Individuals within populations: No evidences of individual specialization in the trophic habits of an opportunistic predator

Abstract

Human activities can affect species in different ways, with some species being unable to cope with the human-driven changes, while others can persist and even benefit from these alterations. The main factors explaining the population dynamics of successful species include their use of food resources derived from human activities and the adaptability in their feeding behavior. Among marine predators, some gull species of the genus Larus, such as the yellow-ledged gull Larus michahellis, are particularly successful within the current context of human-induced global change. While the yellow-ledged gull is considered a generalist and opportunistic predator at the population level, some individuals may show specialisation in feeding habitat and diet. Here, we examined the degree of individual specialisation in the trophic habits and temporal variation in food resource utilisation within a population of this gull in the southwestern Iberian Peninsula. We analysed the isotopic composition (d 15 N and d 13 C) in blood and different feathers molted throughout the annual cycle for reconstructing the diet of individuals and quantifying the degree of individual specialisation. Individuals of this species in the studied population preferably fed on demersal fish throughout the entire annual cycle, with a low consumption of terrestrial prey and human-related resources. Isotopic values also revealed a generalist feeding behavior in this gull species with a very small proportion of individuals acting as specialists throughout the annual cycle. Taking into account the generalist strategy of this population, management measures should mainly be focused on controlling the availability of demersal sources from fishery discards. Future research should prioritise multispecific approaches to study how general or flexible the behavior is in this winning species.

Behavioral rhythms of an opportunistic predator living in anthropogenic landscapes

“…In particular, we reported daily behavior rhythms of yellow-legged gulls in the different habitats fitting within the different schedules of human activity. These results thus support the plasticity of the behavior of this gull species and their dependence on anthropogenic food subsidies [10,41,42].…”

“…Overall, the activity patterns of gulls in the marinerelated habitats (port, estuary, and open sea) encompassed 65% of the foraging activity, with unimodal activity waveforms that peaked at mid-morning. According to previous studies, the yellow-legged gull mainly exploits marine preys [42] with some variation in dietary preferences among breeding colonies [48]. Pelagic and benthic coastal animals have a natural variability in their availability throughout the day due to the endogenous rhythms of physiology and behavior of coastal marine animals, such as vertical migrations associated with oceanographic processes [49].…”

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

“…In particular, we found different demersal fish (e.g. Boops boops, Trisopterus capelanus or Micromesistius poutassou), as well as demersal cephalopods and crustaceans in the stomach contents of the yellow-legged gull chicks (Lopezosa et al, 2019;Mendes et al, 2018;Oro, Genovart, Tavecchia, Fowler, & Martínez-Abraín, 2013;Ramos et al, 2008). Based on the GPS tracking data from adult gulls, these marine prey are only available to the gulls at the port, as a result of discards from fishery activity, and were obtained in the fishing ports and the open sea, two important habitats used by GPS-tracked adults.…”

“…The yellow-legged gull is one clear example of an opportunistic gull that has adapted to living in urban areas (Vidal et al 1998;Sorace and Gustin 2009). This predator has a highly plastic foraging behaviour, which allows efficient exploitation of a great diversity of marine and terrestrial food resources of both natural and human origin (Arizaga et al, 2014;Lopezosa, Forero, Ramírez, & Navarro, 2019;Matos et al, 2018;Navarro et al, 2017). Moreover, this species is considered problematic across its distribution, as it comes into conflict with a variety of human activities (Navarro et al, 2019;Vidal et al, 1998) Barcelona has a number of characteristics that make it attractive to this species.…”

“…Additionally, the urban landscape of Barcelona surrounded by fishing ports, landfill installations, agricultural areas and freshwater habitats presents a diversity of trophic resources that can be exploited by gulls. An important amount of fishing activity is associated with the fishing port of Barcelona, with around 3,200 tons of annual catches (Idescat 2019), providing a large volume of fishing discards, one of the main trophic resources exploited by the yellow-legged gull (Arcos, 2001;Lopezosa et al, 2019;Ramos, Ramírez, Sanpera, Jover, & Ruiz, 2008). Similarly, the availability of organic waste in the city and in nearby garbage processing plants and landfills, also offer important subsidiary food sources for this species (Ramos et al, 2009).…”