The study evaluated the sizes of those birds that passed through the Carnegie Museum's Powdermill ringing station between the years 1961-2007. Each bird was weighed and wing length was measured (wing cord length). The study concluded that many of these birds/species were becoming lighter in weight and exhibiting shorter wing lengths.
More specifically: 83 species were caught during spring migration, 60 of which have shown a decrease in size over the course of the study, weighing less and having shorter wings. Of the 75 measured autumn migrators, 66 had become smaller. 51 of the 65 summer breeding species and 20 of the 26 overwintering species also had a reduction in size.
However, it is important to note that there was no observation of a decline in population size.
Quickly, let's review some common ecological "rules".
Bergmann's Rule - Animals tend to be larger in cold areas compared to those in tropical/warm areas - even within the same species. Think surface area on this one. The larger the body size, the less its surface area is to its total volume, allowing it to conserve more heat. Wolves are a good example of this one.
Allen's Rule - The extremities of animals (ears, legs, etc.) are shorter in cold areas compared to those in warms areas. A good example of this is comparing hares/rabbits - snowshoe hare vs. jackrabbit.
Gloger's Rule - Dark pigments increase in races of animals in warm and humid habitats.
So, this story looks to be a good example of Bergmann's Rule -- Although, I could argue the shorter wings Allen's rule angle. As this change in size and weight has occurred only within the last 50 years, it is thought to be an evolutionary change in response to warmer temperatures. Keep in mind that 50 years is not long to us but in bird-terms we're talking at least 20 generations - a good example of rapid, contemporary evolution.
But once we start talking evolution you know the term "fitness" is going to pop up. In terms of NA birds, they may be finding an optimal body size but it is yet unclear as to the overall fitness of many species in these changing climatic conditions. Evidence from other studies suggests that some species will benefit while others will decline.
Additionally, this change in body size might not actually be due to temperature at all but rather to variables that correlate with temperature, such as food availability and/or metabolic rate.
As with most other interesting science --- more study is needed.
Here's the story link: http://news.bbc.co.uk/earth/hi/earth_news/newsid_8560000/8560694.stm
(image from rps.psu.edu)