North American Songbirds get smaller

I came across an interesting article today.

According to research published in the journal Oikos American songbirds are getting smaller.

Dr Buskirk of the University of Zurich, Mr Robert Mulvihill and Mr Robert Leberman of the Carnegie Museum of Natural History in Rector analysed the records of 486,000 birds, from 102 species, that had been caught and measured at the ringing station from 1961 to 2007.

Of 83 species caught during spring migration, 60 have become smaller over the 46 year study period, weighing less and having shorter wings.
Of the 75 species migrating in autumn, 66 have become smaller.

They found that:

Of 83 species caught during spring migration, 60 have become smaller over the 46 year study period, weighing less and having shorter wings.

Of the 75 species migrating in autumn, 66 have become smaller.

In summer, 51 of 65 breeding species have similarly reduced in size, as have 20 of 26 wintering species.

The changes are not huge, on average the spring migrants have lost 1.3% of mass. Although some species have lost mass at a great rate, Rose-Breasted Grosbreak by 4%.

The birds populations are not dwindling. According to Dr Buskirk

many of these species are apparently doing just fine, but the individual birds are becoming gradually smaller nonetheless

One possibility is that climate change is having an affect. Berman's rule suggests that species evolve into a smaller size as temperatures increase.

Interesting though I thought.