North American birds are getting smaller

Since 1978, North American birds have become smaller, with scientists examining over 70,000 dead birds from 52 species that crashed into Chicago skyscrapers. A study in Ecology Letters found a 2.4% reduction in leg bone length and a 1.3% increase in wing length in 49 species. These changes may be a response to climate change, as birds evolve to have wider wingspans for migration. Animals tend to be smaller in warmer regions to carry less heat and larger in colder areas to conserve warmth.

What is the relationship between animal size and temperature in different regions as mentioned in the study?

What do scientists believe is the reason animals in warmer regions tend to be smaller?