These data support findings linking relative brain size with the ability to persist in novel and changing environments in vertebrate populations, and have important implications for our understanding of recent trends in biodiversity.
|Harpy eagle and chick|
And it's never been clear what brain size in itself is all about. Male and female brains vary by a clear (average) amount -- roughly 10% in humans but varying among species with their sexual dimorphism. But the intelligence of males and females is comparable. And one can argue about whether urban or rural environments, with their different threats and opportunities, vagaries and resources, are more demanding of 'intelligence'. Does being fed by old folks in the park take more intelligence than finding food while worrying about hawks?
Amoeba adapt to changing environments, after all.
But whichever it is, if the underlying assumption is that adapting to city living is the ultimate in adaptability, this suggests that other birds are somehow less adapted or adaptable, but all birds are adapted by definition, and given that environments change all the time, they've all managed to adapt to change. And, there's probably an ascertainment bias here anyway -- a lot more observations have been made on urban birds than non-urban birds, so that we have a lot more evidence of this presumed superior adaptability in these birds than we do on birds that we don't see nearly as much.
A few more things are worth considering here. One is that if this result is indeed correct, this would reflect organismal selection rather than Darwnian natural selection: birds who like the urban environment go there, birds who find it unpleasant or confusing don't. Genes related to environmental response could be partitioned in this way, but there need not be any differential reproductive success.
Of course lifestyles in cities and country, including diet, amount and nature of exercise, and who knows what else may differ during the growth and development of the birds. This would not pertain if the country relatives of the urban species the authors studied are also bigger-brained, but then it would imply that country life was originally responsible.
The authors also found small-brained birds in the urban areas but (clinging to their hypothesis) tried to explain that away by invoking ad hoc (or post hoc) special explanations. That means that the rule, if true, is only a sometime-thing.
In any case, the study (if it can be confirmed in a systematic way, and if what it actually means about brain size can be clarified) is interesting.
And whatever the reason, crows have certainly adapted to city dwelling. But then, crows are in a class of their own.