NEST-SITE SELECTION IN SAVANNA SPARROWS - USING GULLS AS SCARECROWS

Savannah sparrows, Passerculus sandwichensis, breeding on Kent Island, New Brunswick, Canada, have two types of nest predators, one of them (herring gulls, Larus argentatus) abundant but relatively ineffective, the other (American crows, Corvus brachyrhynchos) scarce but highly e ffective. We hypothesized that the net effect for Savannah sparrows of nesting near gulls would be to reduce the overall risk of nest predat ion. Despite being surrounded by predators, the eggs and offspring of sparrows that nested among gulls survived as well during the incubatio n and post-fledging periods as did those of sparrows that did not nest among gulls. During the nestling period, sparrows nesting among gulls had significantly lower predation rates. In defending their own nests from predatory crows, gulls apparently shielded nearby sparrows from the more dangerous predator. Experiments with model predators demonstr ated that sparrows reacted to gulls as potential predators of their eg gs and nestlings. Sparrows apparently recognized crows as a far greate r threat, however. The tendency to nest near gulls appeared not to be heritable or influenced by early experience. Sparrows nesting among gu lls were indistinguishable from sparrows nesting away from gulls in te rms of body size, age and date of nesting. By choosing nest sites in m icrohabitats that gulls avoided, such as dense patches of goldenrod an d blueberry, and by adopting more cautious approaches to their nests, sparrows nesting near gulls reduced their risk of predation by gulls. The density of Savannah sparrow nests was inversely correlated with th e density of gull nests, which suggests that sparrows avoided gulls de spite the apparent advantage in terms of reduced nest predation by cro ws. A strong nesting association between gulls and birds like Savannah sparrows is unlikely to evolve because of the low heritability of the trait, gene flow from other populations where avoiding gulls and othe r potential predators is selectively advantageous, and constraints on short-lived birds in learning to differentiate situations in which a p redator presents a threat from those in which it provides protection. (C) 1997 The Association for the Study of Animal Behaviour.