Mate-choice criteria of Ipswich sparrows: the importance of variability

Mate choice of female Ipswich sparrows, Passerculus sandwichensis princeps, was investigated with respect to a measure of territory quality (size) and a measure of male quality (singing vigour). These two measures were not correlated. Females used both territory size and male song rate to choose mates, but the importance of each trait differed between the 2 study years. In each year, the trait that varied more among males was more important in mate choice. Thus, it appears that mate choice criteria must not only be reliable but also sufficiently variable, although it is not known whether variability is important because it contributes to reliability or because more variable traits are more easily assessed.