A new study by scientists has shown that many birds in the world, particularly in North America, are shrinking, in response to warmer temperatures.

In biology, there is a general rule of thumb that animals tend to become smaller in warmer climates: an idea known as Bergman's Rule.

Usually, this trend can be seen among animal species that live over a range of latitude or altitude, with individuals living at more northern latitudes or higher up cooler mountains being slightly larger than those below, for example.

Quite why this happens is not clear.

To find out, Dr Josh Van Buskirk of the University of Zurich, Switzerland and colleagues Robert Mulvihill and Robert Leberman of the Carnegie Museum of Natural History in Rector, Pennsylvania, US, decided to evaluate the sizes of hundreds of thousands of birds that pass through the Carnegie Museum's Powdermill ringing station, also in Pennsylvania.

They examined the records of 486,000 individual birds that had been caught and measured at the ringing station from 1961 to 2007.

These birds belonged to 102 species, arriving over different seasons.

Each was weighed. It also had the length of its wings measured, recorded as wing chord length, or the distance between the bird's wrist to the tip of the longest primary feather.

Their sample included local resident bird species, overwintering species, and even long distance migrants arriving from the Neotropics.

What they found was striking.

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 out of 26 wintering species.

But some species are losing more weight.

For example, the rose-breasted grosbeak has declined in mass by about 4 percent, while the Kentucky warbler has dropped 3.3 percent in weight and the scarlet tanager 2.3 percent.

The trend is particularly noticeable among those birds that winter in the New World tropics of the Caribbean, Central America and South America.

"The headline finding is that the body sizes of many species of North American birds, mostly songbirds, are gradually becoming smaller," said Dr Buskirk. (ANI)