52 species of migratory birds have become smaller in size thanks to rising temperatures and climate change
Migratory birds have been shrinking in size over the past four decades, driven by a warming climate and rising temperatures.
The length of a lower leg bone in birds called the tarsus has declined by 2.4% across 52 species, according to researchers from the University of Michigan.
The biologists say what is particularly surprising is that the change is "remarkably consistent" across 70,716 North American migratory birds despite the diversity of their natural histories, habitats, and geographic distributions.
The wings of the birds have also gotten a bit longer as a response to a warming climate, according to the analysis of migratory birds that died when they collided with buildings in Chicago.
The team suggests that the body-size reductions are a response to climate warming and that increased wing length may help offset the body-mass losses. The researchers plan to test this idea in a follow-up project.
"Our results suggest that this reduction in body size may be occurring broadly — and consistently — across species. Body size is an important trait for species, and changes in body size may impact how natural systems, which we depend on, function," study's lead author Brian Weeks, an assistant professor at the University of Michigan School for Environment and Sustainability, told MEA WorldWide (MEAWW).
"The thing that was shocking was how consistent it [reduction in body size] was. I was incredibly surprised that all of these species are responding in such similar ways," says Weeks.
According to the biologists, "Mean summer temperature was significantly negatively associated with bird body size." This implies that body size decreased significantly as temperatures warmed.
Temperatures at the birds' summer breeding grounds north of Chicago increased roughly 1 degree Celsius (or 1.8 degrees Fahrenheit) over the course of the study. "Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa," says Weeks.
Since 1978, Field Museum personnel and volunteers have retrieved dead birds that collided with Chicago buildings during spring and fall migrations. For each specimen, multiple body measurements are made.
The birds analyzed in the study are small-bodied songbirds that breed north of Chicago in the summer and migrate through the region in high numbers. Several species of sparrow, warbler and thrush make up the majority of the dataset.
Using this dataset, researchers analyzed trends in body size and shape. They found that, from 1978 through 2016, body size decreased in all 52 species, with statistically significant declines in 49 species. Over the same period, wing length increased significantly in 40 species.
Field Museum ornithologist and collections manager emeritus David E. Willard measured the birds analyzed in the study. Willard measured the tarsus length, bill length, wing length, and body mass. "In birds, tarsus length is considered the most precise single measure of within-species variation in body size," says the study.
All three measures of body size — tarsus length, body mass, and PC1, a standard measure of overall body size that combines several key body-part measurements -- showed statistically significant declines, says the team.
"We found remarkably consistent reductions in body size, leading us to test the hypothesis that increases in temperature over this four-decade period are associated with the observed declines in body size."
"Despite the ecological and phylogenetic diversity among species, we found consistent reductions in all indices of body size (tarsus, mass, and PC1) across species over the course of the study," states the study published in Ecology Letters.
It adds, "These declines represent a mean decline of 2.4% in tarsus length and 2.6% in mass from 1978-2016." In contrast, wing length increased through time across nearly all species. This reflects an increase in wing length of 1.3% from 1978-2016, shows analysis.
The researchers hypothesize that increasing wing length represents a "compensatory adaptation" to maintain migration as reductions in body size has increased the "metabolic cost" of flight.
"Our results suggest that increasing temperatures on the breeding grounds are responsible for body size reductions, either through developmental plasticity (that is, birds end up being smaller when they develop in warmer conditions) or natural selection for reduced body size in warmer summers," Weeks tells MEAWW.
"Better understanding the mechanism by which increasing temperatures are driving body size reductions is an important avenue of future research," he adds.
Within animal species, individuals tend to be smaller in warmer parts of their range, a pattern known as Bergmann's rule. This has led to the hypothesis that as the world warms, animals may get smaller.
However, the evidence supporting the idea remains mixed. The uncertainty is likely due, in part, to lack of datasets like the Field Museum, say experts.
"I think the critical takeaway from this study is that humans are changing the world at an unprecedented rate and scale, and the responses of natural systems can be complex," Weeks shares.
"To understand biotic responses to global change - like climate change - there is a critical need to collect data at large spatial and temporal scales. To do this, I think we need to invest in the institutions — like natural history museums — that collect essential data," he continues.
He adds, "Without data, we will not be able to understand how the world is changing, and without that understanding, we cannot craft well-informed policies to protect the natural systems that are essential to people."
According to researchers, studies of plant and animal response to climate change often focus on shifts in the geographical range of a species or the timing of events such as springtime flowering and migration.
They say the consistency of the body-size declines reported in the new study suggests that such changes should be added to the list of challenges facing wildlife in a rapidly warming world.
"It's clear that there's a third component — changes in body size and shape — that's probably going to interact with changes in range and changes in timing to determine how effectively a species can respond to climate change," says Weeks.