Nest-site selection in Savannah sparrows: using gulls as scarecrows?

Abstract

Savannah sparrows,Passerculus sandwichensisbreeding 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 effective. We hypothesized that the net effect for Savannah sparrows of nesting near gulls would be to reduce the overall risk of nest predation. Despite being surrounded by predators, the eggs and offspring of sparrows that nested among gulls survived as well during the incubation 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 demonstrated that sparrows reacted to gulls as potential predators of their eggs and nestlings. Sparrows apparently recognized crows as a far greater threat, however. The tendency to nest near gulls appeared not to be heritable or influenced by early experience. Sparrows nesting among gulls were indistinguishable from sparrows nesting away from gulls in terms of body size, age and date of nesting. By choosing nest sites in microhabitats that gulls avoided, such as dense patches of goldenrod and 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 the density of gull nests, which suggests that sparrows avoided gulls despite the apparent advantage in terms of reduced nest predation by crows. 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 other potential predators is selectively advantageous, and constraints on short-lived birds in learning to differentiate situations in which a predator presents a threat from those in which it provides protection.