The effect of early life conditions on song traits in male dippers (Cinclus cinclus)

Abstract

Song complexity and singing frequency in male birds are shaped by female choice; they signal male quality because song is costly to develop and produce. The timing of song learning and the development of the brain structures involved occur during a period when chicks are exposed to a number of potential stressors. The quality and quantity of song produced by adults may therefore reflect the level of stress experienced during early life, a theory known as the 'developmental stress hypothesis'. We tested this hypothesis using song recordings and life-history data from an individually marked, long-term study population of wild dippers (Cinclus cinclus). The extent to which early life conditions predict adult song traits was investigated using natal brood size as a measure of sibling competition; the rate of provisioning by parents as a proxy for nutritional stress; and residuals of the linear regression between body mass and tarsus length as a measure of nestling condition. The syllable diversity in the songs of adult males was positively correlated with their body condition as nestlings, but there was no significant correlation with either provisioning rate or brood size. Provisioning rate did, however, predict song rate; males in relatively poor condition as nestlings or those raised in smaller broods which were fed more frequently by their parents sang at a higher rate in adulthood. These results support the developmental stress hypothesis and provide some of the first evidence from a wild bird of how the conditions experienced during early life impact adult song. Song traits may therefore provide females with information regarding both the current condition and developmental history of males.

Fig 1. The relationship between nestling body condition and syllable diversity in adult song.

The line shows the predicted relationship from averaging of the best fitting general linear models; the raw data are plotted as points. The 95% confidence interval is shown in grey.

Fig 2. The relationship between the rate at which nestlings were provisioned and their song rate as adults.

The lines show the predicted relationships from averaging of the best fitting generalised linear models, plotted for ‘solo’ breeding stage, with the 95% confidence intervals shown in grey. (a) The dashed line shows the relationship for birds raised in small broods (< 4.39 nestlings, raw data plotted as circles) and the solid line shows the same relationship for nestlings in large broods (> 4.39 nestlings, raw data plotted as triangles), both plotted for mean nestling body condition. (b) The solid line shows the relationship for nestlings in good condition on day 9 (mean condition + 1 SD) and the dashed line shows the same relationship for nestlings in poor condition on day 9 (mean condition– 1 SD), both plotted for large broods (>4.39 nestlings); the raw data are plotted as points.