A variety of species of migratory birds in the U. S. are being stressed due to global climatic changes, says a new Interior Department report. Apart from this, there are a lot of other environmental factors that are threatening their very species and may lead to their extinction.

About one third of the country's 800 bird species are in danger and are facing decline in population. The other things that are easily noticed about them are their becoming lighter and growing comparatively shorter wings. This is happening due to warm temperatures. This reduction has occurred within last 50 years.

Some of the other factors that are threatening bird population are destruction of habitat, hunting, pesticides, invasive species and loss of wetlands. According to the report, the oceanic birds and shore birds were largely affected by global warming, as it has changed the marine ecosystems and has led to an increase in the sea level as well. Song birds are also getting smaller in size in the U. S.

According to Bergman's Rule in Biology, animals grow smaller in the climates which are comparatively warmer.