Scientist have known for some time that birds that live island-dwelling birds have larger beaks than their closest relatives on the mainland. A recent discovery found that sparrows living in North American salt marshes also have larger beaks than their inland relatives. And now a third habitat can be added to the list of those that harbor birds with big beaks.
Scientists compared 13 species of songbirds that live their entire lives in mangroves (the tropical equivalent of the salt marsh) to their closest relatives that live inland. They found that the mangrove birds had larger beaks than their relatives.
So why do birds in these habitats evolve bigger beaks? These 3 diverse habitats do have something in common. Although there are quite a few individual birds present, there are not very many different kinds.
It is possible that there is reduced competition between different kinds of birds which may open up more foraging niches, making a large beak more useful. It is also possible that a larger beak allows a male to outcompete other males for the female's attention.
Scientists are not entirely sure why these areas spur schnoz growth but now salt marshes and mangroves can be considered to be like islands, at least as far as bill size is concerned.
This article summarizes the information in this publication:
David Luther and Russell Greenberg. The island syndrome in coastal wetland ecosystems: convergent evolution of large bills in mangrove passerines. The Auk 128(2): 201-204
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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