Differences between diet of adult and chick Audouin's Gulls Larus audouinii at the Chafarinas Islands, SW Mediterranean

Abstract

Differences in diet between chicks and adults of Audouin's Gull Larus audouinii were examined in the Chafarinas Islands (SW Mediterranean) during the breeding seasons of 1993 and 1994. The major food type for chicks on the Chafarinas Islands were fish, both epipelagic (some ofthem coming from purse-seine activity) and benthonic (coming from trawler discards), forming a minimum 70%by biomass. The amount of fish fed to young increased with chick age, up to 95% by biomass. Adults also consumed mainly fish (75% by number and 90% by biomass), mostly clupeiforms, Sardina Sardina pilchardus and Gilt Sardine Sardinella aurita . Terrestrial prey were especially given to youngest chicks, probably because of requirements to increase the feeding rates of these chicks . Shorter foraging trips to the near Moroccan coast to feed on terrestrial prey (chiefly arthropods and isopods) mayallow adults especially to guard/brood small chicks, and eventually to increase the feeding rates. Feeding rates significantly decreased with brood size, and also decreasedwith chickage, although this effect was not significant. Weight and size of food items fed to chicks increased with chick age, and fledglings were fed with larger items than those consumed by adults . This was probably related to the increase in gape width with chick age, and the trade-off between food load carried to feed chicks and the distance traveled to catch the fish .