Space use, habitat preferences, and time -activity budgets on non-breeding Dunlin (Calidris alpina pacifica) in the Fraser River delta, British Columbia.

摘要

I investigated aspects of the ecology of non-breeding Dunlin ( Calidris alpina pacifica) in the Fraser River Delta, B. C., the most northerly site supporting a sizeable population in winter (approximately 40,000 birds). I used radio telemetry, direct observation, and Geographical Information Systems to document site fidelity, space use patterns, habitat, preferences, and time-activity budgets of individual non-breeding Dunlin throughout the 24-hour day and twice-daily tidal cycles. Site fidelity and habitat preferences were examined at both regional and local scales. Space use was quantified by estimating home range and core area sizes, and by examining core area placement, macro-habitat choices (marine versus terrestrial), and movement patterns within the home range. By following individuals through time and calculating within-bird means by tide stage, macro-habitat, and time of day, I minimized sampling biases and produced a detailed picture of the individuals' behaviour. Dunlin were trapped in three areas within the Delta during two non-breeding seasons (1995--96 and 1998), and categorized by sex, and, where possible, by age. I used a maximum likelihood mixture model to assign sex based on culmen length. Dunlin were site faithful, both regionally (to the Fraser Delta) and locally (within the Delta). I used compositional analysis to show that Dunlin chose habitats non-randomly at both regional and local scales, and there were differences among sex and site categories. Marine habitats were ranked highest. I assessed marine invertebrate prey densities (large and small annelids, crustaceans, and molluscs) for intertidal micro-habitats throughout the Delta, to examine their relationship with space use by Dunlin. Across sites, marine home range size decreased as prey density within the home range increased, with prey density accounting for 63% of the variance in home range size. Within a single site, both marine home range and core area size decreased as prey density increased, with prey density explaining 89% of the variance in home range size and 80% of the variance in core area size. Dunlin marine core areas contained higher densities of crustaceans and small annelids than did the rest of the home ranges. Most Dunlin also used a range of terrestrial habitats, particularly at night. Soil based agricultural crops were preferred at a regional scale, and pasture was the only agricultural crop that was highly ranked and significantly preferred at both regional and local scales. Dunlin spent on average at least 15.7 hours per 24-hour day foraging (depending on season), and at least another 3 hours per day flying (measured in spring), leaving on average at most 5.3 hours per day for activities such as roosting, preening, vigilance, and aggression. The percentage of time that Dunlin spent feeding did not differ between day and night, nor between marine and terrestrial macro-habitats. Dunlin spent on average at least 7.1 hours foraging at night, of which at least 2.9 occurred in terrestrial habitats, although the relative use of marine and terrestrial habitats varied considerably among individuals. Females spent less time foraging than males, but there was no difference between age classes. Finally, I compared the sex ratios and within-sex body sizes of Dunlin wintering in the Fraser River Delta with those wintering in central California, where ecological variables might favour smaller birds. Both populations were similarly male-biased, even though female Dunlin are larger than males, and I did not find significant within-sex size differences between latitudes.