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June 21, 2021
by Jim Erickson, University of Michigan
When a research team led by the University of Michigan reported last year that North American migratory birds had become smaller and their wings slightly longer over the past four decades, scientists wondered if they saw fingerprints from previous spring migrations.
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Several studies have shown that birds migrate earlier in spring when the world warms up. Perhaps the evolutionary pressures to migrate faster and arrive at breeding sites earlier led to the physical changes observed by the UM-led team.
« We know that bird morphology has a huge impact on the efficiency and speed of flight , so we were curious whether environmental pressures to encourage the spring migration would result in natural selection for longer wings, « said UM evolutionary biologist Marketa Zimova.
In a new study published June 21 in the Journal of Animal Ecology, Zimova and her colleagues are testing for a connection between the observed morphological changes and the earlier spring migration, which is an example of temporal shifts that biologists refer to as phenological changes.
Unexpectedly, they found that the Morphological and phenological changes take place in parallel, but do not seem to be connected or related to one another « We found that birds change size and shape regardless of changes in the timing of their migration, which was surprising, » said Zimova, lead author of the study and postdoctoral fellow at the UM Institute for Global Change Biology .
Both the new study and the 2020 paper describing changes in body size and wing length were based on analyzes of around 70,000 specimens of birds from 52 species in the Field Museum. The birds were collected after they collided with buildings in Chicago during the spring and autumn migrations between 1978 and 2016.
In addition to the findings on the decoupling of morphological and phenological changes, the new study is considered to be the first to provide museum specimens from building collisions used to study long-term trends in the timing of bird migration. Several previous reports relied on data from bird band studies or, more recently, analysis of weather radar records.
The U-M-led team confirmed previous findings on earlier spring migration and provided new insights into autumn bird migration in North America that have been less studied. In particular, they found that the earliest spring migrants now arrive almost five days earlier than they did four decades ago, while the earliest autumn migrants leave for the south around ten days earlier than they used to leaving a week later than earlier, so that the overall length of the autumn migration season has increased significantly.
“It is unusual to have a data set that simultaneously provides insights into several aspects of global change – such as phenology and morphology « said UM evolutionary biologist and ornithologist Ben Winger, a lead author on the study.
» I was impressed that the collision data showed so clear evidence of the advancing spring migration. The collision monitors in Chicago have been collecting this data on bird collisions for 40 years, and now the birds are changing the timing of their migration patterns into paths that were imperceptible until the dataset is examined as a whole, « said Winger, assistant professor at the Institute of Ecology and Science Evolutionary Biology and Assistant Curator at the Museum of Zoology.
Last year, the UM-led team reported in the journal Ecology Letters that almost all of the 52 bird species in their study showed both a decrease in height and a decrease in height over the four-decade period recorded a simultaneous increase in wing length.
At that time they combined the measured body size reductions with warmer temperatures in the breeding areas of the birds. Since smaller bodies can dissipate heat better, smaller birds might gain a competitive advantage and were favored by natural selection. Alternatively, the Reduction in height the E The result of a process known as developmental plasticity, an individual’s ability to change development in response to changing environmental conditions.
The researchers also suggested that the observed increases in wing length helped compensate for the smaller body size so that the birds could maintain their migration by increasing flight efficiency.
In the previous study, however, it was not tested whether the changes in body size and wing length were driven by climate-related shifts in the time of migration. In the new study, they tested on this link.
For each of the 52 species, the researchers estimated temporal trends in morphology and changes in the timing of migration. They then tested associations between species-specific rates of phenological and morphological changes, taking into account the possible effects of migration distance and breeding space.
« Scientifically speaking, this is really the most interesting and novel finding, » said UM evolutionary biologist and ornithologist Brian Weeks, a senior author of the Journal of Animal Ecology’s new study.
Advances in phenology, such as the For example, flowering plants blooming earlier in the spring, and changes in morphology, including a decrease in body size, are among the most commonly described biological responses to global warming.
Many studies of the adaptation of plants and animals to global warming have si I was concerned with either phenological or morphological changes, but few were able to study both at the same time. The depth of the Field Museum dataset enabled the UM-led team to study multiple responses to global warming simultaneously and test for links between them.
« It is often believed that morphological changes are being driven by climate and changes in the timing of migration need to interact to either facilitate or limit adaptive responses to climate change, ”said Weeks, assistant professor at the School for Environment and Sustainability. « But to my knowledge this has never been empirically tested to any significant extent due to a lack of data. »
So if the increased wing length is not responsible for the earlier arrival of migratory birds in Chicago each spring, what then? Previous studies suggest that shorter, less frequent stops while migrating north may be a factor.
“And there could be other adaptations that allow birds to migrate faster that we hadn’t thought of – maybe a physiological adaptation that allows faster flight without the birds overheating and losing too much water, ”said Zimova.
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