North American migratory birds are becoming smaller as the planet warms due to climate change, a new study finds.

Researchers from the University of California, Los Angeles (UCLA) examined more than 30 years of data for adult male birds across 105 avian species that migrate through North America. They found that between 1989 and 2018 the birds' body masses declined by about 0.6% on average, according to an Oct. 27 study in the journal Nature Ecology and Evolution.

The species that "experienced the greatest change over time" was the tree swallow (Tachycineta bicolor), said study lead author Casey Youngflesh, a quantitative ecologist from UCLA and a presidential postdoctoral fellow in the Ecology, Evolution, and Behavior (EEB) Program at Michigan State University. In this songbird, known for its striking iridescent blue feathers, body mass dropped by nearly 3%. Data used in this study came from the Monitoring Avian Productivity and Survivorship program (MAPS), part of the Institute for Bird Populations, a California-based nonprofit that studies bird population decline and has 1,200 bird banding stations throughout North America.

So, what's causing birds to shrink, especially over such a relatively short time period? Scientists suspect that climate change is the most likely culprit, and birds are adapting accordingly. 

In other words, smaller-bodied birds have a larger body-surface-area-to-volume ratio, so they need to expend less energy to keep cool. By comparison, birds with larger bodies are better equipped for conserving heat, according to the study.

Scientists also found that the size of a North American bird is largely dependent on where it resides  even for birds of the same species.

Source: Livescience