Global warming-induced migratory bird body size reduction and shape change

Given that HN has had articles on the decline of the insect population (https://en.wikipedia.org/wiki/Decline_in_insect_populations) and the decline in bird populations (https://www.nytimes.com/2019/09/19/science/bird-populations-...), I am surprised that food supply is not listed as one of the causes of body changed.

There are few details in the abstract but wing length is notoriously difficult to compare across measurers - the standard method of measuring the leading edge of the wing by gently straightening the primaries along a stopped ruler can easily generate differences of 2-3mm for different people measuring the same bird. It can be done for small groups of people who work together - wader ringers (shorebird banders if you are in the Americas) where they deliberately practise to standardise their technique. If the sample sizes are good then the error should be consistent across time so the conclusion might still be sound.

https://en.wikipedia.org/wiki/Bird_measurement#Wing

The good news from this is that nature is splendidly adaptable and able to compensate for changes that occur within a few decades. That says that plants and animals are a lot more robust than we think they are.

UPDATE: One confounding factor is that standards in handling birds are always improving so there's no way of telling whether measurements are comparable across time.

UPDATE: Thanks to throwaway5752 for finding the full text. The birds were measured by a single observer so you can ignore everything I said. Interesting result.

There have been huge changes in habitat during the same period, driven largely by urbanization and changes in agricultural patterns, that dwarf any changes driven by climate. We actually had Ectopistes migratorius go completely extinct by 1901, due to habitat destruction, not climate change.

Birds eat insects, yes? If there's been a reduction in insects then wouldn't that impact birds' development?

I don't have access to the article, and there really isn't anything given in the abstract to actually link the change in body size to global warming. I hope the article does a better job than "warmer temperatures are expected to reduce bird size and here are a bunch of bird species which have gotten smaller over the last 4 decades, therefore climate change."