The influence of diet, habitat, and recreational shooting of prairie dogs on burrowing owl demography
Date
2002
Authors
Woodard, Jason Daniel, author
Van Horne, Beatrice, advisor
Wiens, John A., committee member
Kennedy, Patricia (Patricia L.), committee member
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Abstract
The burrowing owl is a ground-nesting raptor that is in decline across much of its geographic range. Habitat loss and widespread control of fossorial rodents on which burrowing owls regionally rely for nest sites are the primary contributors to owl decline. In addition to reducing breeding and foraging opportunities, habitat loss and eradication programs may result in suboptimal habitat conditions at remaining sites. Identifying the habitat characteristics preferred by burrowing owls and the prey important to successful reproduction are top priorities. Human activity may exacerbate declines in local breeding populations. Isolating and mitigating sources of disturbance to nesting owls is a critical step in achieving conservation goals. Black-tailed prairie dog colonies are frequently occupied by burrowing owls where ranges overlap. In northeastern Colorado, prairie dog colonies provide the majority of suitable nesting habitat. I studied a population of burrowing owls nesting on the Pawnee National Grassland, Weld County, Colorado, to determine how prey use and nest placement affect demography. Specifically, my objectives were to (1) describe local prey use, compare the owl's diet across its geographic range, and relate diet measures to reproduction; (2) identify the habitat features that drive nesting patterns, and evaluate the reproductive consequences of nest placement; and (3) identify the factors influencing recreational shooting of black- tailed prairie dogs, and relate burrowing owl breeding numbers and reproductive output to the presence and intensity of shooting activity. Owls foraged opportunistically for invertebrate prey, using readily available sources to supplement intake of more profitable vertebrate species. Vertebrate use was low and decreased throughout the breeding cycle. Consumption rates may have met minimum dietary levels necessary to initiate nesting, but may not have been sufficient to benefit breeding owls through increased biomass gains. Invertebrates comprised the majority of the diet, a theme common to most diet studies. Their ready abundance and availability provided owls with an attractive alternate food source, and may have buffered owls against the reproductive consequences of short-term food shortage. Nearest-neighbor distance and satellite burrow density were poor predictors of nest placement. Habitat-based models explained little variation in reproductive performance at the nest level. Annual variation was significant. Nesting pairs in 2000 had a higher probability of success and fledged more young per breeding attempt than did their 1999 counterparts. Increased rates of starvation and flooding may have contributed to lower reproduction in 1999. Prairie dog colony size moderately influenced breeding densities and colony reproductive performance. Large colonies supported lower breeding densities, were less successful, and fledged fewer young per breeding attempt than did small colonies. Small colonies may constitute superior habitat if breeding densities are indicative of site quality. High rates of landscape fragmentation and human disturbance may have reduced reproductive performance on large, public sites. Burrow availability did not limit breeding densities, but it may have provided a source of refuge and prey to nesting pairs. No effect of prairie dog activity on measured demographic parameters was evident. Land ownership primarily governed recreational shooting patterns. Shooters did not demonstrate strong preferences for specific colony characteristics, using the majority of available public sites. Colonies subject to recreational shooting activity supported more breeding owls, but nests had lower success rates and fledged fewer young than did nests on colonies not exposed to shooting activity. Reproduction did not exhibit a linear response to recreational shooting intensity suggesting other factors contributed to reproductive variation. Adult owl mortality from recreational shooting invariably resulted in nest failure. Although gunshot trauma was infrequent, the additive effects of breeder loss and reduced reproductive output remain a cause for concern. Increasing the number of small, expanding prairie dog colonies will provide prospective breeders with potentially productive sites. As burrowing owls occupy the majority of colonies on the Pawnee National Grassland, many sites are likely to support nesting pairs. Maintaining a sizable pool of breeders will help to minimize the effects of annual reproductive variability and episodic plague on population viability. Protecting important vertebrate and invertebrate prey sources and reducing shooting activity on colonies historically productive or preferred by burrowing owls are essential to effective management. Incorporating reproductive, survival, and recruitment data into monitoring efforts will provide managers with a clearer picture of overall breeding conditions.
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Subject
Burrowing owl -- Habitat -- Colorado -- Weld County
Prairie dogs -- Control -- Colorado -- Weld County