Evidence of body size plasticity in response to increased minimum nest temperatures in a North American aerial insectivore

Abstract

Many animal species have declined in body size with rising global temperatures, yet the underlying mechanism of these changes is unknown. We designed an experiment to test whether increased minimum temperature during development, comparable to increases over the past 50 years, can induce body size plasticity in developing birds. We installed warming devices in 76 tree swallow (Tachycineta bicolor) nest boxes to increase nest temperatures by up to 1ºC and recorded daily mass for 307 nestlings across two sites. We found that increases in nest minimum temperature resulted in larger nestling mass, with some variation among levels of warming. Nestling mass was influenced most by our two study sites, which consistently differed in insect prey abundance and diversity throughout the 2024 breeding season. Our results suggest that the magnitude of recent global changes in minimum temperature are enough to induce plastic growth in developing animals, but that the driver of overall declines in size, perhaps linked to resource availability, may be overcoming this effect.