Scientists also recorded countryside great tits and found they sang at a lower pitch and, in turn, city birds found the rural droll difficult to understand, the study found.

BBC: Birds in cities 'singing louder'

According to a 1998 study by Louisiana State University, more than 500 million birds fly over the Gulf and enter the United States along coastal areas in Louisiana and Texas each spring.

CNN: Oil spill could be disaster for animals, experts say

An aim of the study is to better understand the part hedgehogs play in the decline of the birds.

BBC: Hedgehogs may have returned to cleared areas of Hebrides

If birds, as well as mammals, have personalities, it may make it easier to study the evolutionary pressures that give rise to such systematically different ways of behaving within a single species.

ECONOMIST: Animal personalities