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Birds Are Shrinking as the Climate Warms — and Small Birds Are Shrinking Faster

As temperatures rise, birds’ bodies are growing smaller, but their wings are growing longer. A new study finds this shift is most pronounced among the tiniest species.

For the research, scientists analyzed four decades of data gathered from migrating birds in Chicago and non-migrating birds in the Amazon rainforest. In both places, birds grew smaller and longer-winged over time, a shift that would help them to stay cool in warmer weather. The new analysis, published in the Proceedings of the National Academy of Sciences, found that smaller birds shrunk faster.

One possible explanation is that smaller species tend to reproduce on shorter time scales — the span of one generation is shorter for a mouse, say, than it is for an elephant — and, as such, they are able to evolve faster. But the new study found no link between generation length and changes in body size.

Another potential explanation is that smaller species tend to have larger populations, meaning there is a greater chance that one member of their population will randomly develop a new, beneficial trait that can be passed on. But scientists found no link between population size and shifts in body size. “Why smaller-bodied species are changing faster is unknown,” authors wrote.

Scientists say future research should investigate why larger birds have been slower to adapt to human-caused climate change. “Larger species have an increased risk of extinction,” they wrote. “Our results suggest that large body size could further exacerbate extinction risk by limiting the potential to adapt to rapid, ongoing anthropogenic change.”

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