Nonrandom pairing by male barn owls (Tyto alba) with respect to a female plumage trait

In socially monogamous species it is rare for females to be more intensely colored than males. The barn owl (Tyto alba) is one of the exceptions, as females usually exhibit more and larger black spots on the plumage. The evolution of sexual dimorphism in plumage traits is commonly assumed to be the result of sexual selection. I therefore examined the prediction that male barn owls do not pair randomly with respect to female plumage spottiness during a 5-year study in Switzerland. The prediction was supported, as males that changed mates acquired a new female that was similarly spotted to the previous one, and pairing with respect to plumage spottiness was positively assortative. Significant repeatability in male pairing was presumably neither the consequence of sharing the same habitats with females displaying a given plumage spottiness nor of morphological characteristics of the males that could influence mate sampling. A resemblance in plumage spottiness between the mates of sons and of their father suggests that repeatability could have resulted from sexual imprinting and/or heritable variance in male preference for spotted females. To test whether males assess female plumage spottiness, I either cut off black spots or small pieces of feathers but not the spots of already mated females. Males mated to females with reduced plumage spottiness fed their brood at a lower cadency and achieved a lower reproductive success than other males. This experiment further suggests that female plumage spottiness is a stimulus for males.