a larger white cheep patch, a grey tail, wings, back and nape. It is also slightly smaller, with a smaller bill but longer tail. The situation is similar for the two related subspecies in the Turkestan tit group. P. m. turkestanicus is like P. m. bokharensis but with a larger bill and darker upperparts. P. m. ferghanensis is like P. m. bokharensis but with a smaller bill, darker grey on the flanks and a more yellow wash on the juvenile birds. The colour of the male bird's breast has been shown to correlate with stronger sperm, and is one way that the male demonstrates his reproductive superiority to females. Higher levels of carotenoid increase the intensity of the yellow of the breast its colour, and also enable the sperm to better withstand the onslaught of free radicals. Carotenoids cannot be synthesized by the bird and have to be obtained from food, so a bright colour in a male demonstrates his ability to obtain good nutrition. However, the saturation of the yellow colour is also influenced by environmental factors, such as weather conditions. The width of the male's ventral stripe, which varies with individual, is selected for by females, with higher quality females apparently selecting males with wider stripes. Voice The great tit is, like other tits, a vocal bird, and has up to 40 types of calls and songs. The calls are generally the same between the sexes, but the male is much more vocal and the female rarely calls. Soft single notes such as "pit", "spick", or "chit" are used as contact calls. A loud "tink" is used by adult males as an alarm or in territorial disputes. One of the most familiar is a "teacher, teacher", often likened to a squeaky wheelbarrow wheel, which is used in proclaiming ownership of a territory. In former times, English folk considered the "saw-sharpening" call to be a foretelling of rain. There is little geographic variation in calls, but tits from the two south Asian groups recently split from the great tit do not recognise or react to the calls of the temperate great tits. One explanation for the great tit's wide repertoire is the Beau Geste hypothesis. The eponymous hero of the novel propped dead soldiers against the battlements to give the impression that his fort was better defended than was really the case. Similarly, the multiplicity of calls gives the