Physical changes in birds evolved separately from earlier migrations

Migratory birds in North America have been getting smaller with longer wings. After making this discovery, a team of experts at the University of Michigan set out to investigate whether the physical changes in migratory birds are tied to earlier spring migrations.

The researchers theorized that the pressure to migrate faster and arrive at breeding grounds earlier may have driven the morphological changes.

“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 study lead author Marketa Zimova.

The study revealed that the phenological change to earlier spring migration had occurred parallel to the physical changes in birds, but appeared 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.

The investigation was based on the analysis of 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.

The experts determined that 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.

In addition, the last of the fall migrants are now departing about a week later, which means that 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 study senior author Ben Winger.

“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.”

Last year, the U-M-led team reported that nearly all of the 52 bird species they examined had experienced declines in body size and increases in wing length over a period of four decades. 

In the current study, the team analyzed whether the changes in body size and wing length were driven by climate-related shifts in migration timing.  They did this by testing for associations between species-specific rates of phenological and morphological change.

The researchers found no evidence that the rates of migratory changes are predictive of rates of concurrent changes in morphological traits.

“Scientifically, this is really the most interesting and novel finding,” said study senior author Professor Brian Weeks.

The depth of the Field Museum dataset enabled the U-M-led team to examine multiple responses to climate warming simultaneously and to test for connections between them.

“It is often assumed that morphological changes driven by climate and changes in the timing of migration must interact to either facilitate or constrain adaptive responses to climate change,” said Professor Weeks. “But this has never to my knowledge been tested empirically at a significant scale, until now, due to lack of data.”

Previous studies suggest that the earlier arrival of migratory birds in Chicago each spring is linked to shorter, less frequent stopovers during the northbound trek may be a factor.

“And there might be other adjustments that allow birds to migrate faster that we haven’t thought about–maybe some physiological adaptation that might allow faster flight without causing the birds to overheat and lose too much water,” said Zimova.

The study is published in the Journal of Animal Ecology.

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By Chrissy Sexton, Earth.com Staff Writer