Where, when and why do Lesser Black-backed Gulls forage in cities?

For many species human activities alter the environment they encounter on a daily basis, often affecting wildlife adversely because of a loss in resource availability. Some species, however, manage to forage successfully in human-altered environments (Chace & Walsh 2006, Sol et al 2013) and have expanded their foraging activity to include anthropogenic resources where they can cause a nuisance for humans. A range of species have capitalised on anthropogenic resources available in urban areas, by either foraging and breeding in urban areas or spending much of their time in more natural areas but at times venture into urban areas to forage. Species successful in human-altered environments are typically characterised by phenotypic plasticity and a generalist diet (Griffin et al. 2017). Moreover, long-lived species with large home ranges that they need to navigate and species living in socially complex groups often possess higher cognitive skills enabling them to learn to successfully exploit anthropogenic resources (Barrett et al. 2018). While we have gained some understanding what wildlife is more likely to forage in human-altered environments, we lack, a deeper understanding of the ecological consequences of foraging on anthropogenic resources for the individuals.
Our understanding of how and why animals make decisions to forage in urban environments is incomplete. Anthropogenic foods in urban areas are thought to be high-energy, predictable food sources (Griffin et al 2017), but they can be nutrient and protein deficient and temporarily unavailable (Grémillet et al. 2008, Murray et al. 2015). On the other hand, the costs of searching for and handling food as well risks (e.g. disturbance, competition, predation) likely differ between foraging on anthropogenic and natural resources (reviewed in Fehlmann et al. 2021). The most beneficial resource to forage on depends on the trade-off between the different costs and benefits of alternative foraging strategies (Houston et al. 1993). If the benefits of foraging in their natural habitat are relatively poor and fall below some threshold, then animals may be pushed into urban areas in search of food. Alternatively, peaks in rich anthropogenic food that allow for an effective gathering of needed resources may pull animals into urban areas, regardless of the foraging in the natural habitat. In order to better understand the foraging decisions of generalist foragers in human-altered landscapes we need a better understanding of the costs and benefits for alternative foraging resources. Foraging in urban environments can cause conflicts with humans and a better knowledge of the decisions where to forage improves our understanding of a species’ adaptation to urban and natural foraging opportunities and provides insight for how to manage conflicts caused by urban animals.
This PhD project will address this question for large gulls (Larus sp.) that increasingly use urban habitat giving raise to conflicts with humans (Pais de Faria et al. 2022). Gulls, as generalist foragers using large home ranges and breeding colonially, are a good example of species successfully exploiting anthropogenic foods. While gulls in urban areas increase, populations at traditional natural sites declined dramatically over the same time period making them species of conservation concern and a conservation dilemma. It is yet unclear whether gulls benefit from anthropogenic food sources (e.g., Pons & Migot 1995, Weiser & Powell 2010) or not (e.g., Pierotti & Annett 1991, O’Hanlon et al. 2017). The project will compare energetic and nutritional benefits and costs and risks of foraging on different food resources and explore whether the costs and benefits vary through the breeding season. This provides valuable insights for the conservation management of gull populations as well as the human-gull conflicts.

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Image Captions

Foraging Lesser Black-backed Gull (image: Sam Hobson)


The project aims to combine conventional tracking studies with detailed behavioural observations to obtain a more detailed picture of habitat use than conventional tracking studies cannot achieve. The student will tag breeding Lesser Black-backed Gulls with lightweight, solar-powered and remotely downloadable GPS tags (n = 20, e.g. ICARUS tags) both from urban and natural breeding sites within the same general area using established methods (e.g., Langley et al. 2022, O’Hanlon et al. 2022). In the Scottish central belt Lesser Black-backed Gull is the dominant gull species using urban space. Breeding birds will be caught in the early incubation stage in May of the first year. Tagged birds will identify what habitats are used by birds. For the type of GPS tag we plan to use, most of them are expected to yield results for a full annual cycle allowing us to track changes in foraging decisions through the part of the annual cycle the birds are in Scotland; the Lesser Black-backed Gull is a migratory species with most individuals leaving the study area in winter. Throughout the period the birds are in the study area, the student will carry out behavioural observations of foraging Lesser Black-backed Gulls in the habitats identified by the tracked birds. Lesser Black-backed Gulls forage in open, typically terrestrial and intertidal habitats where they can be readily observed.
In order to compare energy and nutrition intake across different habitats, the behavioural observations will be used to identify food type and frequency of food intake. From known energy and nutrient composition of different food types this allows calculating energy and nutrient intake across different habitats. This information alongside with evaluating the diet and nesting success of individual birds (through pellets collected from individual nests, Pennycott et al. 2020) can also be used to evaluate the impact of different diets on reproductive performance using the framework of nutritional geometry (e.g., Tait et al. 2014).
To assess the costs of foraging in the different habitats the student will combine information on search and handling times and how much time they lost to forage because of disturbance, anti-predator vigilance and competition with others) from the behavioural observations along with information on travel time and distance from the tracked birds using that habitat (e.g., Langley et al. 2022, O’Hanlon et al. 2022).
Evaluating intake vs costs will allow comparing the profitability of foraging in different habitats within the study area and whether the profitability of the habitats relative to each other varies through the season. This will provide us with a map of habitat profitability depending on breeding site location, and whether it suggests Lesser Black-backed gulls are pulled or pushed into human-altered habitats.
Habitat use analyses using the most up-to-date Species-Habitat-Association models (Matthipolous et al. 2020) will test whether Lesser Black-backed Gulls use according to their relative probability as assessed by the foraging profitability analyses. This will test the hypothesis that foraging profitability is an important driver of habitat use in breeding Lesser Black-backed Gulls, or whether other factors will also need to be considered in future.
Contingency plan: Tracking data are also available for 25 Lesser Black-backed Gulls over three years in the proposed study area that will be made available to the student, and in case of access limitation to breeding colonies in the first year of study (e.g. because of ongoing HPAI outbreak) can form the basis of selecting suitable habitats for the collection of behavioural data. There is also a rich literature on diets and reproductive output at the colony level that could be used for the nutritional geometry approach across colonies in year 1, that then could be compared with a similar analysis across individuals within the same colony in year 2 if colonies can be accessed.

Project Timeline

Year 1

Literature review on foraging behaviour and resource use in gulls
Training in tag deployment on gulls and acquiring of relevant licences
Designing a field work plan for the first breeding season
Tag deployment on urban- and rural nesting Lesser Black-backed Gulls within the same region
Collating information on energy and nutrient content of food types typically consumed by gulls
Collecting movement, behavioural, diet and reproductive output data in the field

Year 2

Training in the analyses of movement and behavioural data
Analyses of behavioural observations identifying foraging profitability for each habitat that gulls used for foraging
Establish analyses routines for gull movement data acquired in year 1 and identify habitat use patterns and interpret initial patterns
Apply nutritional geometry approach on field data collected in year 1Present first results at a relevant national ecological or ornithological conference
Adjusting field work plans for year 2
Collecting additional movement and behavioural data in the field

Year 3

Adding year 2 data to analyses of foraging profitability and habitat use and explore temporal variation (within and between year) in foraging profitability and habitat use
Explore the match between patterns in foraging profitability and use by foraging gulls across habitat types assessed in this study
Write-up manuscript on habitat use and compare foraging behaviour of Lesser Black-backed gulls foraging in different habitats with relation to anthropogenic resources.
Present results at a relevant international ecological or ornithological conference

Year 3.5

Finalising analyses and write up thesis

& Skills

This project will integrate movement and behavioural data to explore an issue of conservation concern. Skills in analyses of movement data have become very important in ecological careers but rarely are they combined with independent behavioural data. The supervisory team combines expertise in both fields and have experience with wildlife using man-made habitats and can provide a unique environment for a student to develop a combination of these skills.
Large gulls are an interesting conservation problem as their overall numbers significantly declined over recent decades but their numbers in urban areas has increased where they cause considerable human-gull conflicts and a conflict between eco-tourists and conservationist aiming to protect gulls and humans adversely affected by increasing numbers of gulls (local residents, shops, recreation). In addition to an improved general understanding of the ecological consequences on individuals by foraging on anthropogenic resources, the project can show potential new solutions to the gull conservation dilemma that future conservation programs can follow up.

References & further reading

Barrett LP, Stanton LA, Benson-Amram S. 2018. The cognition of “nuisance” species. Animal Behaviour 147: 167–177.
Chace JF, Walsh JJ. 2006. Urban effects on native avifauna: A review. Landscape and Urban Planning 74: 46–69.
Fehlmann G, O’Riain MJ, Fürtbauer I, King AJ. 2021. Behavioral Causes, Ecological Consequences, and Management Challenges Associated with Wildlife Foraging in Human-Modified Landscapes. BioScience 71: 40–54.
Grémillet D, Pichegru L, Kuntz G, Woakes AG, Wilkinson S, Crawford RJM, Ryan PG. 2008. A junk-food hypothesis for gannets feeding on fishery waste. Proceedings of the Royal Society B 275: 1149–1156.
Griffin AS, Netto K, Peneaux C. 2017. Neophilia, innovation and learning in an urbanized world: A critical evaluation of mixed findings. Current Opinion in Behavioral Sciences 16: 15–22.
Houston AI, McNamara JM, Hutchinsom JMC. 1993. General results concerning the trade-off between gaining energy and avoiding predation. Philos Trans R Soc B-Biol Sci. 341: 375–97.
Langley LP, Bearhop S, Burton NHK, Banks AN, Frayling T, Thaxter CB, Clewley GD, Scragg E & Votier SC. 2022. Urban and coastal breeding lesser black-backed gulls (Larus fuscus) segregate by foraging habitat. Ibis https://doi.org/10.1111/ibi.13109
Matthiopoulos, J., Fieberg, J.R & Aarts, G. 2020. Species-Habitat Associations: Spatial data, predictive models, and ecological insights. University of Minnesota Libraries Publishing.
Murray M, Cembrowski A, Latham ADM, Lukasik VM, Pruss S, St Clair CC. 2015. Greater consumption of protein-poor anthropogenic food by urban relative to rural coyotes increases diet breadth and potential for human-wildlife conflict. Ecography 38: 1235–42.
O’Hanlon NJ, McGill RAR, Nager RG. 2017. Increased use of intertidal resources benefits breeding success in a generalist gull species. Marine Ecology Progress Series 574: 193-210.
O’Hanlon NJ, Thaxter CB, Burton NHK, Grant D, Clark NA, Clewley GD, Conway GJ, Barber LJ, McGill RAR & Nager RG. 2022. Habitat selection and specialisation of Herring Gulls during the non-breeding season. Front. Mar. Sci. 9: 816881.
Pais de Faria J, Lopes CS, Kroc E, Blight LK, Nager RG. 2022. Urban gulls living with humans. In: Ramos, J. A. and Pereira, L. (eds.) Seabird Biodiversity and Human Activities. Series: Aquatic sciences, 1. CRC Press: Boca Raton, Florida, United States, pp. 90-105.
Pennycott TW, Grant D & Nager RG. 2020. Earthworms in the diet of Herring Gulls Larus argentatus breeding on an off-shore island, Bird Study, 67: 131-134.
Pierotti R, Annett CA. 1991. Diet Choice in the Herring Gull: Constraints Imposed by Reproductive and Ecological Factors. Ecology 72: 319-328.
Pons J-M, Migot P. 1995. Life-history strategy of the herring gull: changes in survival and fecundity in a population subjected to various feeding conditions. Journal of Animal Ecology 64: 592-599.
Sol D, Lapiedra O, González-Lagos C. 2013. Behavioural adjustments for a life in the city. Animal Behaviour 85: 1101–1112.
Tait AH, Raubenheimer D, Stockin KA et al. 2014. Nutritional geometry and macronutrient variation in the diets of gannets: the challenges in marine field studies. Mar Biol 161, 2791–2801.
Weiser EL, Powell AN. 2010. Does Garbage in the Diet Improve Reproductive Output of Glaucous Gulls? Condor 112: 530-538.

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