We used δ2HF values of adult thrushes sampled at 25 breeding sites as prior information for assigning first-time breeders to their natal site. We calculated the minimum distance birds moved from their natal to first breeding site and fit these data to three statistical distributions for characterizing the importance of long-distance dispersal: the exponential, Weibull and half-Cauchy. Finally, we assessed differences in the probability of dispersal across the breeding range and through time to understand spatio-temporal variation in demographic connectivity.
The δ2HF values of first-time breeders were lower compared with those of adults, a difference that was greater at the southern compared with northern breeding range extreme. Assignment tests accounting for age differences in δ2HF suggested that most birds dispersed < 200 km from their natal area and within the centre of the breeding range, whereas comparatively few individuals dispersed up to 700 km. A Weibull distribution provided the best fit to these data. Two of three corrections for age differences in δ2HF indicated that natal dispersal probability declined by 30–38% from 1996 to 2005.
Our findings suggest that estimating natal dispersal with δ2HF measurements may contribute to understanding the resilience of geographically isolated Bicknell's thrush populations. Declining natal dispersal may be symptomatic of observed population declines and could compound this trend by limiting demographic exchange between habitat patches predicted to be increasingly isolated by natural and anthropogenic habitat changes.
Colin E. Studds, Kent P. McFarland, Yves Aubry, Christopher C. Rimmer, Keith A. Hobson, Peter P. Marra and Leonard I. Wassenaar Stable-hydrogen isotope measures of natal dispersal reflect observed population declines in a threatened migratory songbird Diversity and DistributionsArticle first published online: 9 JUN 2012 | DOI: 10.1111/j.1472-4642.2012.00931.x