The importance of marine resources in the diet of urban gulls

Abstract

The availability of anthropogenic food subsidies has promoted an increase in generalist opportunistic gull species, which currently breed and forage on predictable anthropogenic resources (e.g. landfills). Here we investigated whether marine resources are still important to urban-dwelling gulls. We studied 4 natural and 2 urban yellow-legged gull Larus michahellis colonies and compared (1) diet composition (through pellet analysis) and (2) isotopic niches of adults and chicks, (3) diet delivered to chicks of different ages, and (4) fatty acid (FA) composition of fledglings, in order to assess diet composition, diversity and quality, and the relevance of marine prey for natural and urban gull populations. Adult urban gulls consumed considerably lower proportions of marine prey when compared to gulls from natural colonies; however, they fed their younger chicks (<20 d old) mostly with fish, representing 61-80% of their chick food deliveries. Refuse items were mostly delivered to chicks older than 20 d. Overall, urban isotopic niches were not completely distinct from those of natural colonies, in some cases sharing ca. 50% of their niche space. Fledglings from the most urbanized colony presented overall higher FA concentrations and diversity, but they were lacking some omega-3 FAs relevant to their physiology. Our results highlight the importance of marine resources in the diet of urban gulls, particularly during early chick rearing, the relevance of food sources in the area around the breeding colonies and the fact that urban gulls benefit from year-round reliable anthropogenic food resources.

Fatty acids composition in yellow-legged (Larus michahellis) and lesser black-backed (Larus fuscus) gulls from natural and urban habitats in relation to the ingestion of anthropogenic materials

“…However, individuals from PBGaia (the most urbanized location) exhibited particularly higher percentages of the FA C18:1n-9, accounting for 44% of the total FAs for that rescue centre vs. 28% in RIAS (the least urbanized site) and 34% in LxCRAS. Urban gulls from PBGaia should have a highly diverse diet, including the presence of anthropogenic food items in their diets such as remnants of human meals, as reported for other gulls using and relying on urban habitats (Egunez et al, 2018;Huig et al, 2016;Pais de Faria et al, 2021b;Real et al, 2017). Unfavourable physiological states characterized by loss of body mass or periodic fasting associated with breeding, moult or migration, which can be enhanced by a nutritionally poorer diet in urban habitats, may result in the selective mobilization of certain FAs, and de novo biosynthesis of other FAs like C16:0 and C18:0, as well as their respective products C16:1n-7 and C18:1n-9 (Williams and Buck, 2010).…”

“…This FA may be stored in the adipose tissue or used rapidly as an energy substrate (Williams and Buck, 2010). Palmitic acid was the most abundant FA found in plasma of YLG fledglings in Porto urban breeding colony and in Berlenga natural breeding colony (Pais de Faria et al, 2021b). By being biosynthesised de novo by birds, both SFAs and MUFAs relative levels can be controlled to a larger extent than the levels of PUFAs and HUFAs (Isaksson et al, 2017), therefore these SFAs and MUFAs results are likely a consequence of metabolic regulation combined with habitat specific diet.…”

“…Still, the major factor affecting FA composition is diet and, although in smaller amounts, SFAs and MUFAs are also obtained from diet (Iverson et al, 2007). Despite the highly diverse diet reported for urban gulls from Porto, they still relied on marine resources throughout the year (Pais de Faria et al, 2021b), which also may help in understanding the higher C18:1n-9 proportions in PBGaia urban gulls.…”

“…Large gulls Laridae, among them the yellow-legged gull (YLG; Larus michahellis) and the lesser black-backed gull (LBBG; Larus fuscus) have become more common in urban areas, with established breeding populations around the world, benefiting from a more temperate and stable microclimate and fewer natural predators than in natural habitats (Auman et al, 2008;Huig et al, 2016;Méndez et al, 2020;Pais de Faria et al, 2021a;Spelt et al, 2019). As opportunistic foragers, gulls use a wide variety of foraging habitats and strategies, being capable of exploiting different food types, especially anthropogenic food remains collected in landfills and within urban habitats (Gyimesi et al, 2016;Matos et al, 2018;Pais de Faria et al, 2021b;Parra-Torres et al, 2020;Ramos et al, 2009;Spelt et al, 2019). This resulted in an increase of their urban population numbers over the last few years (Duhem et al, 2008;Nager and O'Hanlon, 2016;Vidal et al, 1998).…”

“…FA signatures of fledgling gulls are known to differ between urban and natural habitats (Pais de Faria et al, 2021b), however, variation in FA composition has rarely been investigated in the context of urbanization, with the exception of passerines (e.g. Andersson et al, 2015;Isaksson et al, 2017).…”