American songbirds evolve with forests

Scarlet Tanagers, pictured above, were one of the many varieties of songbird studied by researchers to determine how these birds responded to changes in their habitats.
Scarlet Tanagers, pictured above, were one of the many varieties of songbird studied by researchers to determine how these birds responded to changes in their habitats.Getty Images
/ Source: Discovery Channel

Eastern North American songbirds are a pretty adaptable bunch, says a scientist who discovered some remarkable changes in their wings over the last 100 years.

A close look at museum collections of 851 songbird specimens belonging to 21 species shows that most of the birds evolved pointier wings after their forests were fragmented by clear-cutting. Others in re-foresting areas evolved less-pointy wings. The reason for the wing changes: nothing less than the drive to procreate.

Pointier wings can help birds who are long-distance commuters fly more efficiently. More rounded wings, however, are better over short distances.

"I've been studying the effects of (forest) fragmentation," said Andre Desrochers of Quebec's Universite Laval and the Laboratory of Ornithology at Cornell University. Roads, rivers, clear-cuts and other gaps can break up songbird habitats.

"To me, it has become apparent that fragmentation is a really big problem," he added. "If you (as a songbird) are in a fragmented habitat, you have more chance of being without a mate."

To see if forest changes were causing evolutionary shifts in the bodies of birds, Desrochers carefully measured museum specimens collected from 1900 to 2008. What he found supported the idea that birds adapted quickly to forest changes. His results were published in the latest issue of the journal Ecology.

Desrochers figures that the reason for the wing changes is that as forests are broken up, it gets harder for songbirds to find mates. There is what's called "selective pressure" on birds to travel farther — from forest fragment to forest fragment — in order to find their one and only. Birds that can do this better tend to succeed in mating and pass on their more efficient mate-searching traits — in this case pointier wings — to their offspring.

Once the forests re-grow, however, there is no selective pressure for those pointy wings. In fact, there may even be a greater need for the birds to develop shorter wings for more maneuverability.

Put another way, the birds are essentially being bred for different wing shapes in the same way a dog, rabbit or other domestic animal is deliberately bred by humans for a particular desired trait or set of traits.

"The pattern is fairly clear cut," agreed Michael Brooke, curator of ornithology at the Cambridge University Museum of Zoology in the U.K. "Few people would have guessed that this result would come from museum collections. This study depended on 100 years of acquisition."

One of the lessons from the study, says Desrochers, is that species are not as static. "The assumption that species do not respond adaptively to rapid environmental change caused by humans is frequent and probably wrong in many cases, and several authors have warned that this may lead to species mismanagement," he concluded.

All of the specimens Desrochers studied were from the Cornell University Museum of Vertebrates and the Canadian Museum of Nature. The collections covered the the period from 1900 to 2008 and included such birds as the Eastern Meadowlark, Field Sparrow, Mourning Warbler, Scarlet Tanager, Hooded Warbler, Pine Warbler and Boreal Chickadee.