Inheritance and variation in eggshell patterning in the great tit Parus major

The inheritance of patterns on avian eggshells is central to understanding the evolution of traits such as egg mimicry (e.g. in cuckoos). Yet little is known about the inheritance, or indeed function, of eggshell patterns. It has long been believed that the evolution of eggshell pattern mimicry required that patterns
be determined by genes situated on the female-specific W chromosome. However, it has never been demonstrated for any bird that egg pattern traits (rather than ground colour) are female sex linked, or indeed that they are inherited. We studied the inheritance of three measures of egg-pigment patterns in a wild great tit population. Egg patterns were female specific but unrelated to female attributes such as age or condition and showed only weak environmental effects. Eggs of daughters resembled those of both their mothers and maternal grandmothers, but not of their paternal grandmothers. We conclude that this is the first demonstration of female sex-linked inheritance of avian eggshell patterning, so raising the probability that such a system operates in egg mimics and their hosts.