Observed declines in body size have differential effects on survival and recruitment, but no effect on population growth in tropical birds

Declines in body size can be an advantageous physiological response to warming temperatures, or a result of physiological and nutritional stress. Either way, studies often assume that these climate‐induced trait changes have important implications for fitness and demography. We leveraged almost three decades of capture‐mark‐recapture data of 51 bird species in Panama to examine if body size has changed through time, how sensitive body size is to changes in weather, and if body size impacts population demography. We evaluated two metrics of body size, structural size (wing length), and body condition (residual body mass). Over the study, wing length changed in varying directions for 88% of species (23 decrease, 22 increase), but the effects were weak, and change was only significant for two species. Conversely, body condition declined for 88% of species (45), effects were stronger, and that change was significant for 22% of species (11). This suggests that nutritional stress is likely the cause of changes in body size, not an adaptive response to warming. Precipitation metrics impacted body condition across three of our four feeding guilds, while wing length was only impacted by weather metrics for two guilds. This suggests that body condition is more sensitive to change in weather metrics compared to wing length. Lastly, we found that the impact of changes in body size on survival and recruitment was variable across species, but these relationships were in the opposite direction, ultimately resulting in no change in population growth for all but one species. Thus, while different stages (adult survival and recruitment) of populations may be impacted by body size, populations appear to be buffered from changes. The lack of an effect on population growth rate suggests that populations may be more resilient to changes in body size, with implications for population persistence under expected climate change. One way animals respond to climate change is through changes in body size; however, the direction of change, drivers, and consequences for populations is often unclear. Studying 51 bird species in Panama, we found birds are generally declining in body condition and weather influences foraging guilds differently, suggesting that nutritional stress may be a cause of declining body size and guilds may be experiencing different selection pressures. Changes in body size had opposing effects on adult survival and recruitment, while population growth rate was buff