The Use of a Breeding Synchrony Index

People who study mating systems or behavioural aspects of sperm competition in birds are aware of the importance of breeding synchrony in a population. Birkhead and Moller (1992) summarize some ideas on how breeding synchrony, by affecting the operational sex ratio, may have an influence on for example copulation behaviour and mate guarding. In general, the idea is that the more synchronous the population is, the fewer options males have to attract extra mates and become polygynous (see also Oring 1982) or to engage in extra-pair copulations. If we want to understand the effect of breeding synchrony when comparing behaviour between both species and populations of the same species, we need a measure of synchrony for the population under study. Many researchers have these necessary data, but they are rarely presented in their papers. Bj6rklund and Westman (1986) developed a breeding synchrony index, which takes into account differences in population size and duration of the fertile period. However, their index suffers from a minor error in the formula. By presenting here the correct index formula, I hope that people will be encouraged to use it, so that more data on breeding synchrony become available and that these data can be more easily compared. The correct formula for the synchrony index (SI) is: