Assortative Mating in Urban-breeding Merlins

We examined assortative mating behavior in 149 pairs of Merlins (Falco columbarius) breeding in Saskatoon, Saskatchewan, Canada from 1985 through 1990. Yearlings paired with yearlings and adults (2-plus years old) paired with adults more frequently than expected. Among adult pairs, however, there was no significant assortment by age. The productivity of pairs which included a yearling member was significantly lower than that of pairs with two adult birds. Similarly, pairs with two adults hatched their young on average nine days earlier than pairs with at least one yearling. There was no significant assortment of pairs based on body size with the exception of tail length. Along with differences in the timing of breeding activities, one other possible proximate mechanism promoting the distinction between adults and yearlings when choosing a mate was the difference in plumage between adult and yearling males.