Why Nesting Earlier May Not Be Enough for Northern Tree Swallows in a Warming World

By Conor Taff, Research Associate, Cornell University

As documented by birdwatchers and numerous scientific studies across the world, birds are laying eggs earlier in the spring as the climate warms. The speed at which a species can adjust their breeding timing to changing conditions (like temperature, breeding season length, or climatic variability) might play a role in determining whether their numbers are holding steady or declining. And many species experience enormous differences across their range, such as the vast breeding range of the Tree Swallow, which spans more than 31 degrees of latitude.

In a new study (Taff et al. 2026), my colleagues and I investigated whether different populations of Tree Swallows responded similarly to half a century of climate change, and whether different responses were related to changes in population abundance. Answering these questions requires decades of carefully-collected nest records from many populations across an entire species’ range, but very few species in the world have this kind of data. Tree Swallows are one of the few species with enough data to study range-wide patterns, but even so, it took a big collaboration to pull all of the necessary data together. Combining observations from long-term research sites and NestWatch allowed us to examine 94,873 nests from 1966–2024, spanning most of the Tree Swallow breeding range.

Knowing When to Nest is Tricky

We found that Tree Swallows lay earlier in warmer springs, at a rate of about one day earlier for every 1℃ during the three weeks before laying starts. But what we did not expect was that the sensitivity to spring temperature was essentially the same everywhere. Meaning, a population in Tennessee and a population in Alaska adjust their timing by the same amount in response to the same amount of warming, despite breeding in completely different climates. Tree Swallows appear to be working from a single, broadly shared template across the continent, which may be linked to their strong dependence on flying insect emergence in order to successfully raise their young.

If every population responds the same way, you might think they would all be doing equally well, or equally poorly. In fact, we found that despite a consistent response to temperature, there are big differences in population stability, with northern populations showing pronounced declines while southern populations have been stable or increasing. One possible explanation for these differences is that the nests that fledged the most young were laid earliest. In other words, even after adjusting breeding timing, most birds are still laying later than appears to be ideal, but this pattern depends strongly on the weather experienced each season. In cold springs there is an ideal time frame to nest, with costs to nesting either too early or too late. In warm springs, that time frame is earlier, so that there is pressure to breed even earlier across the entire breeding range. Northern populations have experienced the most warming, but they are also more likely to experience occasional cold spells during spring. Balancing the benefits of breeding earlier against the risks of cold exposure may be especially challenging.

Northern Limits

We also found that northern populations may be running into a timing constraint that southern populations don’t face. When we added migration arrival dates estimated from eBird checklists, a striking pattern emerged. Arrival dates get later as you move north, exactly as you would expect. But average egg-laying dates, and the temperature-sensitive window that precedes them, stop shifting later above about 45°N. The consequence is a time squeeze. In the southern part of the range, Tree Swallows have roughly eight weeks between arriving and laying their first egg. In the far north, that gap narrows to under three weeks. Northern birds are not less responsive to a warm spring, but they are working against a much shorter deadline, with less time to recover from migration, claim a nest box or natural cavity, and build up the body condition needed to produce eggs.

Three further lines of evidence support the idea of greater vulnerability in the north. First, the critical pre-laying window has warmed across the whole range, but it has warmed the most at the highest latitudes. Second, as a consequence of faster warming, laying dates have advanced faster in the north; since 1975 we see almost no change at the southernmost sites, but up to a 10-day advance in the northernmost ones. Third, Breeding Bird Survey estimates of population abundance show a clear latitudinal gradient, with southern populations generally increasing and northern populations declining over the past half century. We found that the populations that have adjusted their breeding dates the most are the same ones that have lost the most birds. Altogether, our results suggest that timing conflicts and climate exposure for northern populations may be contributing to these patterns, despite a similar behavioral response to temperature. We must note that our data can’t definitively demonstrate the cause of population declines, and aerial insectivores like Tree Swallows are also vulnerable to a variety of other challenges that could differ across their range, including changes in insect availability, pesticides, and conditions outside the breeding season.

These results influence where and how we look for climate adaptation and consequences. None of our findings would have been visible from any single study site, no matter how carefully the study was run. One clear conclusion of our study is that some patterns can only be detected by large-scale comparison across latitudes, requiring nest records from locations beyond the scope of any one research group. Long-term and widespread monitoring programs like NestWatch are critical to answer questions like these. The value of these data continues to grow the longer they are collected, and the nests that are being monitored across the ranges of many species right now will be the best resource to understand the next 50 years of population responses to ongoing climate change.

Reference:

• Taff, C. C., J. R. Shipley, D. R. Ardia, D. Aborn, L. Albert, M. Bélisle, A. Belmaker, L. L. Berzins, T. Blake, F. Bonier, H. C. Brewer, M. W. Butler, K. Cameron, S. B. Case, D. Chang van Oordt, R. G. Clark, E. D. Clotfelter, A. R. Cox, R. D. Dawson, E. P. Derryberry, A. M. Diaz Bohorquez, P. O. Dunn, V. Ferretti, A. M. Forsman, M. Fuirst, D. Garant, D. R. Garrett, J. Gutiérrez, J. C. Hagelin, B. M. Hardt, M. E. Harris, K. Horton, C. Houle, J. L. Houtz, P. L. Jones, K. C. Jordan, A. S. Kindel, R. Klaver, S. A. Knutie, K. S. Lauck, M. P. Lombardo, S. C. Lougheed, A. C. Love, S. A. Mackenzie, J. P. McCarty, A. E. McKellar, N. Mejia, C. A. Morrissey, M. L. Nahom, D. R. Norris, L. M. Para, F. Pelletier, C. K. Porter, W. B. Rendell, E. A. Riddell, J. W. Rivers, R. J. Robertson, A. Rose, K. A. Rosvall, T. A. Ryan, R. P. Shannon, D. Shutler, V. F. Simons, M. Stanback, C. E. Tarwater, P. A. Thorpe, M. W. Tingley, C. L. Tischer, B. A. Tonelli, M. L. Truan, C. W. Twining, J. J. Uehling, C. Vleck, D. Vleck, M. L. Watson, N. T. Wheelwright, L. A. Whittingham, D. W. Winkler, C. Youngflesh, C. Zimmer, and M. N. Vitousek. 2026. Divergent population trajectories despite similar response to temperature in a widespread aerial insectivore. Proceedings of the National Academy of Sciences of the United States of America 123:e2601817123. https://doi.org/10.1073/pnas.2601817123.