Climate Change Causing Birds to Shrink

Birds collected after flying into Chicago’s McCormick Place. (Karen Bean/Field Museum)

Image: Karen Bean/Field Museum


In September we wrote about a study that revealed that bird populations have declined nearly 30 percent — a loss of 3 billion birds — in the last fifty years. While scientists don’t quite know what’s caused this decline, new evidence shows that in addition to species decline, North American migratory bids have been getting smaller over the past four decades, and their wingspan wider. The changes appear to be a response to a warming climate.

CNN reported that a staggering 600 million birds die every year in the United States after colliding with tall buildings. And Chicago, with its skyscrapers and location on a major migration path, is perhaps the biggest killer.

The Stats: Dave Willard, a collections manager emeritus at the Field Museum has collected the dead birds (since 1978!) who have flown into skyscrapers and explained that this has yielded important scientific data.

A paper was published based on Willard’s data:

  • For the analysis, the biologists used 70,716 dead birds representing 52 species — including thrushes, sparrows and warblers — that Willard had logged between 1978 and 2016. Of those species, 49 saw statistically significant declines in body size. In particular, the length of the tarsus or lower leg bone, shrank by 2.4%.
  • Meanwhile, wing length showed a mean increase of 1.3%, with the species showing the fastest declines in tarsus length also showing the most rapid gains in wing length.

The bird’s wingspans may have increased to compensate for smaller bodies that produce less energy for the incredibly long distances the birds travel during their migrations.

One of the study authors, Benjamin Winger of U. Mich,  explained to the Washington Post that “Warming temperatures seem to be having a pretty consistent and almost universal effect on a large number of different species, regardless of other aspects of their biology.”


Why This Matters: One of the scariest things about climate change is that we truly have no way of knowing the breadth of effects it will have on our planet. Because of our activity, we’re forcing animals to try and adapt to a rapidly warming planet–which means that some will be successful and others will perish.

While scientists don’t fully know why birds are shrinking in size they’ve speculated that hot temperatures during their breeding season stresses young birds and don’t allow them to grow as large. Regardless, we don’t know what sort of reverberations smaller birds have across ecosystems, e.g. what it might mean for their predators.


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