Stable‐hydrogen isotope measures of natal dispersal reflect observed population declines in a threatened migratory songbird

AIM: Measuring dispersal is crucial for estimating demographic rates that inform conservation plans for rare and threatened species. We evaluated natal dispersal patterns in Bicknell's thrush (Catharus bicknelli) across most of the breeding range using a 10‐year data set of stable‐hydrogen isotope ratios in feathers (δ²HF) grown on the natal area and sampled 1 year later at the first breeding site. LOCATION: North‐eastern United States and south‐eastern Canada. METHODS: We used δ²HFvalues 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. RESULTS: The δ²HFvalues 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 δ²HFsuggested 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 δ²HFindicated that natal dispersal probability declined by 30–38% from 1996 to 2005. MAIN CONCLUSIONS: Our findings suggest that estimating natal dispersal with δ²HFmeasurements 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.