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 published online June 21 2021 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.

Advances in phenology, such as flowering plants blooming earlier in the spring, and changes in morphology, including body size reductions, are among the most commonly described biological responses to globally warming temperatures.

But in this case, the U-M researchers unexpectedly 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 to examine long-term trends in bird migration timing. Several previous reports relied on data from bird-banding studies or, more recently, the analysis of weather radar records.

The U-M-led team confirmed previous findings about earlier spring migration and provided new insights about fall bird migrations in North America, which have been less studied. Specifically, they found that the earliest spring migrants are now arriving nearly five days sooner than they did four decades ago, while the earliest fall migrants are heading south about 10 days earlier than they used to.

Notably, the last fall stragglers now depart about a week later than they used to so that, overall, the duration of the fall migration season has been stretched considerably.

“It is unusual to have a dataset that can provide insights into multiple aspects of global change – such as phenology and morphology – at the same time,” said U-M evolutionary biologist and ornithologist Ben Winger, a senior author of the study.

“I was impressed that the collision data so clearly showed evidence of advancing spring migration. The collision monitors in Chicago have been collecting these data on bird building collisions for 40 years and, meanwhile, the birds have been changing the timing of their migratory patterns in ways that were imperceptible until the dataset as a whole was examined,” said Winger, an assistant professor in the Department of Ecology and Evolutionary Biology and an assistant curator at the Museum of Zoology.

In addition to Zimova, Weeks and Winger, the other author of the Journal of Animal Ecology study is David Willard of the Field Museum, the ornithologist and collections manager emeritus who measured all 70,716 birds analyzed in the study. The Field Museum dataset has been a bonanza for bird researchers and has led to several recent publications.

Read full Michigan News press release

Michigan Public Radio Stateside interview