Evidence of non‑random mating in a colour polymorphic raptor, the Booted Eagle

Abstract

Sexual selection and non-random mating are considered, among others, determinant mechanisms for the maintenance of genetic colour polymorphism in some bird species. We analyse the mechanisms, which, in parallel with Mendelian inheritance, may be acting in the maintenance and evolution of the morph ratio in a two-morph raptor species, using observational data of successful breeding individuals and their offspring from long-term studies conducted in three Spanish populations. Our results showed that the dark offspring produced in breeding events involving mixed-morph adult pairs far exceeds the expected value under the Hardy–Weinberg equilibrium, especially in the case of pairs formed by a light male and a dark female. In addition, the low number of dark eaglets born from pairs formed by light individuals (indistinctly homozygous or heterozygous) indicates that the number of breeding events of heterozygous (both the male and female) light morph pairs, was much lower than expected. As the plausible existence of a transmission ratio distortion phenomenon in heterozygous light morph males does not, alone, explain the disproportionate number of dark eaglets observed, our results suggest that one or two selective mating phenomena may be occurring in this polymorphic system. The first one could be a disassortative mating process whereby heterozygous light males preferentially mate with dark females, based on the imprint of the colour morph of their mother. The second phenomenon would only affect light morph individuals, which would preferentially mate with heterozygous individuals of the opposite sex, selected according to secondary sexual characteristics or behavioural traits that are unknown at the moment