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."
The fuzziness of all supposedly absolute taxonomic distinctions - Stephen Jay Gould (1977) "Ever Since Darwin: Reflections in Natural History".
Species and subspecies are but a convenient fiction - Kees van Deemter (2010), "In praise of vagueness". Biology is messy