Can birds tell us about rising global temperatures?

By Isaac Rush

Male House Sparrow, David Quanrud

In January of 2023, my team and I, a group of student researchers from the University of Kansas, made our way across the American southwest, capturing and sampling birds in places like Salt Lake City, Death Valley, Los Angeles, and Tucson. We would arrive at our sites just before sunrise, set up our nets, and start capturing and processing birds throughout the day. Specifically, we were sampling House Sparrows, those common drab birds you often see in the parking lot of your local grocery store or around your feeders. You might not consider this bird to be very special, but this backyard species can tell us a lot about how birds are adjusting to rapidly rising global temperatures.

There are two well-known patterns in the animal kingdom described over 150 years ago. In 1833, a German biologist, Carl Bergmann, observed that animals farther north tended to be larger in size than their cousins in the south. He proposed this was because northern animals are better able to retain their heat with their large, stout bodies in colder environments, and that southern animals are better able to dissipate heat with their small, lanky bodies in warmer environments. In 1847, another German biologist, Constantin Gloger, observed that birds in warmer and wetter environments, like the tropics, were often darker in color than birds in colder and drier areas. There are several proposed reasons why this pattern could be observed, like for camouflage in dense foliage or for protection from harmful ultraviolet light, however the reasons are still not entirely clear.

These patterns from the mid-1800s, hence called Bergmann’s rule and Gloger’s rule, have been observed across the world even today. Recently, scientists have thought that these patterns will change in response to rising global temperatures. Biologists predict that birds will get darker in color and smaller in size due to climate change. If they change, and by how much is not currently known, so we intend to address this gap in knowledge.

House Sparrows are an ideal species to study these patterns. They are just about everywhere across the US, Mexico, and Canada. They don’t migrate like other birds; they stick close to where they were born for their entire life. House Sparrows are also not native to the US. They traveled with humans from western Europe around 160 years ago and quickly spread across the continent. Additionally, back in the ‘60s, two biologists, Richard Johnston and Robert Selander, trapped over 4,000 House Sparrows and collected their bodies for a museum specimen collection. They preserved the skeletons and feathers of all these birds, and now today I can go back into that collection, measure the birds’ limbs, and photograph the birds’ feathers.

House Sparrows in a museum collection.

Museum collections are an underutilized resource for research. Many people don’t know that behind the scenes there is a vault of information: thousands of beautifully preserved specimens in temperature and light controlled cabinets. Some of these specimens are never seen by the public. Those collections could even contain examples of species long extinct, remembered only through what was left behind. Properly preserved specimens can provide a trove of information over long periods of time, showing us an intimate history of an animal’s distribution and condition. Specimens decades old can be used in modern analyses—sometimes analyses the collectors never dreamed of. Johnston and Selander likely never imagined a grad student would use their specimens to help predict how birds will respond to rising global temperatures.

Those historical House Sparrow specimens can then be compared to today’s birds, revealing how they have changed in the past 60 years. To do this, my team and I traveled across the US, resampling 18 of the original sites from this historical study, creating a contemporary matching collection. At each site, we aimed to sample 40 House Sparrows—20 males and 20 females—using mist nets, fine mesh nets meant to capture birds without harm. After capturing the birds, we can measure their limbs, photograph their feathers, and conduct genetic studies, just like with the historical specimens.

Setting up the mist net at Tucson Audubon; House Sparrow in hand.

One of the original sites in the historical study was in Tucson. The Tucson Audubon Society was kind enough to let us work with them and utilize their headquarters as one of our sampling sites. It was by far one of the best sites I have had the pleasure to collect at. The House Sparrows came in at a steady stream that allowed me and my team to work effectively with them. However, House Sparrows are surprisingly clever, and they can quickly figure out where our nets are placed; we had to move our nets periodically so they wouldn’t get used to them. We also caught several other beautiful species of birds that we carefully released back into the wild, like Verdin and Yellow-rumped Warbler. The Tucson Audubon Society also has several species of hummingbirds around their property, like Anna’s, Broad-billed, and Costa’s Hummingbirds. Most were keen and agile enough to avoid getting caught, but some hit the nets and we were able to observe them up close before gently releasing them. A young Cooper’s Hawk got himself caught in our net as well, probably thinking he was getting himself an easy lunch.

Non-target Curve-billed Thrasher and juvenile Cooper’s Hawk were caught.

We were able to reach our target number at the Tucson Audubon site and we’d like to thank them for allowing us to use their space. The trip was a success overall, and we returned with 200 samples from across the American southwest for our study. Hopefully in the coming years we will be able to assess how our rapidly changing climate is affecting House Sparrows, and possibly birds overall, with these valuable samples.

Isaac Rush is a PhD student at North Dakota State University studying morphometrics in House Sparrows and managing the university museum collection.