| Abstract |
- Dispersal is integral to our understanding of the life history and population biology of many vertebrates, but difficulties in detecting long distance movements have complicated its study. Moreover, studies of factors affecting dispersal are often unable to determine the relative contributions of variables such as nesting success, mate fidelity, and nest site fidelity. I examined the effects of nest depredation on dispersal in comparison to successful nests and nests that failed for other reasons. Additionally, I investigated a suite of biological factors affecting within-season and between-season breeding dispersal by burrowing owls (Athene cunicularia) in California, attempting to partition the effects of these covariates and to deal with long distance detectibility issues. For both types of dispersal, I divided dispersal into two components; dispersal probability and dispersal distance. I used experimental and observational approaches to investigate within-season dispersal in two contrasting environments; a large grassland and an agricultural landscape. I found that the factors affecting dispersal probability and dispersal distance were different, supporting my decision to examine each separately. Of the factors investigated, dispersal probability was influenced most by study area, mate fidelity, and nesting success. The proportion of individuals dispersing tended to be greater for owls that lost their mate due to death or dispersal (60%, 6 of 10) and owls whose nests were depredated (50%, 10 of 20) than for owls that did not lose their mates (33%, 6 of 18) and owls whose nests were successful (17%, 1 of 6), respectively. The results from an experiment where we removed eggs from pairs of owls to simulate nest depredation were consistent with the observational results, suggesting that owls whose nests were depredated may have been more likely to disperse than control owls. The reactions of owls from depredated nests, however, did not appear to differ from those whose nests failed for other reasons. In contrast, owl dispersal distance was most affected by owl gender, and to a lesser degree by study area and nesting success. Dispersal distance was greater for female owls (median = 1575 m, n = 13) than male owls (median = 417 m, n = 11), greater for owls from the grassland area (median= 939 m, n = 9) compared to owls from the agricultural area (median = 829 m, n = 15), and greater for owls whose nests had failed (median = 1018 m, n = 17) than for owls that successfully bred (median 475 m, n = 7). Nest depredation, however, did not appear to increase dispersal distance. The geometric models performed poorly at approximating within-season dispersal distance, indicating that many owls disperse farther than predicted by a "first is best" model. I speculate that the distribution of within-season dispersal distances by burrowing owls is related to the densities of suitable territories and mates, which are more variable than predicted by a geometric model within a breeding season. I used data from band resightings and nesting success (1998-2003) to examine factors related to between-season breeding dispersal by burrowing owls in an agricultural environment. Of the factors investigated, nesting success appeared to have the greatest effect on burrowing owl dispersal. The proportion of individuals dispersing was greater for owls whose nests had failed (68%, 28 of 41) than owls whose nests were successful (27%, 58 of 212). Similarly, dispersal distance was greater for owls whose nest failed (mean = 745 ± 175 m, n = 28) than owls with successful nests (mean = 340 ± 36 m,n = 58). The owls exhibited high rates of nest site and mate fidelity between breeding seasons. There was evidence that previous experience at a breeding site may have reduced dispersal probability and that unpaired owls may have been more likely to disperse and dispersed slightly greater distances than those that retained their mates. Nesting success, however, appeared to be the major factor contributing to burrowing owl breeding dispersal after controlling for nest site and mate fidelity, particularly for male owls. Despite the complexity of the dispersal process, a geometric model provided a reasonably good fit to the distribution of between-season breeding dispersal distances at relatively short distances, but failed to predict a small percentage of long distance dispersals. Geometric models appeared to be a better fit for the distribution of between-season breeding dispersal distances than within-season breeding dispersal distances. Factors affecting within-season dispersal were generally similar to those affecting between-season dispersal. Both within-season and between-season breeding dispersal were affected by nesting success and mate fidelity, but the effects of these factors differed between the two types of breeding dispersal, suggesting that time constraints and competition play a larger role in within-season dispersal than between season dispersal. In addition, both studies supported a difference in dispersal behavior, in which the factors that affected dispersal probability were distinct from those that affected dispersal distance. These results help determine the relative contributions of nesting success, mate fidelity, and nest site fidelity to avian dispersal, offer some evidence that the effects of nest depredation are not distinct from the effects of nest failure in general, and provide further support for the division of dispersal into dispersal probability and dispersal distance.
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