A possible case of contemporary selection leading to a decrease in sexual plumage dimorphism in a grassland-breeding shorebird

Abstract

In sexually dimorphic species, males with more exaggerated plumage ornamentation generally have higher body condition, are preferred by females, and have higher reproductive output. In contrast to the majority of studies, we describe that less ornamented males of the monogamous and sexually dimorphic black-tailed godwit Limosa limosa were larger and heavier during late incubation than more ornamented males and were mated with females that laid larger eggs. In females, we detected no such correlations. If paler and larger males indeed have higher fitness, this suggests ongoing selection for a reduction in male ornamentation, leading to a decrease in plumage ornamentation. We found evidence for such a change since 1840: Male ornamentation in museum specimens became progressively less in all our measured plumage traits. One explanation for this could be that formerly sexually selected plumage traits are now selected against and that the sexual plumage dimorphism of WestEuropean breeding black-tailed godwits might be waning. Concomitant with this change in plumage coloration, this species has experienced enormous human-mediated changes in both breeding and nonbreeding habitat, and we suggest that these might have led to increased costs or reduced fitness benefits for highly ornamented male godwits. Key words: contemporary evolution, egg size, Limosa limosa limosa, meadow bird, plumage coloration, sexual selection. [Behav Ecol]