Almost 40 years of data reveal that, thanks to climate change, North America’s migratory birds are shrinking whilst their wings are growing longer — effects that may be occurring all around the world
NOTE: This piece was a Forbes Editor’s pick.
A recently published study reveals that migratory birds in North America are shrinking, thanks to warming temperatures caused by climate change. In addition to losing body weight, the birds’ legs are growing shorter whilst their wings are growing longer.
A 40 year look at how climate change is affecting North American birds
“It started as a very casual study — someone mentioned that birds sometimes run into McCormick Place, and I was curious, so I went for a walk around the building one morning,” study co-author, ornithologist David Willard, who now is collections manager emeritus at Chicago’s Field Museum, said in a press release issued by the Museum.
McCormick Place, an enormous building overlooking Chicago’s lakefront, is the largest and most flexible use convention center in North America. Conveniently located within walking distance from the Field Museum, its vast windows are a conspicuous killer of migratory birds. (According to Dr. Willard, McCormick Place dimmed its nighttime lights about 20 years ago and the resulting bird deaths there dropped by about three-quarters.)
“I found a couple dead birds and I brought them back to the museum — I’ve always wondered if there had been no birds that morning whether I would have ever bothered to go back.”
What began as a simple look-see soon developed into an obsession. Arising as early as 3:30 every morning in spring and autumn, Dr. Willard and a team of volunteers from the Chicago Bird Collision Monitors collected more than 100,000 birds that had collided during the previous night with McCormack Place and other buildings in downtown Chicago that demarcate the lake front.
They rescued and rehabbed the few individuals that were still alive, but for nearly all of the birds, it was too late. Chicago’s glass buildings had done their deadly job. The volunteers ended up collecting so many dead birds between 1978 and 2016 that these specimens now comprise as much as 20% of the museum’s bird collections.
Dr. Willard measured each and every bird collected — an amazing feat in itself as well as an important feature of this particular study because the resulting data were collected by only one individual, which adds to their consistency. In total, Dr. Willard measured 70,716 birds representing 52 species for this study and entered those data by hand into a ledger. These study birds were mostly common migratory songbirds such as sparrows, warblers and thrushes that breed in colder regions of North America and overwinter in warmer areas in Central and South America, although there were a few rarities, such as a lone yellow rail, an especially elusive wetlands bird, and the state’s first confirmed record of a Brewer’s sparrow, a small migratory songbird that’s more common in western North America.
“When we began collecting the data analyzed in this study, we were addressing a few simple questions about year-to-year and season-to-season variation in birds,” Dr. Willard said in a press release. However, data analysis revealed that these birds’ body size differences were more than simple seasonal variations. They followed a trend: regardless of species, the birds were becoming smaller.
“It’s a matter of millimeters, tenths of millimeters — it’s not something you know is happening until the analysis,” Dr. Willard added. He noted that because these changes were measured in such a large sample, and in one involving diverse bird species, indicates these trends are real and widespread.
Between 1978 and 2016, the birds’ body sizes gradually decreased. For example, 49 of 52 species of birds lost an average of 2.6% of their body mass, and their lower leg bone length decreased by 2.4% across species (Figure 2). Surprisingly, 40 of 52 of the bird species’ wingspans increased by 1.3% during the same time (ref). Further, the researchers found that species with the fastest declines in leg bone length also showed the most rapid gains in wing length.
The researchers also collected temperature, precipitation, vegetation and other environmental factors during the same time period, and found that warmer temperatures were the strongest predictor of smaller body size.
Why are we seeing such dramatic shifts in the body shapes and sizes of North American migratory birds?
The researchers suspect that these changes in body size and shape are related to climate change. Basically, as climate change causes northern latitudes to become warmer, birds with smaller bodies that are breeding in these regions are better able shed excess heat. However, smaller bodies makes it difficult for these birds to produce enough energy to fuel their long semi-annual migrations so, the researchers propose, their wings are becoming longer to compensate.
These observations are consistent with Bergmann’s rule, which describes how populations of animal species with large geographic distributions are typically observed to have a larger body size in colder environments, or in higher latitudes, compared to their counterparts living in warmer regions — a trait helps them stay warm in cold places. This body size-climate relationship was widely “known” by the historical scientific community, but this pattern was first formally described by German physiologist, Carl Bergmann in 1847. Although there are a few exceptions to Bergmann’s Rule, it generally applies to most mammals and birds (i.e.; ref & ref).
Birds are canaries in the climate change coal mine that humans created
There is absolutely no debate in the scientific community regarding whether the planet is warming, nor about the ultimate cause of this warming. As people rapidly change the world, we are causing consistent, widespread — and often subtle — changes in all living beings that share their planet with us. But without long-term, broadly-based studies such as this, it’s difficult to document precisely how climate change is affecting living populations around the world.
But this study’s findings fit what has been reported by other research teams in other places. For example, this study bolsters findings by a 2017 study in zebra finches breeding in a range of temperatures across Australia. That study reported that warmer temperatures during breeding periods resulted in smaller birds (ref).
But precisely how might warmer temperatures create smaller birds? In addition to increased heat, it is possible that other effects of climate change, such as food limitation or heat stress, could lead to reduced body size.
This study adds more evidence to our growing concern for the overall long-term health of North America’s birds: a short time ago, another team of researchers documented a 29% decline in North American migratory birds over the past 40 years — a decline that is estimated to translate into a loss of 2.9 billion birds (more here).
That research was quickly followed by another study by a different team in Hawaii that documented how dramatic population declines are affecting the distinctiveness of songbirds’ songs — which could negatively affect their annual reproductive rates because song is critically important for attracting mates and maintaining a breeding territory (more here).
Most climate change studies of animals and plants focus on shifts in geographic range, or in the timing of migration or springtime flowering. However, this study suggests that changes in body shape and size, which have been consistently overlooked, are also important aspects of how global warming is affecting wildlife.
This study is a valuable reminder that museum collections hold a vast treasure trove of knowledge and discovery for present and future scholars to study in ways that may not have been imagined when specimens were first being collected. It also highlights the critical importance of museums that curate and care for biological collections that provide data for studies such as this one, and the immense value of long-term studies, which can reveal and document worrying patterns caused by climate change.
Brian C. Weeks, David E. Willard, Marketa Zimova, Aspen A. Ellis, Max L. Witynski, Mary Hennen and Benjamin M. Winger (2019). Shared morphological consequences of global warming in North American migratory birds, Ecology Letters, published online on 4 December 2019 before print | doi: 10.1111/ele.13434
Originally published at Forbes on 9 December 2019.