Millions of migratory birds collide with buildings in the city of Chicago each migration season. They are affected by the bright lights and the weather, and inadvertently crash into the buildings in the dark and fall to the ground. The corpses of birds killed in this way were collected between 1978 and 2016, and this dataset has been used in a new study and has revealed some interesting trends.
Many species of migratory birds have shrunk in size as the temperatures have increased during the past 40 years. The sizes of sparrows, songbirds and thrushes from the Chicago dataset have decreased significantly in that time, even though the absolute changes have been small. And the reduction in body size is so pervasive a phenomenon that scientists have suggested it may be a universal response to warming conditions.
However, the new research study, published today in Ecology Letters, indicates that not all birds are equally affected when it comes to reductions in body size. Birds with bigger brains relative to body size have not shrunk as much as their lesser brained counterparts.
Study lead author Justin Baldwin is a PhD candidate in the lab of Carlos Botero, assistant professor of Biology at Washington University in St. Louis. “As temperatures warm, body sizes are decreasing,” said Baldwin. “But larger-brained species are declining less strongly than small-brained species.”
The researchers analyzed information on some 70,000 birds that died when they collided with buildings in Chicago between 1978 and 2016. They also supplemented this vast dataset with brain volume measurements and lifespan data that they collected for 49 of the 52 species.
They found that birds with very big brains relative to body size have shown only about one-third of the decrease in body size that other bird species have undergone in that time period. In birds, relative brain size is often considered an indicator of behavioral flexibility, and having a larger relative brain capacity has advantages for birds.
“Relative brain size correlates with increased learning ability, increased memory, longer lifespans and more stable population dynamics,” said Baldwin. “In this case, a bigger-brained species of bird might be able to reduce its exposure to warming temperatures by seeking out microhabitats with cooler temperatures, for example.”
This is the first time that scientists have been able to show a direct link between cognition and phenotypic responses to climate change.
“One of the first things that jumps out to me from these findings is that we can already see that climate change is having a disproportionate effect on species that have less capacity to deal with environmental change through their behavior,” said Professor Botero.
“This doesn’t mean that climate change is not affecting brainy birds, or that brainy birds are going to do just fine. What our findings suggest is that climate change can have a much stronger effect on the less-brainy birds.”
North America has lost nearly one-third of its birdlife in the last half-century, with migratory species experiencing particularly acute declines. The findings of this recent study, therefore, have important practical implications for management and conservation strategies.
“Rapid changes in the environment often produce a few winners and a whole lot of losers, which is really unfortunate,” said Professor Botero. “Many wildlife populations have moved toward colder places as the planet has warmed. Selection forces those that don’t move to adapt, for example, by changing their body size.”
Baldwin’s analysis reveals that smaller-brained species might be experiencing particularly strong natural selection pressure currently, a fact that planners may need to take into account for conservation management.
“When it comes to climate change mitigation and planning, a major goal is to maintain population-level connectivity,” said Baldwin. “We want to allow species to move toward the poles or upslope in elevation, to keep up with warming climates. Our findings suggest that this type of intervention could be especially important for smaller-brained species.”