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The black-backed gull in northern New Zealand:
evidence of changing diet from stable isotope analysis of bone and feathers
Mel Galbraith 1 Matt Rayner 2 Sarah Bury 3 Graham Jones 1 Diane Fraser 1
1 Unitec Institute of Technology 2 Auckland War Memorial Museum 3 National Institute of Water and Atmosphere
Spoilt for Choice – foods of the black-backed gull (Larus dominicanus) in urban Auckland
Acknowledgements
We gratefully acknowledge the Royal Society of New Zealand for suppor:ng this study through the Science Teaching Leadership Programme. Thanks to 360 Discovery Cruises for travel support to Tiri:ri Matangi Island, and to Graeme Jones, Unitec Ins:tute of Technology, for iden:fica:on assistance.
Leanne Watson1, Mel Galbraith2, Diane Fraser2, Kayla Nicholson2 1 Ma:po Primary School, Te Atatu Peninsula, Auckland, New Zealand
leannew@staff.ma:po.school.nz 2 Department of Natural Sciences, Unitec Ins:tute of Technology, Auckland
References
Galbraith, M; Krzyżosiak, J.; Aguilar, G.; Jones G.; Oliver, R. 2015. Changes in the breeding status of the southern black-backed gull (Larus dominicanus) colonies on Rangitoto Island, Hauraki Gulf, New Zealand. Notornis 62: 192-201.
Higgins, P.J.; Davies, S.J.J.F. (Eds). 1996. Handbook of Australian, New Zealand and Antarc<c birds. Volume 3: Snipe to Pigeons. Melbourne: Oxford University Press.
Nogales, M.; Zonfrillo, B.; Monaghan, P. 1995. Diets of adult and chick herring gulls Larus argentatus on Ailsa Craig, south west Scotland. Seabird 17: 56-63.
Spaans, A.L. 1971. On the feeding ecology of the herring gull Larus argentatus Pont. in the northern part of The Netherlands.
Ardea 59:73-188.
Discussion
Our results support black-backed gulls as characteris:c omnivores and top predators. Marine fish and molluscs and small birds are important in the diet at all sites. We suspect the presence of small birds in the diet may be small fledglings falling from nests and may be seasonal, however further studies would need to be undertaken to confirm this.
Our results suggest that the diet of black-backed gulls closest to Auckland City is influenced most by the urban environment. The presence of chicken and beef bones in pellets suggest that the gulls are scavenging on human leeovers. Gulls roos:ng closer to urban environs have a more varied diet sugges:ng they are exploi:ng human leeovers at a higher rate that birds on the islands. The presence of plant material in the diet of gulls at Western Springs may be due to the Gulls feeding on terrestrial arthropods on grassland. The presence of freshwater shrimp and one eel jaw are indica:ve of the Gull’s being opportunis:c feeders.
However, chicken is common to pellets from all sites, even on Tiri:ri Matangi some distance from urban habitats. Field observa:ons of chicken bones scahered around the sites support this. This suggests that black-backed gulls will expend energy travelling longer distances to readily available, high energy food. Although the data is only comparable due to a certain degree due to the fact that soe bodied animals do not appear in pellets (Nogales 1995), similar studies of western gulls Larus occidentalis (Anneh and Pieroi 1989) and of herring gulls L. argentatus (Spaans 1971) concluded that pellets accurately reflect the adult dietary composi:ons.
Results
As expected of a top predator and omnivore, the results show a range of food in the gull’s diet. City-based birds showed a par:cularly varied diet, including terrestrial beetles, shrimps and eel. Fish was the predominant food source for gulls on Tiri:ri Matangi, in contrast with small birds being the most dominant in pellets from Western Springs. Chicken and small birds were a common food source at all sites. Beef bones were present in pellets from Western Springs.
Method
Many of the foods gulls take and/or ingest contain indiges:ble maher, eg. bones, exoskeletons, feathers. This maher may be discarded either as a regurgitated pellet, or simply lee as prey ‘remains’. Analysis of regurgitated pellets is an accepted method to inves:gate the foods being consumed by the gulls (Nogales et al. 1995). Pellets and prey remains are generally concentrated around roost and breeding sites as these are loca:ons where significant :me is spent by the gulls.
In this preliminary study, pellets were collected from three sites - Western Springs Park (an inner city communal roost site), Rangitoto and Tiri:ri Matangi Islands. The pellets were soeened in water and dissected under microscopes to iden:fy the food remnants present. A comparison was made between the pellets collected at the different sites.
Introduc8on
The southern black-backed gull (Larus dominicanus) is a common species found throughout New Zealand, and par:cuarly abundant in ci:es. Roost and breeding sites are found around Auckland City, on Rangitoto Island and Tiri:ri Matangi Island. Both historical increase and decline of the black-backed gull’s popula:on size in the Auckland region has been ahributed to human ac:vity, especially the changing availability of food from human sources (Galbraith et al. 2015).
Gulls are generally both marine and terrestrial feeders, as well as being omnivorous predators and scavengers (Higgins & Davies 1996). A preliminary comparison of the food of the species in areas both distant and close to the Auckland urban environment has been undertaken.
Observa<on of discarded chicken bones at a roost site.
0%
10%
20%
30%
40%
50%
60%
70% Tiri:ri Matangi
0% 10% 20%
30% 40% 50%
60% 70% 80%
90% Rangitoto
0%
10%
20%
30%
40%
50%
60% Western Springs
Regurgitated pellet containing a small bird Regurgitated pellet containing marine crustaceans.
Birds New Zealand conference 2015
0 % 5 % 1 0%
1 5%
2 0%
2 5%
3 0%
3 5%
4 0%
Fish Aqu a tic
ar thro p od M ollus c Sm all Bird C hick en M am ma l T erre strial Arth rop o d P la nt
M ate rial La rge Bird Oth er
Fr eq uen cy
? Food sources of black-backed gulls in Auckland from
analysis of regurgitated pellets (n=371)
Why the black-backed gull?
• Gulls are both marine and terrestrial feeders – likely to reflect changes environmental quality at relatively small scales.
• Black-backed gull an ideal species to investigate the long-term impacts of local environmental changes:
o resident coastal seabird;
o generalist marine and terrestrial feeder;
o common in the Auckland region.
Ø Has the stable isotope signature of gulls in the Auckland
area changed over time?
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• Gulls are both marine and terrestrial feeders – likely to reflect changes environmental quality at relatively small scales.
• Black-backed gull an ideal species to investigate the long-term impacts of local environmental changes:
o resident coastal seabird;
o generalist marine and terrestrial feeder;
o common in the Auckland region.
Ø Has the stable isotope signature of gulls in the Auckland area changed over time?
Two sample sources:
• feathers – AM collection & contemporary samples;
• sub-fossil bones (pre-human marker) – AM collection.
0 1 2 3 4 5 6 7 8
19 00 19 09 19 20 19 30 19 40 19 50 19 60 19 70 19 80 19 90 20 00 20 10 20 20
Nu m b er o f sp ec im en s
Collection dates of Auckland MuseumLarus dominicanus specimens
(n=33) 10
1 7 10 4 1
Bone sample ages (yBP, ± 20-24 )
4571 3753 3153 2279 1596
R² = 0.1065
0 5 1 0 1 5 2 0
1 90 0 1 92 0 1 94 0 1 96 0 1 98 0 2 00 0 2 02 0
R² = 0.2705 -2 5
-2 0 -1 5 -1 0 -5 0
pre-human
pre-human δN15
δC13
- small drop in trophic level
- shift towards terrestrial feeding
-13 -11
(Weiser & Powell 2011)
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(M c K in n o n 1 9 9 7 )
Pre-human vegetation
(M c K in n o n 1 9 9 7 )
(R o b e rts o n e t a l. 2 0 0 7 )
Summary: - small drop in trophic level;
- trend towards an increase terrestrial foods.
References:
Svyatko, S. n.d. Information about stable carbon and nitrogen isotope analysis. http://chrono.qub.ac.uk/Publications/IRMS/
Weiser, EL; Powell, AN. 2011. Evaluating gull diets: a comparison of conventional methods and stable isotope analysis. J. Field Ornithology 82(3): 297-310.
Acknowledgements:
• Unitec Institute of Technology research funding;
• Nisbet Research Award, The Waterbird Society, USA;
• Royal Society of New Zealand Science Teacher Leadership programme;
• NIWA & GNS laboratories;
• field volunteers.
[Research carried out under DOC permits 39910-RES and AK-32734-RES ]
Environmental and
Animal Sciences
