Smallest birds change the fastest


A new study led by the University of Michigan and published in Proceedings of the National Academy of Sciences has found the rate of change in biometrics is fastest in smaller species of birds.

The study analysed data from two previous research papers. Together, the papers provided data on 86,000 individual birds as part of research into body size and wing length.

One of the studies measured 52 species of migrant birds from individuals killed after hitting buildings in Chicago and the other was a trap-and-release project using mistnets to focus on 77 sedentary species in the Amazon. The studies covered roughly the same period of time.

Golden-crowned Kinglet was the smallest species examined as part of the Chicago dataset, weighing an average of 5.47 g (Darran Rickards).

Golden-crowned Kinglet (5.47 g) and Fork-tailed Woodnymph (4.08 g) were the smallest species studied in Chicago and the Amazon respectively, with Common Grackle (107.9 g) and Amazonian Motmot (131 g) the largest.

Despite the very different locations and species studied in each project, the new study has used to the data to show that both datasets display widespread increases in wing length in line with decreases in body size.

Furthermore, the data show that the smaller the species, the faster the rate of decrease in body size has occurred, and the faster the increase in wing length.

Benjamin Winger, assistant professor of ecology and evolutionary biology at University of Michigan, said: "The relationships between body size and rates of change are remarkably consistent across both datasets. However, the biological mechanism underlying the observed link between body size and rates of morphological change requires further investigation."

The authors of the previous studies in Chicago and the Amazon proposed a link between increasing temperatures over the last 40 years and the measured changes in biometrics. This raises the possibility that body size and wing length could be important responses to climate change.

Smaller-bodied species may be adapting more quickly to evolutionary pressures presented by climate change, but the researchers from the University of Michigan said the data did not allow them to test whether the changes in measurements were related to natural selection.

Brian Weeks, co-author of the study, said: "If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both. Either way, body size appears to be a primary mediator of birds' responses to contemporary climate change."

Marketa Zimova, lead author of the study, said: "Our results suggest that large body size could further exacerbate extinction risk by limiting the potential to adapt to rapid, ongoing anthropogenic change.

"In contrast, the body-size effect on evolutionary rates might increase persistence of small taxa if their rapidly changing morphology reflects a faster adaptive response to changing conditions."

A total of 70,716 individual birds were collected as casualites from collisions with buildings in the Chicago study, but David Willard, co-author of the new study, said it is "extremely gratifying to see data from these birds analysed for a better understanding of the factors driving these changes."

The similar, extensive datasets provided a rare chance to investigate whether body size and generational length affected the response of birds to fast environmental changes. Generational length is the average age at which organisms produce offspring, with shorter-lived organisms generally reproducing more quickly and predicted to evolve faster than larger ones.

The researchers found no relationship between generation length and rate of change in North American birds, and generation-length data was not available for the South American species.

However, a clear relationship existed between the mean body size of a species and the rate of change in body mass and wing length.

The authors said: "Body size may be a valuable predictor of adaptive capacity and the extent to which contemporary evolution may reduce risk of extinction among species."



Zimova, M, Weeks, B C, Willard, D E, & Winger, B M. 2023. Body size predicts the rate of contemporary morphological change in birds. Proceedings of the National Academy of Sciences. DOI: https://doi.org/10.1073/pnas.2206971120