June 21 (UPI) -- Previous studies have shown birds are departing on their spring migrations earlier and earlier as the planet warms. Additionally, morphological surveys suggest the wings of birds are getting longer, while their bodies are getting smaller.
The authors of the new paper, published Monday in the Journal of Animal Ecology, wanted to find out if these climate-driven changes are related.
"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," study author Marketa Zimova, an evolutionary biologist at the University of Michigan, said in a press release.
The analysis showed that though both the morphological and migrational shifts are happening concurrently, the changes aren't related.
"We found that birds are changing in size and shape independently of changes in their migration timing, which was surprising," said Zimova.
For the study, researchers relied on a massive, multi-decade dataset compiled by ornithologists at the Field Museum in Chicago, where scientists have been collecting and measuring the dimensions of birds killed in collisions with buildings since the 1970s.
The dataset features body size and wing length measurements for some 70,000 bird specimens from 52 species.
The Field Museum's long-term data collection project has also helped scientists study changes in the timing of fall and spring migrations.
"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 senior study author Ben Winger.
"I was impressed that the collision data so clearly showed evidence of advancing spring migration," said Winger, an ornithologist and evolutionary biologist at Michigan.
Without the large, longterm dataset, the changes in wing and body size, as well the shifts in migrational timing, would be imperceptible. It's only when examining the full 40-year dataset that the changes become apparent.
Researchers have previously suggested that smaller bodies might help birds dissipate heat, while longer wings could aid migrational fitness.
But until now, scientists had never tested whether climate-related shifts in migration timing were directly responsible for morphological changes.
For the new study, scientists looked at whether links between temporal shifts in morphology and migrational timing could be used to predict species-specific changes. Their analysis, which accounted for migratory distance and breeding latitude of different species, revealed no such link.
Changes in migration timing, migratory distance and breeding latitude all failed to accurately predict species-specific changes in morphological traits.
"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 senior author Brian Weeks.
"But this has never to my knowledge been tested empirically at a significant scale, until now, due to lack of data," said Weeks, evolutionary biologist and ornithologist at Michigan.
The authors of the new study suggest less frequent migrational stopovers, not longer wings, may explain the earlier arrival of migrating birds, though more research is needed.
"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," Zimova said.