Credit: Johan Träff/S. R. Evans et al./Nature Ecol. Evol.

Warming of the spring breeding season may have reversed the direction of natural selection in a bird species.

Male collared flycatchers (Ficedula albicollis; pictured) have a conspicuous white patch on their foreheads and females tend to mate with those with larger patches. Simon Evans and Lars Gustafsson at Uppsala University in Sweden measured the forehead patches of males breeding in nest boxes on Gotland, an island in the Baltic Sea, from 1981 to 2014.

They found that patch size declined notably during the 34-year study. Statistical models showed that selection switched from favouring larger patches to smaller ones during the 1990s. This reversal coincided with a 1.5 °C rise in average temperatures during the spring breeding season over the course of the study.

The males may be trading off their ornamentation against other traits that improve survival in a changing climate, the authors suggest.

Nature Ecol. Evol. 1, 0039 (2017)