Fertilization in birds

The 9672 known species of birds (Sibley 1994) are widely distributed over every global habitat and, as might therefore be expected, display great diversity in their morphology, physiology and behaviour. This diversity is perhaps best manifested in avian breeding systems. In terms of mating behaviour, some species breed in small polygynous groups, others are relatively solitary, meeting only to mate in a ‘lek’, whilst more than 90% of avian species form breeding pairs, either in isolation or in colonies (Lack 1968). Notwithstanding, ‘cuckoldry’ is a normal feature of the mating behaviour of many of these apparently ‘monogamous’ pairs (Birkhead and Møller 1992a).