As temperatures continue to rise around the world, some species have no choice but to sweat it out in place, while others are deciding to pack it up and move toward cooler temperatures. 

Following ideal climatic conditions is an important strategy for wildlife capable of moving long distances – rather than adapting (or succumbing) to new warmer conditions inside their original range boundaries, species can sometimes shift their ranges, typically to higher latitudes or higher elevations, to follow the conditions ideal for their survival. Doing so allows the species to mitigate at least some portion of the environmental changes they would have experienced had they stayed in place. 

Researchers have long expected that birds, as highly mobile animals, should have a strong capacity for niche tracking, and many species have in fact been documented shifting their geographic ranges over the past few decades. But the little-studied question remained: are they actually able to reduce their exposure to climate change through this strategy? 

A new paper by Jeremy Cohen, Associate Research Scientist at Yale University, and Walter Jetz, a Professor at Yale in the Department of Ecology and Evolutionary Biology, investigates the extent to which North American birds can actually retain their niches through these range shifts. The researchers found that during both summer and winter, birds that moved long distances north did better than other species at reducing their exposure to warming temperatures, but nearly all species still experienced significant warming – even those that moved 100-200 miles north. The findings were published this week in the journal Nature Ecology and Evolution.

“While numerous studies over the past few decades have found that species have been moving north or up mountains to escape climate change, the consequences of these large-scale movements have been almost entirely ignored, and it’s often been assumed that these species were able to avoid climate change entirely,” said Cohen. “Given the enormous amount of bird observations available thanks to citizen science initiatives (e.g., eBird) over the past few decades, we had the data to explore whether birds have been able to reduce their climate change exposure by escaping to cooler places. If birds can’t keep pace with climate change despite being highly mobile, the outlook isn’t great for reptiles, amphibians, plants and other taxonomic groups that can’t move very far.”

The trove of observational data available for birds in North America is ideal for this kind of study. In all, the researchers pulled tens of millions of bird observations from the Global Biodiversity Information Facility (GBIF) database across 406 species that breed or overwinter in the United States or Canada over a twenty year period. Observation points recorded between 2000 and 2004 were used as a baseline for each species, defining the average conditions experienced by the species across its range at the beginning of the study. 

The researchers used this baseline to project what temperature the species would have experienced by 2020 if they had stayed in place, then assessed the actual temperatures experienced at the real observation points (which were often further north) for the species between 2000 and 2020. Comparing the projected and actual climate change each species experienced revealed how much, if at all, each species was able to mitigate warming temperatures by moving north or up mountains The researchers also adjusted their estimates based on the movement of human observers, which dictates where birds might be observed. This analysis was carried out for both the summer and winter seasons, made possible by the fact that citizen science data is collected year-round. 

“Unfortunately, we found that most bird species have been unable to keep pace with warming conditions, especially during the winter,” said Cohen. “Typically, only the species that moved incredibly far – 200 or more miles north in only 20 years – were able to keep up with changing conditions. Meanwhile, moving up mountains didn’t allow many species to escape warming.” 

The results are troublesome. The researchers found that while birds are moving north at a substantial pace, these efforts have not been sufficient to outpace warming temperatures. 

Over the summer months, the researchers found that birds were able to avoid, on average, about half of the expected 2.4°C warming by shifting their ranges an average of 43 miles northward. Though this is substantial, nearly two thirds of the species still experienced a temperature shift greater than 1°C. 

The wintertime results were even more extreme. On average, bird species experienced nearly 4°C of warming over two decades in the winter months – and that was after mitigating just half a degree of the expected warming by shifting, on average, 44 miles northward. Interestingly, even though they were exposed to more warming in the winter than summer, species on average mitigated much more warming through their movements during summer. “This suggests that species may be working harder to limit their exposure to unusually hot conditions during the summer, where those conditions might be closer to the upper limits of what they can tolerate, as compared to winter where unusually warm conditions – i.e., mild weather – are less likely to be harmful,” said Cohen.

However, the authors stress that species varied quite a bit in their movement patterns and ability to reduce their exposure; some species made large range shifts northward and mitigated almost all of their expected warming, while others more or less stayed in place and experienced the full brunt of the warming climate. For example, the blue-winged warbler (featured in the cover photo) would have experienced 2.6°C warming in just twenty years if they remained stationary; but, by moving north by nearly 100 miles, they avoided the majority of that warming and were labeled  a niche retainer by the researchers. On the other hand, some species, like the Nashville warbler, are niche shifters – they stayed within their original ranges and experienced nearly the complete expected warming. Unsurprisingly, species that have a greater ability to disperse over large distances – based on their migration patterns or physiology – were more likely than other birds to avoid much of the warming experienced in their original ranges.

Complicating the picture for birds is the issue of habitat cover – how can a bird move north to avoid warming temperatures if the habitat it requires, such as forests, grasslands, or lakes, doesn’t exist to the north? Or if, like much of the prairie region, habitat is too fragmented to feasibly make the jump? The researchers stress that future studies should incorporate this important variable to get a better picture of how birds are adjusting to climate change.

“Our results indicate that most birds are unable to keep up with climate change during both the summer and winter seasons, and with the pace of climate change only accelerating, the gap between the environments that birds are forced to live in and that which they are historically adapted to will continue to grow,” said Cohen.

If even birds, some of the most mobile organisms on the planet, can’t keep up with the pace of the warming climate, what are other species going to do? 

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