When a University of Michigan-led research team reported last year that North American migratory birds have been getting smaller over the past four decades and that their wings have gotten a bit longer, the scientists wondered if they were seeing the fingerprint of earlier spring migrations. Multiple studies have demonstrated that birds are migrating earlier in the spring as the world warms. Perhaps the evolutionary pressure to migrate faster and arrive at breeding grounds earlier led to the physical changes the U-M-led team observed. "We know that bird morphology has a major effect on the efficiency and speed of flight, so we became curious whether the environmental pressure to advance spring migration would lead to natural selection for longer wings," said U-M evolutionary biologist Marketa Zimova. In a new study scheduled for publication June 21 in the Journal of Animal Ecology, Zimova and her colleagues test for a link between the observed morphological changes and earlier spring migration, which is an example of timing shifts biologists call phenological changes. Unexpectedly, they found that the morphological and phenological changes are happening in parallel but appear to be unrelated or "decoupled." "We found that birds are changing in size and shape independently of changes in their migration timing, which was surprising," said Zimova, lead author of the study and a postdoctoral researcher at the U-M Institute for Global Change Biology. Both the new study and the 2020 paper that described the changes in body size and wing length were based on analyses of some 70,000 bird specimens from 52 species at the Field Museum. The birds were collected after colliding with Chicago buildings during spring and fall migrations between 1978 and 2016. In addition to its finding about the decoupling of morphological and phenological changes, the new study is believed to be the first to use museum specimens from building collisions... |