Scale-of-choice effect in the assortative mating by multiple ornamental and non-ornamental characters in the black-headed gull

Abstract

Assortative mating is a tendency to mate with phenotypically similar individuals as a consequence of either selection acting on mate preferences or individual constraints resulting from temporal and spatial segregation. The aim of this study was to examine the patterns of assortative mating in a plumage-monomorphic bird species, the black-headed gull Chroicocephalus ridibundus. For this purpose, we captured 217 pairs of gulls breeding in two colonies in northern Poland over six years and assessed whether they mated assortatively based on morphology, condition, individual genetic diversity, and expression of ornaments. An analysis of raw data (colonies and years combined) provided support for assortative mating by specific structural characters (head and wing morphology) and condition (size-corrected body mass). However, these relationships lost significance after removing inter-colony and inter-annual variation in the traits, suggesting that they were primarily driven by spatial and temporal segregation of individuals. We also found support for assortative mating by the size, but not coloration, of two melanin-based plumage ornaments (hood and wingtip). These correlations were retained after removing variation between colonies and years, suggesting that they could be due to directional mate preferences. No evidence was found for assortative mating by physiological measures of condition (blood hemoglobin concentration and plasma metabolite concentrations) and heterozygosity of neutrally selected microsatellite loci. Our study provides rare evidence for the scale-of-choice effect in assortative mating, where different variants of a given trait are distributed non-randomly across spatial or temporal scales and pooling them into a single heterogeneous sample may produce false-positive results. Animals often choose their reproductive partners non-randomly, preferring to mate with individuals that are similar by certain phenotypic characters, which results in an assortative pattern of mating. However, similar patterns can occur as a result of non-random distribution of individuals across spatial and temporal scales. Here, we examined the patterns of assortative mating in a colonial waterbird, the black-headed gull Chroicocephalus ridibundus. We found that assortative mating by morphological characters and condition was apparent only in a heterogeneous sample, where data from different colonies and years were pooled together. In contrast, assortative mating by the size of melanin-based plumage ornaments (hood and wingtip) was retained after removing inter-colony and inter-annual variation in these traits. Our study provides a rare example of the scale-of-choice effect in assortative mating of birds.