The breeding success of the Damara Tern Sterna balaenarum in the restricted diamond mining area

of southern NamibiaJustine Braby, Les Underhill, Rob Simmons and Jean Paul Roux

Damara Tern Sterna balaenarum

• Small seabird (52g) that breeds in small, isolated colonies (predominantly) along the desert coastlines of Namibia and South Africa

• Classified as Near-threatened under the IUCN red-data list (world population - 13,500)

• Migrates to West African countries like Gabon, Cameroon, Nigeria and Benin during the non-breeding season

• The Damara Tern is a near-endemic breeder to the region

• It has held special conservation status among Namibian conservation authorities for 25 years and has in recent years become a flagship species of the Namibian coastline

• Will be classified as a Specially Protected species under the draft Parks and Wildlife Management Bill of the Ministry of Environment and Tourism

BREEDING AREAS STUDIED

• Elizabeth Bay – the main study site

• Grosse Bucht – outside restricted area and used as a control for foraging

• Marmora Pan (Pocket Beaches)

• Anigab Pan (Hottentot’s Bay)

Elizabeth Bay

FEEDING:

• Simmons (2005) suggested that the introduction of sediment into the bay has been detrimental to breeding Damara Terns

• Control area is Grosse Bucht (22km north)

• Preliminary analysis (Elizabeth Bay, n=91; Grosse Bucht, n=70) showed no significant difference in foraging success between Grosse Bucht and Elizabeth Bay (χ21 = 1.158, P=0.2819)

• Further analysis will be completed once sediment discharge data has been received from Elizabeth Bay mine (and a greater sample size at both areas is collected)

Adult populations and breeding success

• During the height of breeding season (mid-December 2007) a minimum of 405 Damara Terns were found in the Sperrgebiet: Hottentot’s Bay (n=287), Marmora Pan (n=68), Elizabeth Bay and surrounds (n=32), Grossebucht (n=18)

• Elizabeth Bay had the smallest nest number (13), Hottentot’s Bay (Anigab Pan) had by far the greatest number of nests (80)

Nest Totals for Season 2007/08

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

Hottentot's Bay Elizabeth Bay Grosse Bucht Marmora Pan

Breeding locality

Num

ber o

f nes

ts

Breeding success

• Nests were deemed successful when chicks fledged• Only nests with known fates are shown

0%

20%

40%

60%

80%

100%

Hottentot'sBay

ElizabethBay

GrosseBucht

MarmoraPan

OtherFailures

OtherPredations

JackalPredation

Fledging

Egg measurements

• The mean length and breadth (mm) at Hottentot’s Bay were l=32.58, b=24.16, n=37, Grossebucht l=33.51, b=24.07, n=26, Elizabeth Bay l=33.09, b=24.34, n=10, and Marmora Pan l=33.24, b=23.85, n=50

• There was no significant difference in egg volume between the different colonies (F3,128=0.63, P>0.05)

• Mean volume (ml) was 9.214, SD=0.572 (range 7.590 – 10.812)

Egg length versus breadth (mm)

30

31

32

33

34

35

36

37

38

22 22.5 23 23.5 24 24.5 25 25.5 26

Egg breadth (mm)

Eg

g l

eng

th (

mm

)

Chick condition• Chick condition is determined by weight related to structural size

(length of head+bill) (Jan Veen’s unpublished method)• No significant difference between colonies (F3,52=2.1935, P>0.05),

this may be due to small sample sizes• Hottentots Bay colony lowest mean standardized condition (-0.502) and Grosse Bucht highest (0.376) indicating possible

relationship between condition and distance to sea

0

10

20

30

40

50

60

25 30 35 40 45 50

Chick Head Length (mm)

Ch

ick

Mas

s (g

)

Other colonies

Hottentot Bay (Anigab Pan)

Linear (Other colonies )

Linear (Hottentot Bay (AnigabPan))

Key findings• Nest number has decreased at Elizabeth Bay, but increased at all

other colonies

• Chick condition was not lowest at Elizabeth Bay

• There seems to be a trade-off between predation rates and distance to the sea, where in breeding areas closer to foraging sites chicks can be fed more frequently but predation levels are higher, whereas in breeding areas further from foraging sites chicks may not be fed as frequently (and adults may work harder to feed chicks as flying distances between colonies and feeding areas are much greater), but predation rates are much lower and chicks have a greater chance at survival to fledging

• More data needs to be collected to analyse breeding success, chick growth rates, foraging success as well as adult condition at different colonies. Thus far there have been no significant differences, apart from the smaller colony size, between the mined area, Elizabeth Bay, and the non-mined sites

Acknowledgements

• Megan Murgatroyd for her help in the collection of data during the past field season

• Namdeb for their continued support on the project• MET, especially Trygve Cooper and MFMR, especially

Jean-Paul Roux and Jessica Kemper• Animal Demography Unit, Les Underhill and Sue Kuyper

(UCT)• Percy FitzPatrick Institute, Rob Simmons (UCT)• Elizabeth Bay Mine, and especially Urban Burger, Senior

Geologist at Namdeb for his support on the project• Environmental Unit at Namdeb (Ronel van der Merwe,

Ursula Witbooi)