bill temperatures gradually returned to normal. Vasoconstriction of blood vessels in the bill not only serves as an energy saving mechanism, but also reduces the amount of heat transferred from core body tissues to the skin (via cutaneous vasodilation), which, in turn, reduces heat loss rate by lowering skin temperature relative to the environment. Relationship with humans The great tit is a popular garden bird due to its acrobatic performances when feeding on nuts or seed. Its willingness to move into nest boxes has made it a valuable study subject in ornithology; it has been particularly useful as a model for the study of the evolution of various life-history traits, particularly clutch size. A study of a literature database search found 1,349 articles relating to Parus major for the period between 1969 and 2002. The great tit has generally adjusted to human modifications of the environment. It is more common and has better breeding success in areas with undisturbed forest cover, but it has adapted to human modified habitats. It can be very common in urban areas. For example, the breeding population in the city of Sheffield (a city of half a million people) has been estimated at some 17,000 individuals. In adapting to human environments its song has been observed to change in noise-polluted urban environments. In areas with low frequency background noise pollution, the song has a higher frequency than in quieter areas. This tit has expanded its range, moving northwards into Scandinavia and Scotland, and south into Israel and Egypt. The total population is estimated at between 300–1,100 million birds in a range of 32.4 million km2 (12.5 million sq mi). While there have been some localised declines in population in areas with poorer quality habitats, its large range and high numbers mean that the great tit is not considered to be threatened, and it is classed as least concern on the IUCN Red List. References External links RSPB page Song sample BBC fact file Ageing and sexing (PDF; 2.5 MB) by Javier Blasco-Zumeta & Gerd-Michael Heinze Feathers of great tit (Parus major) Great tit videos, photos & sounds on the Internet Bird Collection Titbox and the jay - video great tit Birds of Eurasia great tit great tit