Distribution of arrival dates in a migratory bird in relation to environmental conditions, natural selection and sexual selection

Frequency distributions of arrival date in migratory birds will determine the level of competition for mates and other limiting resources, but the factors associated with the shape of such distributions have never previously been analysed. Using a long-term data set from 1971-2003 on arrival date in a Danish population of barn swallows Hirundo rustica, I analysed the determinants of the shape of the frequency distributions of arrival date. Mean, coefficient of variation, skewness and kurtosis of arrival date all changed during the course of the study, more in males than in females. The shape of the frequency distributions of arrival date was related to environmental conditions during spring migration as reflected by the Normalized Difference Vegetation Index in Algeria in March-May, which decreased significantly during the study. During years with adverse environmental conditions in Algeria a larger proportion of barn swallows died, thereby affecting the frequency distribution of arrival dates. The shape of the frequency distributions of arrival date was also related to environmental conditions in the breeding area as reflected by mean temperature in April, which increased significantly during the study. The descriptors of the shape of the frequency distributions of arrival date changed in response to a micro-evolutionary change in the size of a secondary sexual character; the tail length of males. These findings suggest that the shapes of frequency distributions of arrival date are affected by both environmental and micro-evolutionary responses to selection.