The natal dispersal of tree swallows in a continuous mainland environment

1. To minimize study-area artefacts in a study of natal dispersal distances in tree swallows, we intensively banded nestlings and adults in nest-boxes in upstate New York and recruited and trained 70 volunteer banders within a 400 km radius of Ithaca. We banded 26 567 nestlings in the years 1985 through 1998, and captured 4774 adults at the nest, 630 of which had been banded as nestlings and were recaptured the year after fledging. 2. To correct for spatial variation in capture intensity, we resampled the distribution of all boxes where an adult capture was made under uniform, exponential and Cauchy null hypotheses. Compared to the null distributions, the frequency of observed dispersal events was significantly higher at 0-10 km and lower for all larger dispersal distances. The Cauchy distribution came closest to approximating the observed dispersal distance distribution. 3. Dispersal distances were sensitive to the distribution of nesting sites, and all measurements of dispersal distance distributions must be seen as being habitat-specific. We could detect no effect of dispersal distance on the subsequent timing of breeding and no effect of the timing of fledging on dispersal distance. The sexual differences in raw mean dispersal distances (8.38 km for females, 2.44 km for males) are similar to those reported for other species. 4. While it is tempting to conclude that studies in smaller areas have not missed a great deal, even a study area of 10 km extent (substantially larger than most) would have missed 11% of the dispersing birds detected and 95% of the range of distances recorded. Despite this sizeable component that would have been missed with a smaller study area, the relatively low frequencies of long-distance dispersal overall reinforce the conclusion that tree swallows, and probably most other migratory passerines, generally disperse much less far from their natal sites than the distances of their annual migrations might lead one to expect.