Habitat utilisation, feeding tactics and age related feeding efficiency in the Caspian Gull Larus cachinnans

Abstract

The feeding behaviour of the Caspian Gull Larus cachinnans was analysed in southern Poland in 2001. During the pre-breeding period, most birds foraged on a refuse dump and some foraged in a river valley. During incubation, similar numbers of birds foraged on fishponds, gravel pits and the refuse dump. During the chick-rearing period, fishponds were the most important foraging grounds. The foraging success of three main foraging tactics was analysed: digging on refuse, fishing and kleptoparasitism. We found that digging success was higher in juveniles than in immature or adult birds. However, older birds moved and ate more items per unit of time than juveniles, which indicates that older birds improved their energy gain simply by a higher speed of searching. The opposite was found for fishing success. As juvenile birds made fewer attempts than immature or adult birds, fishing success was higher in adults. Adults and immature birds interrupted more attacks than juveniles, which indicates that older birds were better able to assess the probability of fish catching than juveniles. Kleptoparasitism was observed almost exclusively on the refuse dump during the pre-breeding period. Young birds kleptoparasitised more frequently than adults, but they had a lower rate of success. However, the lower success in young birds was due to victim choice, rather than differences in flight skills. Young birds kleptoparasitised Black-headed Gulls Larus ridibundus and Jackdaws Corvus monedula more frequently than adults, but none of the attacks towards these species was successful. Generally, Caspian Gulls kleptoparasitised conspecifics more often than expected from species frequency. Only attacks towards conspecifics yielded any success.

Response of young and adult birds to the same environmental variables and different spatial scales during post breeding period

“…The gulls captured fish varying between 5 and 47 cm (Gwiazda 2004;Gwiazda et al 2011, Skórka unpublished). The size of fish captured does not differ between adult, immature and juvenile birds (Skorka and Wojcik 2008).…”

“…First, in larger settlements gulls may use refuse-tips as foraging habitat. Caspian gulls, however, gather at refuse tips in larger numbers only in late autumn and winter (Skorka and Wojcik 2008). The second possibility is that humans have direct negative effect on abundance, for example in waterbodies surrounded by dense settlements more people visiting/walking/fishing may disturb foraging birds (Fernández-Juricic and Tellería 2000).…”

“…However, the social learning may possess substantial costs for young birds. They are usually subordinate to adults and less-efficient foragers; thus, when in a foraging habitat patch, they may suffer from lower food intake due to competition, kleptoparasitism, aggression or food depletion (Gochfeld and Burger 1981;Carroll and Cramer 1985;Skorka and Wojcik 2008). The drawback of the competitive advantage of adults disappears if young birds seek food patches unoccupied by adults.…”

An invasive gull displaces native waterbirds to breeding habitats more exposed to native predators

“…At the reservoir under study, numerous fish were found at nests of Caspian gulls, indicating that this was a major food for them. Skórka and Wójcik (2008) clearly showed that this species foraged mostly on fish farms and hunted for fish, while the proximity of fish farms is one of the major factors affecting its successful colonization of inland reservoirs (Lenda et al 2010). Therefore, it is possible that the availability of alternative food resources in the form of the fish, bred in Poland's numerous fish farms, may be a factor that limits the direct predatory impact of this invasive gull on native waterbirds.…”

Effects of maternal traits and individual behavior on the foraging strategies and provisioning rates of an income breeder, the Antarctic fur seal

“…Foraging efficiency (energy gained per unit effort) increases with age and experience in many species (Galbraith et al 1999, Daunt et al 2007, Skorka & Wojcik 2008, potentially because an individual's ability to find and handle prey improves with experience (Orians 1981, Galbraith et al 1999. In central place foragers, experience may influence the point at which the individual decides to return to their offspring.…”