-plaining helping-at-the-nest among communally breed-ing birds (see Brown 1987; Emlen 1991), we knowalmost nothing about the provisioning `rule-of-thumb'that helpers use to regulate their chick-feeding e�ort.This is especially interesting given how much we knowabout parental provisioning. Theoretical and empirical

Behav Ecol Sociobiol (1998) 42: 423±429 Ó Springer-Verlag 1998

J. Wright (&)School of Biological Sciences, University of Wales,Bangor, Gwynedd, LL57 2UW, UKe-mail: j.wright@bangor.ac.uk, Tel:+44-1248-382313,Fax: +44-1248-371644

studies have demonstrated plausible evolutionarily sta-ble strategies of cooperation in systems where more thanone parent invests in provisioning the same brood (e.g.Houston and Davies 1985; Wright and Cuthill 1989).Such cooperative investment in chick feeding is almostcertainly implemented by individual parents using chickbegging to regulate their provisioning e�ort, in order tocope adaptively with short-term changes in brood sizeand partner e�ort (e.g. Wright and Cuthill 1990). Recentwork concerning the evolution of begging as a reliablesignal of chick hunger or `need' for food has con®rmedthe adaptive nature of parental provisioning based onsome measure of chick-begging e�ort (e.g. Godfray1991; Cotton et al. 1996). The question now is whetherhelpers-at-the-nest use these same cues to regulate theirinvestment in the brood.

The chick-feeding rules of helpers should re¯ect theirevolutionary interests in provisioning nestlings. Kin se-lection (hypothesis a) predicts that helpers should actlike parents, increasing their chick-feeding e�ort in ac-cordance with the net ®tness returns from investment inthe brood. Hence, more related chicks should be fed athigher rates, and helping should be adjusted accordingto both its energetic cost and current brood demand forfood (i.e. begging e�ort of the brood, itself re¯ectingnumber, age and condition of young). Non-kin-selectedhelping in the form of pseudo-reciprocity and/or aug-mentation of group size (hypothesis b) produces verysimilar predictions. As with kin-selected helping, suchmutualistic bene®ts predict that like parents helperswould use begging as a signal of brood need, becauseany investment by helpers in chick-feeding e�ort in-volves the same types of costs to the feeder and bene®tsto the chicks in terms of future ®tness and survival. Incontrast, if chick-feeding operates as a signal, wherebyhelping enhances group membership and/or socialbonds (hypothesis c), the role of chick begging is lessclear. Whilst it may be advantageous to be seen to helpfeed chicks even more when they really need it, anyhelper which displays parent-like reactions to beggingsignals of brood demand might leave itself open toparasitism of its feeding e�ort, and hence exploitationby both chicks and parents (i.e. we have yet to con®rmthe conditions for the evolutionary stability of such`altruism as a signal'; Wright, in press b). Finally,helping as experience (hypothesis d) or as non-adaptivebehaviour (hypothesis e) predicts parent-like responsesin helpers, because such behaviour is supposed to be amanifestation of the same parental rule-of-thumb sim-ply being expressed earlier in the ontogeny of theseyounger individuals. However, given the limited ®tnessbene®ts from helping under such hypotheses, individualcontributions to chick feeding would need to involvesimilarly limited costs, which is unlikely in many sys-tems. Therefore, helping on the same scale as parentalcare is unlikely, and any parental type of response tochick begging would be maladaptive and again exposehelpers to potential exploitation by the chicks and theirparents.

This paper uses data from communally nestinggroups of Arabian babblers (Turdoides squamiceps) tocompare the provisioning rules of parents and helperswhen responding to natural and experimental changes inapparent brood demand in the form of chick-beggingnoise. Observational data include individual visit rates,type and size of prey delivered, foraging location of in-dividuals within the group and the relative amount ofnon-provisioning behaviours.

Methods

The study population

The Arabian babbler is a group territorial, communally breedingthrush-like bird that builds open nests in thorn bushes, in theArabian and Sinai deserts. Groups range from 2 to 22 individualsof both sexes, with age-related linear dominance hierarchies withineach sex class. Each group usually contains only one breeding pair,although subordinate individuals seem to obtain some reproduc-tion if they are unrelated to the dominant breeder of the oppositesex (i.e. there is inbreeding avoidance). Young birds delay dispersalfor 1±3 years whilst acting as helpers to their parents, althoughhelpers in newly formed or restructured groups may be less closelyrelated to the breeding pair (for more details see Zahavi 1988; 1989;1990; Lundy et al., in press).

The study site at Hatzeva comprises a 25-km2 area of desert,located 30 km south of the Dead Sea in the Arava rift valley inIsrael. Twenty groups of Arabian babblers have been continuouslystudied there since 1971 by Prof. Amotz Zahavi and students fromTel Aviv University. For at least the last 5 years, up to 40 groupshave been continuously monitored on a weekly basis and habitu-ated to human observers. All birds in the present study were in-dividually colour ringed and their family histories known.

Experimental procedure

Nesting attempts were observed from hatching until the chicks¯edged at around 13 days. Chick begging vocalizations were ®rstrecorded using a DAT tape recorder (HHB PortaDat PDR100) anda small tie-clip microphone (Sony ECM-77B) placed 0.5 m abovethe open nest cup. The recorded sound was then edited, ®ltered toremove low-frequency wind noise, and ampli®ed on a computer(Macintosh PowerBook 180, using SoundEdit v.2.05 software) toproduce a 30-s burst of continuous and urgent begging vocaliza-tions. This begging sound was then re-recorded onto a DAT taperecorder as a seamless repeated loop, and broadcast into the nestusing a speaker (Panasonic RP-SP80) placed against the outsidewall of the nest.

Decibel levels during natural chick begging and broadcast play-backs were monitored using a sound meter (Castle GA206) held ata standard 2 m unobstructed horizontal distance from the nest.Upon arrival of a bird at the nest, the volume of the play-back wasincreased rapidly and in time with the begging movements of chicksin the nest. Natural broods produced an average sound level of52.0 dB (SE � 0.4 dB), but an average level of 63.5 dB(SE � 0.2 dB) was used during broadcast, the latter being equiv-alent to that achieved by a very hungry brood of three babblerchicks just prior to ¯edging. Play-backs were restricted to broods ofat least 5 days of age, because very young chicks did not producesu�cient begging noise. Begging calls changed markedly with chickage, from simple unvaryingly high pitched `peep' calls in youngchicks, to loud `peaked' calls of varying frequencies and overtonesin older chicks. Interestingly, towards ¯edging, chick begging callsin Arabian babblers gradually approach the sound and structure ofthe long-distance `contact call' of adult birds. To control for these

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changes with chick age, begging noise was recorded in the lateafternoon each day and a new play-back tape manufactured thatevening for broadcast back into the same nest on the followingmorning and afternoon.

Control observations were carried out on the day preceding andthe day following each experimental play-back day, therebyequalising the mean age of chicks between treatments. This control-experimental-control design for each play-back trial was repeatedbetween one and three times during the nestling period, dependingupon the particular group (e.g. nest predation reduced the numberof full trials possible in some groups). Groups used varied in thenumber of helpers, brood size and number of trials completed(Table 1). Only `simple' family groups were used, where helperswere the o�spring of at least the breeding female and generallysired by the current breeding male as well (i.e. no helper was in-volved in reproductive competition).

Data collection

Data were collected simultaneously by two observers, between 1March and 30 June 1995, for 3 h every morning (starting within anhour of sunrise) and 2 h every afternoon (®nishing within an hourof sunset). One observer using binoculars (Leica 8 ´ 20 BCA) atminimum focal distance sat 3 m from the nest and recorded (1)individual bird feeding visit rate, (2) size (classed 0±3) and type(larval or adult invertebrate, vertebrate or plant material) of preydelivered and (3) sound level of begging at the nest. Previous workon this population has con®rmed that these behaviours are unaf-fected by this method of relatively close observation (Wright 1997).The second observer collected foraging data by walking with thegroup through the territory and taking scan samples every 15 minto record (1) individual bird location, (2) individual behaviour(0 � `in nest tree', 1 � `acting as sentinel', 2 � `actively forag-ing', 3 � `territorial patrol', 4 � `inter-group territorial encoun-ter', and 5 � `preening') and (3) a ®ve-point scoring of groupdispersion (1 � all birds together, 2 � group split between twolocations, 3 � birds in more than two clusters, 4 � very few birdstogether, 5 � individual birds widely dispersed throughout terri-tory). Bird location was estimated using proximity to knownmarkers within the territory, and recorded as a compass directionplus distance in metres from the nest tree. As babbler territoriesconsist of open desert habitat, and groups were often small andclumped, it was usually possible to obtain the location and identityof most birds by merely using binoculars (mean proportion ofgroup members missed � 19%, range � 0±60% per scan). Fortwo of the eight nests, it was not possible to collect foraging datadue to the lack of a second observer at the time.

Analysis

For each bird on each day, the mean number of visits and thenumber of prey of di�erent types delivered were calculated per

hour, whilst prey sizes and distance from nest were reduced tomeans per visit. For the foraging data, it was necessary to controlfor di�erences in the number of unsuccessful scans where observersfailed to see birds. The data are therefore presented as mean pro-portions per bird per successful scan. Sample sizes for visit ratedata were eight groups, 45 individual birds, and 44 days of ob-servation. For foraging data, sample sizes were slightly lower withsix groups, 36 birds, and 32 days of observation. Each day andchick age was divided into two according to observation sessions inthe morning (as whole number of days) and evening (as number ofdays plus 0.5). The unit of analysis was either chick age, individualbird or group as appropriate.

In some minor classes of prey and behaviour, there were too fewcases for full statistical analyses. Therefore, results are presentedonly for the `larval invertebrate' and `adult invertebrate' prey types,and for `in nest tree', `acting as sentinel' and `actively foraging'behaviours. For analyses of the experimental treatment, all datawere reduced to two means per individual bird: one for controlsand one for experimental play-backs. A repeated-measures ANO-VA was then used with play-back treatment as the repeated mea-sure, with individual class of bird within the group (i.e. sex anddominance rank) as a factor, and the interaction between the twoalso included in the model. The foraging data on proportion ofsuccessful scans in which behaviours were seen were arcsine-squareroot transformed to normalise them before ANOVA. Two-tailed P-values are given throughout.

Results

Visit rate data

There was a signi®cant positive e�ect of chick age on themean visit rate per bird (r2 � 0.50, F1,14 � 15.96,P � 0.001), and the mean begging sound levels pro-duced by these broods also increased with chick age(Fig. 1: r2 � 0.41, F1,14 � 9.81, P � 0.007). A re-gression of the e�ect of begging sound level on nest visitrate was performed for each bird separately, and overallthe estimated slopes of these regressions were positiveand signi®cantly greater than zero (t44 � 7.61,P < 0.001). However, there were no signi®cant di�er-ences between the slopes of the di�erent classes of birdswithin groups (i.e. the e�ect of sex and/or dominance

Fig. 1 The e�ect of chick age on the mean number of feeding visitsper bird, all birds in all groups combined (best-®t line:y � 1.27 + 0.09x). The mean begging noise values (dB) of allbroods combined are given above each point

Table 1 Group size, brood size and the number of begging play-backs (i.e. 3-day control-experimental-control trials) completed forthe eight babbler groups included in the study. The last letter ofgroup indentities denotes ®rst (A) and second (B) brood of theseason. Note that all birds in all groups were sampled and includedin all tests

Group Group size Brood size Number of trials

ATDA 6 3 1BOTA 5 3 2MBSA 4 3 2MZRB 5 4 3STVB 8 3 2SVAA 3 2 2WSZB 9 4 3ZOMB 5 3 3

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rank: F9,35 � 1.35, P � 0.246). Therefore, all classes ofbird fed older, noisy broods at greater rates.

The e�ect of the experimental play-backs of beggingsound was to increase signi®cantly individual visit ratesto the nest (Fig. 2a, Table 2a). However, there was nosigni®cant di�erence between the visit rates of di�erentclasses of bird within groups, and no signi®cant inter-action between class of bird and the e�ect of the play-backs (Fig. 2b). Therefore, all classes bird fed at similarrates and exhibited similar increases in visit rate inresponse to the begging play-backs.

Size and type of prey delivered

As with visit rates, the mean size of prey delivered tonests increased signi®cantly during the begging play-backs as compared with control observations, and againthere was no interaction between class of bird and thee�ect of the play-backs (Table 2). However, mean size ofprey delivered di�ered signi®cantly between the indi-vidual classes of bird within groups, tending to decreasefor birds of lower dominance rank (Fig. 3).

The delivery rate of adult invertebrate prey re¯ectedchanges in overall visit rate, increasing signi®cantlyduring play-backs as compared to controls, with nosigni®cant di�erences being found between di�erentclasses of bird, and no signi®cant interaction betweenbird and play-back (Table 2a). In contrast, the deliveryrate of larval invertebrate prey did not increase duringthe experimental play-backs, with no signi®cant e�ect ofclass of bird, nor any interaction between the two factors(Table 2). Therefore, for all classes of bird, the increasedvisit rates in response to begging play-backs were theresult of an increased delivery rate of adult invertebrateprey, but not larval, invertebrate prey.

Foraging data

During experimental play-backs, birds foraged signi®-cantly closer to the nest than during control observa-tions (Table 2: play-back mean � 143.5 m, SE � 9.3;control mean � 176.6 m, SE � 10.1). However, therewas no signi®cant di�erence in the distances at which

Table 2 The results of ANOVA, with begging play-back treatment(Play-back) as a repeated measure, individual class of bird withinthe group (Bird) as a factor, and the interaction between the two(Play-back*Bird). F-ratios, degrees of freedom and P-values aregiven for visit data tested (Visit rate the mean visit rate per bird,Prey size the mean size of prey, Adult prey the rate of delivery ofadult invertebrate prey, Larval prey the rate of delivery of larvalinvertebrate prey) and foraging data tested [Distance mean distancefrom the nest tree; and proportion of 0.25-h scan samples whenbirds were seen in the nest tree (In nest tree), as sentinels (Sentinel)or foraging (Foraging)]

Play-back Bird Play-Back*Bird

F1,35 P F9,35 P F9,35 P

Visit rate 23.90 < 0.001 1.59 0.158 1.00 0.455Prey size 7.10 0.012 2.27 0.040 1.48 0.195Adult prey 6.26 0.018 0.97 0.486 0.74 0.674Larval prey 0.23 0.853 0.23 0.633 0.80 0.605

F1,26 P F9,26 P F9,26 P

Distance 6.70 0.016 0.47 0.879 0.48 0.875In nest tree 0.13 0.717 1.20 0.343 1.15 0.369Sentinel 6.83 0.015 0.58 0.804 1.35 0.267Foraging 1.00 0.327 0.48 0.873 0.79 0.629

Fig. 2a,b Mean visit rates (�SE) for each experimental day for allgroups combined (a), and each class of bird within groups (b) (M1alpha male, F1 alpha female, M2 beta male, F2 beta female, etc.) forcontrol and experimental begging play-back treatments. The numberof birds in each class is given at the base of the control bars

Fig. 3 Mean size of prey delivered (�SE) for each class of bird withingroups (M1 alpha male, F1 alpha female, M2 beta male, F2 � betafemale, etc.)

426

di�erent classes of bird foraged, and no interaction be-tween individual class of bird and the play-back e�ect.The mean level of group dispersion throughout the ter-ritory did not di�er between play-back and control days(repeated F1,5 � 0.54, P � 0.494). So, although allbirds foraged closer to the nest during play-backs, thisdid not result in individuals within groups being moretightly clumped in the same part of the territory.

The experimental play-backs and individual class ofbird had no signi®cant e�ect upon the proportion ofscans in which birds were seen `in the nest tree' or `ac-tively foraging' (Table 2). Birds were signi®cantly lesslikely to act as sentinels during the experimental play-backs, but this e�ect was relatively minor in terms of theproportion of total time involved in sentinel behaviour(mean control scans � 6.1%; mean play-back scans �5.2%).

Discussion

Arabian babblers appear to be similar to many, if notall, monogamous bird species in that they adjust theirprovisioning rate according to the begging noise in thenest (e.g. Bengtsson and Ryde n 1983; Wright and Cut-hill 1990), a reasonable response if the begging signals ofchicks are positively related to their nutritional `need'for food (Godfray 1991; Cotton et al. 1996). The resultsfrom this study also suggest that there is a strong chickage component in the use of begging noise as a signal.Experimentally supplemented begging noise elicitedprovisioning responses similar to those used when adultsfed older broods. Visit rates increased, and larger preyand higher proportions of adult invertebrate prey typeswere provided, as would be suitable for older, largerchicks with more fully developed digestive systems (i.e.more prey with relatively indigestible invertebrate exo-skeletons). It was notable that following an hour or twoof play-backs and such increased food delivery, chicksceased to beg and lay satiated in the bottom of the nest.

Babblers provisioning at a greater rate in response tobegging play-backs reduced their foraging distance fromthe nest, provided more and larger prey items (e.g. adultinvertebrates, which were possibly easier to gather), andalso reduced other non-foraging behaviour such as`acting as a sentinel' against predators. As in parentalprovisioning (see Wright and Cuthill 1989, 1990;Markman et al. 1995; Wright et al., in press, and refer-ences therein), these changes in foraging behaviour ap-pear adaptive in that they allow increased quantities offood to be delivered to the nest, but at the possible ex-pense of the nutritional quality of chick diets. All birdsspent the majority of their time either foraging or de-livering food items to the nest, with an average of justover 10% of their time being spent in all other behav-ioural activities combined. Thus, as in most bird species(Bryant and Westerterp 1980; Drent and Daan 1980; forhelpers see Heinsohn and Cockburn 1994), provisioning

of nestlings in Arabian babblers appears to be an ener-getically costly and time-consuming process, with anyspare time being used primarily for self-feeding (seeKacelnik and Cuthill 1990; Martins and Wright 1993).This heavy energetic cost makes it unlikely that helping-at-the-nest in Arabian babblers is simply an unselectedbyproduct of parental chick-feeding behaviours (Jamie-son 1986, 1989; Jamieson and Craig 1987), or that it isuseful only as a learning experience in chick feeding(Rowley 1977; see Brown 1987). However, this secondpossibility maybe true for newly ¯edged young in thisspecies which sometimes feed the chicks in later broodsat a low level during their own natal season.

Arabian babblers were single-prey loaders in nearlyall observations of nestling provisioning. Any small preyitems are eaten by the adult birds themselves whilstforaging (rare exceptions to this involve the collectionand delivery to the nest of numerous small invertebrates,such as ants). There was evidence that more dominantindividuals of both sexes delivered larger prey items,which could have been the result of older, and thereforemore experienced, birds being more able to catch anddismantle larger prey types for delivery to the nest. Thiswas especially noticeable in the ®eld when it involvedlarger species such as crickets (Orthoptera) and geckos(Vertebrata).

The most interesting and surprising result has to bethe almost complete lack of di�erences between theforaging and provisioning behaviours of helpers-at-the-nest and parents in the groups tested. Indeed, withoutadditional information concerning group structures,helpers and parents would have been indistinguishableon the basis of their chick-feeding behaviours alone.Much of the variation in absolute feeding rates betweenclasses of bird in Fig. 2b is due to the low e�ort of birdsranked three and above, which is purely an e�ect of their(by de®nition) coming from larger groups in which allbirds perform equally few feeds (see Wright 1997, inpress a). The increase in visits as the brood got older andnoisier showed no di�erences between classes of birdwithin groups. This lack of e�ect was mirrored in all ofthe results from the experimental play-backs, whichcaused consistent increases in the foraging and provi-sioning behaviours of helpers and parents alike. Inter-estingly, observations from the same study siteconcerning the division of food within the brood con®rmthat both parents and helpers have similar provisioning`rules-of-thumb', based again on chick-begging e�ort(Ostreiher 1997). As in many monogamous bird species(e.g. McRae et al. 1993; Kacelnik et al. 1995; Cottonet al. 1996), all age and sex classes of Arabian babblersallocate more food to the chick that pushes its nestmatesout of the way and begs most conspicuously.

In the Arabian babbler, helpers-at-the-nest clearlyfollow a strategy of adaptive investment in dependento�spring, just as parents do in other species of bird(Lack 1968; Trivers 1972). Investment by more than oneparent at the same nest has been shown to be evolu-tionarily stable, and to match the behavioural strategies

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of care that we see in the ®eld for both biparental andmultiparental systems (e.g. Houston and Davies 1985;Wright and Cuthill 1989). According to the data pre-sented here, helpers in the Arabian babbler use the samechick-feeding strategies that are necessary for coopera-tive investment in young in the nest. However, theseadjustments in parental and helper work rates shownhere occurred in parallel, with all classes of bird in-creasing and decreasing their e�orts together in responseto changes in brood demand. Interestingly, we nowknow from further experimental work that such ad-justments can be made by di�erent classes of groupmember independently (J. Wright and N.B. Dinge-manse, unpublished data). Evolutionary strategies ofcooperative investment predict incomplete compensato-ry adjustments in individual provisioning e�ort follow-ing changes in the chick-feeding e�ort of other groupmembers (see Houston and Davies 1985). Compensatoryresponses in chick feeding have been demonstratedthrough the temporary removal of birds in cooperativesystems (e.g. Hatchwell and Davies 1990; Hatchwell andRussell 1996), but such results could always re¯ectadaptive responses to the death or desertion of othergroup members. As yet, incomplete compensation inresponses to decreases in the provisioning e�ort of col-laborators still present have only been demonstrated inexperiments on pairs of monogamous birds (e.g. Wrightand Cuthill 1989; Markman et al. 1995).

Contrary to earlier reports, the present study providesno evidence that Arabian babbler helpers use chick-feeding as an altruistic signal to `show o�' and gain so-cial prestige within the group (Zahavi 1974, 1977, 1990).There were no observations of `interference' betweenhelpers when at the nest (sensu Carlisle and Zahavi 1986;Zahavi 1995), and no other evidence that behavioursrelated to dominance or prestige are involved in chickfeeding (see also Wright 1997, in press b). Hence, al-though these results do not provide a critical test, thedata presented here and elsewhere on Arabian babblers(Wright 1997, in press a) do not appear to provide anyparticular support for the idea that helping-at-the-nest isused as a signal to strengthen social bonds within thegroup, such as `pay-to-stay' (Gaston 1978), `socialprestige' (Carlisle and Zahavi 1986; Zahavi 1974, 1977,1990, 1995) or `kinship deceit' (Conner and Curry 1995).

In the present study of `simple' family groups, help-ers-at-the-nest were always full, or at least, half sibs ofthe young in the nest. Overall, less than one-third of thispopulation is made up of `complex' non-family groups,and even in such groups, helpers were nearly always®rst- or second-order relatives of the young in the nest(see Lundy et al., in press). With such high helper-o�-spring relatedness, there is obvious potential for kinselection to favour helping-at-the-nest in this species(Hamilton 1964), especially given the abundant evidencefor its role in the helping behaviour of other communallybreeding bird species (see Brown 1987; Emlen 1991).However, on the basis of the evidence presented here, wecannot rule out the additional possibility that some of

the investment bene®t to helpers comes from pseudo-reciprocity or augmenting group sizes (Woolfenden andFitzpatrick 1978, 1984; Ligon 1981; Conner 1986, 1995;Brown 1987). As with kin selection, helping for thepurposes of augmenting group size would produce thesame patterns of investment seen here, based upon en-ergetic costs of chick feeding and begging signals ofbrood demand. Unlike kin selection, such helping e�ortshould mirror the future direct ®tness bene®ts to begained from survival of the young, rather than theirdegree of relatedness, but such data are beyond thescope of this paper. We know that larger babbler groupsfeed at greater rates and produce more young, withlower chick-feeding e�ort expended per helper (Wright,in press a). More helpers may also increase the proba-bility of groups winning territorial con¯icts with con-speci®cs, or reduce predation risk via more e�ectiveearly warning sentinel and group-mobbing behaviours(Zahavi 1989). Further evidence is obviously required inthis species to quantify the potential mutual advantagesto helpers from increasing group size.

In conclusion, provisioning strategies used by help-ers-at-the-nest in Arabian babblers appear remarkablysimilar to those used by parent birds in this and otherspecies. Both natural and experimental increases in ap-parent brood demand (i.e. begging) caused greaterquantities of prey to be delivered. To do so, both parentsand helpers foraged closer to the nest and delivered moreadult invertebrate prey types (at the possible expense oftheir own self-feeding rates and the nutritional quality ofchick diets). In almost all chick-feeding and foragingbehaviours in the present study, parents and helperswere observationally and statistically indistinguishable.From previous theoretical and empirical studies onmonogamous birds, it is likely that this parental-typechick-feeding strategy shown by helpers-at-the-nest inArabian babblers has evolved as a strategy of coopera-tive investment by helpers in the ®tness of the young inthe nest, for the purposes of inclusive ®tness or mu-tualistic bene®ts to be gained in the future within alarger group.

Acknowledgements I am very grateful to Amotz Zahavi for al-lowing me to work on his wonderful birds, as well as for muchstimulating conversation and hospitality during my time in Israel.Thanks to Amy McKinven for help in data collection, Ani Kazem,Kim Lundy, Mark Elgar and two anonymous referees for com-ments on the manuscript, and to Avner Anava and everyone atHatzeva for discussion and generous hospitality. For precious desk-space in Cambridge and comments on earlier versions of this work,thanks to Nick Davies. This study was funded by The BlausteinInternational Center for Desert Studies, Sede Boker Campus, BenGurion University, Israel, and a fellowship from NERC, U.K.

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