Are Nightjars Bigger in the North? Tracking Birds Across Continents to Understand a 175-Year-Old Rule
A study on how different species of migratory nightjars organize themselves across the annual cycle in relation to latitudinal and environmental gradients.
By Aaron Skinner
Background
For nearly two centuries, biologists have been fascinated by a simple but surprisingly powerful pattern in nature: animals tend to be larger in colder places. This idea, known as Bergmann’s rule, was first proposed in 1847 and has since been observed across many groups of warm-blooded animals, including birds and mammals. The traditional explanation is intuitive. Larger bodies retain heat more efficiently, which may provide an advantage in colder environments.
But as with many broad patterns in biology, the reality is more complicated. Temperature is not the only factor that changes across the globe. Food availability, seasonality, and even the demands of long-distance migration can all vary with geography. As a result, scientists have debated for decades what drives these patterns in body size. Is it really about staying warm, or are other environmental pressures more important?
Migratory birds add an extra layer of complexity. Unlike species that live in one place year-round, migrants experience very different environments over the course of a year. A bird might breed in a temperate, northern forest but spend the winter in a tropical savanna thousands of kilometers away. This raises an important question: which part of the annual cycle matters most for shaping body size?
Nightjars are an ideal group to explore this question. These nocturnal, insect-eating birds undertake long migrations and are adapted to extreme and variable environments. They tolerate heat well, cope with periods of low food availability, and even enter short bouts of torpor to conserve energy. Because of these adaptations, it has long been unclear whether nightjars would follow Bergmann’s rule at all.
What did we look for?
We set out to answer three main questions. First, do migratory nightjars follow Bergmann’s rule? In other words, are individuals from colder, higher-latitude regions consistently larger than those from warmer areas? Second, if such a pattern exists, what is the most likely cause? We evaluated several competing explanations, including:
Temperature regulation: larger bodies help conserve heat in colder climates
Productivity: differences in food availability influence growth and size
Seasonality: harsher seasonal environments favor larger individuals that can better withstand periods of scarcity
Migration distance: the physical demands of long-distance travel shape body size
Geographic position: latitude and longitude may capture multiple environmental gradients at once
Finally, we asked whether conditions on the breeding grounds or the wintering grounds play a stronger role in shaping body size. This distinction is especially important for migratory species but has rarely been tested directly.
How did we go about it?
To answer these questions, we combined large-scale field data with modern tracking technology. We captured and measured over 7,000 individual birds of three species of migratory nightjars: The Eastern whip-poor-will, Common nighthawk, and European nightjar, across much of the continental United States, Canada, and Europe. We measured body size, and attached tiny GPS tracking devices to a subset of individuals to understand where they spent their winters. These data were critical for linking individuals to environmental conditions throughout the full annual cycle. We then paired each bird’s location with environmental data, including temperature, vegetation productivity, precipitation, seasonal variability, and migration distance (where available) to answer the three questions we posed above.
What did we find?
All three species showed clear evidence of Bergmann’s rule – individuals from higher latitudes were consistently larger than those from lower latitudes, providing the first strong evidence that nightjars follow this pattern. This was somewhat surprising. Given their unique adaptations to extreme conditions, we expected that nightjars might deviate from Bergmann’s rule. Instead, they conform to it quite closely.
When we examined potential mechanisms, a more nuanced picture emerged.
We found some support for the traditional temperature-based explanation, but it was not the whole story. Instead, geographic variables such as latitude and longitude were the strongest predictors of body size. These variables likely capture a combination of underlying environmental gradients, including temperature, productivity, and seasonality.
Finally, environmental conditions on the breeding grounds were much stronger predictors of body size than conditions on the wintering grounds. This suggests that the environments where birds reproduce play a dominant role in shaping their morphology. In contrast, conditions on the wintering grounds had relatively little explanatory power. Despite spending a large portion of the year away from their breeding sites, these environments appear to have less influence on overall body size.
What does this mean?
Our results help clarify several long-standing questions in ecology regarding Bergmann’s rule. First, nightjars do follow Bergmann’s rule. Species that respond to spatial temperature gradients are generally more capable of altering morphology in response to increasing temperatures associated with climate change. This suggests that nightjars may be capable of tracking changing conditions through shifts in body size. whether such responses can keep pace with the rapid rate of current climate change remains uncertain.
Second, our study shows that the breeding season is the critical window for shaping body size in nightjars. This suggests that changes in climate, habitat, or resource availability on the breeding grounds could have disproportionate impacts on morphology and, potentially, fitness.
Finally, we found that geographic variables were the strongest predictors of nightjar body size, which lends credibility to nearly two centuries of past research that has predominantly leveraged geographic information from the breeding grounds. At the same time, it highlights that these geographic patterns likely reflect multiple underlying environmental processes, rather than a single force like temperature alone.
More broadly, our work highlights the value of combining traditional field measurements with modern tracking technology. By linking individual birds to specific locations across their annual cycle, we can begin to untangle the complex interactions between environment and evolution.
ChatGPT was used to assist with proofreading and language polishing. All content was thoroughly reviewed and revised by the author, who take full responsibility for the final text.
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