Many migrant birds are becoming smaller
This huge study has just been published:
"Researchers analysed 70,716 specimens from 52 North American migratory bird species collected over 40 years. The birds had died after colliding with buildings in Chicago, Illinois. The authors say the study is the largest of its kind and that the findings are important to understanding how animals will adapt to climate change."
Looks like a classic illustration of Allen's rule (animals in colder climate have shorter limbs) and Bergmann's rule (warm-blooded animals in colder climate are larger).
Note, that these well-known rules developed in the 19. century might not exist. So many animals don't conform to these rules or were not studied that the rules might be cherry picking.
But I wouldn't be surprised that the results may be e.g. due to habitat fragmentation (smaller size to survive in small habitat patches, and longer wings to search for patches of suiitable habitat).
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