Morphological change in North American birds driven by abiotic variation over time, latitude, and elevation

Casey Youngflesh and Morgan W. Tingley, Ecology & Evolutionary Biology, University of California, Los Angeles, CA, James Saracco and Rodney B. Siegel, The Institute for Bird Populations, Point Reyes Station, CA

Background/Question/Methods
Morphology is both a cause and a consequence of how organisms interact with their environments. Morphological variation over time and space can lend insight into the role that the environment plays in shaping observed morphological patterns. However, the specific drivers of this variation remain elusive. For example, evidence for and causal factors behind Bergmann’s Rule, the notion that animals are generally larger at higher latitudes, continue to be debated. Given the thermoregulatory pressures that rapidly increasing temperatures may have on animals, changing climatic conditions may also play a substantial role in this regard. Still other factors, such as the effect of elevation on morphology, have heretofore not been explored at large scales.
Using data from more than 300,000 captures from 111 bird species derived from the Monitoring Avian Productivity and Survivorship program, we assessed intraspecific changes in North American bird morphology over 29 years, at locations spanning 40 degrees of latitude and 3 km of elevation. We derived indices of overall bird size and relative wing length based on empirical allometric morphological scaling relationships and used hierarchical Bayesian models to quantify spatiotemporal changes in these indices.

Results/Conclusions
Across the wide breadth of species considered, birds have generally decreased in size over the last several decades. Decreases in size were associated with elevated summer temperatures on breeding grounds, suggesting that warming temperatures are resulting in smaller individuals over time. In support of Bergmann’s Rule, birds at colder, more northerly latitudes were larger compared to more southerly locations. Birds at more northerly latitudes also tended to have longer wings as did birds captured at higher elevations. The latitudinal effect is likely due to the longer distances that birds at more northerly locations generally travel during migration, while the elevational effect is likely the result of decreased air density at higher elevations. Longer wings would be expected to be beneficial at higher latitudes and elevations, as they facilitate increased lift, potentially resulting in more efficient flight. This work highlights how spatiotemporal processes drive large-scale morphological variation, with implications for better understanding morphological differences within and among species and the impacts of rapid climate change on animal morphology.