Sexual selection predicts advancement of avian spring migration in response to climate change

Abstract

Global warming has led to earlier spring arrival of migratory birds, but the extent of this advancement varies greatly among species, and it remains uncertain to what degree these changes are phenotypically plastic responses or microevolutionary adaptations to changing environmental conditions. We suggest that sexual selection could help to understand this variation, since early spring arrival of males is favoured by female choice. Climate change could weaken the strength of natural selection opposing sexual selection for early migration, which would predict greatest advancement in species with stronger female choice. We test this hypothesis comparatively by investigating the degree of long-term change in spring passage at two ringing stations in northern Europe in relation to a synthetic estimate of the strength of female choice, composed of degree of extra-pair paternity, relative testes size and degree of sexually dichromatic plumage colouration. We found that species with a stronger index of sexual selection have indeed advanced their date of spring passage to a greater extent. This relationship was stronger for the changes in the median passage date of the whole population than for changes in the timing of first-arriving individuals, suggesting that selection has not only acted on protandrous males. These results suggest that sexual selection may have an impact on the responses of organisms to climate change, and knowledge of a species' mating system might help to inform attempts at predicting these.

Figure 1

The annual change (measured in days per year) in the date at which 50% of the annual passage of migratory birds had been trapped on Christiansø and Heligoland in relation to their degree of sexual selection, with independent data points as (a) species and (b) phylogenetically independent contrasts. Data point labels on (a) refer to the following species: 1, Acrocephalus palustris (Christiansø only); 2, Acrocephalus schoenobaenus (Heligoland only); 3, Acrocephalus scirpaceus; 4, Emberiza schoeniclus (Christiansø only); 5, Ficedula hypoleuca; 6, Phylloscopus sibilatrix (Christiansø only); 7, Phylloscopus trochilus; 8, Prunella modularis and 9, Turdus merula. In (b), the x-axis is shown as a dotted line to assist interpretation of the contrasts.

Figure 2

The annual change (measured in days per year) in the date at which 5% of the annual passage of migratory birds had been trapped on Christiansø and Heligoland in relation to their degree of sexual selection, with independent data points as (a) species and (b) phylogenetically independent contrasts. Data point labels are as for figure 1a. In (b), the x-axis is shown as a dotted line to assist interpretation of the contrasts.