CAUSES AND CONSEQUENCES OF TREE SWALLOW (TACHYCINETA BICOLOR) DISPERSAL IN SASKATCHEWAN

Poor breeding success often increases dispersal, but there is conflicting evidence about the putative adaptive benefits of dispersal behavior. Thus, observational and experimental data were analyzed to test whether breeding success (number of young fledging) influenced breeding and natal dispersal by Tree Swallows (Tachycineta bicolor), using information from 1,302 nests over 12 years. Success was experimentally altered at 291 randomly selected nest boxes by adding or removing three eggs (representing ±50% of modal clutch size). Finally, we tested whether dispersal distance, controlling for antecedent breeding success, had consequences for subsequent breeding success. Contrary to predictions, (1) adult and natal dispersal distances were not significantly influenced by breeding success, and (2) manipulating success had no effect on either breeding or natal dispersal. Moreover, after controlling for antecedent breeding success, dispersal distance had no influence on subsequent breeding success. In most years of our study, >85% of nest initiations produced at least one fledgling, and individual boxes did not consistently have low or high fledgling production. Hence, swallows may have little to gain by dispersing out of our nest-box grid, and box-specific success did not provide a reliable cue about where to disperse within the grid. Thus, remaining in the grid for subsequent breeding attempts, regardless of fledgling production, could be interpreted as adaptive. Nonetheless, most adults remained close to the nest they had used in previous years, which suggests that some unmeasured influence, such as social interactions among neighbors, led to nonrandom dispersal.