Breeding Yellow‐legged Gulls increase consumption of terrestrial prey after landfill closure

Abstract

The ecology of opportunistic foragers can be highly dependent on anthropogenic food sources, such as landfills, resulting in changes in several ecological and demographic aspects. The total closure of several landfill sites and the use of deterrence systems to prevent access to the remaining open landfill sites in a region in the northern Iberian Peninsula provided an excellent opportunity to evaluate the consequences of landfills on the trophic ecology of an opportunistic forager, the Yellow‐legged Gull Larus michahellis, using these resources. Based on stable isotope analyses, we used mixing models to estimate changes in diet before and after the closure of the majority of landfills in the breeding and the non‐breeding season. We found a decrease in the consumption of food from landfills after their closure, which was accompanied by an increase in feeding on terrestrial prey (mostly earthworms), but only in the breeding season. Interestingly, we observed no increase in marine prey consumption after the landfill closures. In winter there was a decrease in terrestrial prey consumption, whereas the consumption of marine and, despite their reduce availability, landfill resources increased. Thus, and unlike when all the landfills were open, we detected significant trophic differences between breeding and non‐breeding seasons. Additionally, we found significant trophic differences among colonies that were quite close to each other, but not between breeding adults and chicks. In conclusion, landfill closure or the use of deterrence systems had a direct impact on the trophic ecology of Yellow‐legged Gulls; loss of refuse was mainly compensated for by prey of terrestrial origin when breeding, but not in winter. Thus, this species may experience foraging constraints in winter with potential effects on other life‐history aspects including their dispersal, breeding and survival that needs further evaluation.

Evaluating the effect of distance to different food subsidies on the trophic ecology of an opportunistic seabird species

“…They were ringed (in order to avoid pseudo‐replications in possible subsequent visits to the colonies where we were not able to take all the samples in 1 day), and 4‐6 mantle feathers were taken from each chick for the stable isotope analysis ( δ 15 N and δ 13 C). Previous studies carried out in three of these colonies reported no differences between adults' and chicks' diet in the breeding period (Zorrozua et al , ), suggesting that the diet of the chicks may be also used to infer adults' foraging ecology.…”

“…The recent closure of several landfills within the region has resulted in a decrease in the consumption of landfill prey, which has been found to be replaced in the breeding season by an increase in consumption of prey of terrestrial origin, but not of marine origin (Zorrozua et al , ). Our results suggest that harbours may not be suitable as a main alternative source of food for those colonies situated at a considerable distance from them, while at the same time, they are compatible with an increasing exploitation of terrestrial habitats during the summer.…”

“…Our results suggest that harbours may not be suitable as a main alternative source of food for those colonies situated at a considerable distance from them, while at the same time, they are compatible with an increasing exploitation of terrestrial habitats during the summer. In winter, however, these gulls would be more likely to substitute the lack of landfill food by increasing the amount of marine prey they consume (Ramos et al , ; Zorrozua et al , ) since in the non‐breeding period, they are able to remain for longer periods in the vicinity of the harbours and thus compete more effectively for fish discards.…”

“…Consequently, potentially important food subsidies may have a relatively small area of influence, which would result in only a limited impact from the large geographic‐scale perspective. In such a scenario, it might be expected that the differential exploitation of trophic resources by physically close colonies would be reflected in the average diet at the colony level (Zorrozua et al , ).…”

Adapting to urban ecosystems: unravelling the foraging ecology of an opportunistic predator living in cities

“…reduction in the number of landfills or measures to impede access by wildlife to these facilities; Landfill Directive 1999) conducted during the last few decades in the metropolitan area of Barcelona have likely reduced the accessibility of this human-related subsidiary resource to the yellow-legged gull. This situation could force that gulls consume more terrestrial prey items (Zorrozua et al 2020) that acquire relatively easy in the urban areas predating on urban birds. In fact, we could not rule out the possibility that some of the organic resources of human origin or mammals may be obtained inside the city in the urban parks or touristic beach areas (Navarro et al 2016;Schwartz et al 2018).…”