Habitat selection and hatching success of Eurasian Oystercatchers in relation to nesting Yellow-legged Gulls and human presence

We studied nest habitat selection and hatching success of Eurasian Oysterca tchers (Haematopus ostralegus) breeding inside and outside colonies of Yell ow-legged Gulls (Larus cachinnans) on barrier and dredge islands of the Po Delta, Italy. Oystercatchers nested on most of the islands, independently o f habitat, distance from mainland and occurrence of gulls. Percentage of eg gs hatched was significantly lower in clutches inside gull colonies than ou tside. Gulls were the main predators of eggs, accounting for 42-64% of egg loss; late oystercatcher breeders were more affected than were early breede rs. Nevertheless, gulls and oystercatchers were significantly associated, a nd numbers of breeding pairs of the two species were positively correlated on the islands. This appears due to similar habitat requirements, since oth er possible explanations seem unlikely.