Passerine birds on islands tend to have larger bills than their mainland relatives. The morphological shift may be related to reduced interspecific and increased intraspecific competition. Emberizid sparrows in North American salt marshes also show consistently greater bill size. We tested the hypothesis that passerines restricted to mangrove forests, another continental system with low species diversity and high population densities, also have larger bills than their closest nontidal relatives. We found a consistent pantropical pattern of longer and deeper bills in passerine birds restricted to mangroves. These results indicate that disproportionately longer bills in relation to body size in passerines restricted to coastal saline habitats, just like those found on islands, seems to be a general ecological rule. The similar pattern in bill morphology suggests that ecological and evolutionary processes thought to occur only in island systems might also occur in some continental systems.
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Vol. 128 • No. 2