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    Population assessment of burrowing owls nesting on black-tailed prairie dog colonies in Colorado
    (Colorado State University. Libraries, 2024) Albright, Sarah, author; Kendall, William, advisor; Conrey, Reesa, committee member; Pejchar, Liba, committee member; Boone, Randall, committee member
    In North America, grassland birds have experienced steeper and more widespread declines than any other avian guild due to habitat loss resulting from grassland conversion to cropland, increasing urban and energy development, and climate change (Knopf 1994, Askins et al. 2007). The historical area of native grasslands has decreased by 62% since the 1800s and contributed to the loss of nearly 40% of grassland bird populations since 1966 (Wilsey et al. 2019). Heterogeneity in climate, grazing, and fire across the landscape have resulted in the existence of different grassland types that vary in structure and composition. The shortgrass prairie is the driest and warmest of the Great Plains grasslands and is dominated by low-growing perennial grasses, forbs, and shrubs. The shortgrass prairie provides vital nesting and foraging habitat for many grassland birds. In Colorado, approximately 50% of the historic shortgrass prairie has been converted to other land uses (Neely et al. 2006). The partial loss of shortgrass prairie habitat has ecological consequences including loss of native vegetation and decreases in populations of grassland species, including grassland birds. Black-tailed prairie dogs (Cynomys ludovicianus) are important drivers of ecosystem function in the shortgrass prairie because their colonial social structure, burrowing and foraging behaviors alter the landscape and provide areas of shorter vegetation and burrow systems that support increased biodiversity of animals and plants (Cully et al. 2010). Black-tailed prairie dogs function as a keystone species in shortgrass prairie ecosystems and create important breeding and foraging habitat for grassland birds including western burrowing owls (Athene cunicularia hypugaea: Smith and Lomolino 2004). The western burrowing owl is a small diurnal raptor that lives in grasslands, deserts, and other open habitats. It is a partially migratory species where populations in the southern parts of its range in the southwestern United States, Mexico, and portions of Central and South America are typically year-round residents. Migratory populations occur in the grasslands of North America, arriving in early spring to start breeding as far north as Canada and departing in late August to return to their wintering grounds in the southwestern United States and Mexico (Poulin et al. 2011). Burrowing owls typically nest in burrows dug by rodents such as prairie dogs and ground squirrels. In eastern Colorado, burrowing owls almost exclusively nest on black-tailed prairie dog colonies. Benefits of nesting on prairie dog colonies include increased predator detection from alarm calls, decreased predation due to the dilution effect, and reduced vegetation height. Black-tailed prairie dog populations have experienced an estimated decline of 90-98% since 1900 due to sylvatic plague outbreaks and habitat loss and alteration by human development (Miller et al. 1994, Desmond et al. 2000). Since prairie dog colonies provide critical habitat for burrowing owls and other species, population decline contributes to decreased availability of burrowing owl nesting habitat. Conservation status of the burrowing owl varies across its range. It is a species of conservation concern in the western United States, threatened in Mexico, and endangered in Canada (Sheffield 1997). The western burrowing owl is currently listed as a state-threatened species in Colorado and is designated as a Tier 1 Species of Greatest Conservation Need in Colorado's State Wildlife Action Plan (Colorado Parks and Wildlife 2015). The last burrowing owl population assessment in Colorado was conducted in 2005 (Tipton et al. 2008, 2009) and since then, only local surveys limited in spatial and temporal extent have been conducted. This has prompted the need for an updated population assessment of burrowing owls nesting in eastern Colorado, where the majority of Colorado's burrowing owls breed on black-tailed prairie dog colonies. In this study, we provide an updated status assessment for burrowing owls on Colorado's eastern plains and seek to expand the current understanding of which black-tailed prairie dog colony attributes have the highest value for burrowing owl occupancy, density, and productivity. We specifically examined how colony size, activity status, and vegetation characteristics influence these population parameters on 175 survey plots throughout eastern Colorado. We surveyed some of the same plots using similar methodology as Tipton et al. (2008, 2009) in their 2005 study, facilitating comparisons of burrowing owl populations 17–18 years later. The first chapter describes the distribution of burrowing owls nesting on black-tailed prairie dog colonies in eastern Colorado and serves to examine which black-tailed prairie dog colony characteristics drive the use of a colony by burrowing owls and the probability of successful reproduction. The second chapter focuses on burrowing owl density, productivity, and abundance in eastern Colorado to determine how many burrowing owls are present on occupied colonies and how productive they are on colonies where they do reproduce. The value of describing these components of burrowing owl populations in separate chapters comes from estimating and identifying the drivers of burrowing owl occupancy in chapter 1, then shifting to the finer scale of density to determine if the drivers of burrowing owl distribution are also driving density, productivity, and abundance. We used a black-tailed prairie dog colony shapefile prepared by the Colorado Natural Heritage Program for Colorado Parks and Wildlife in 2020 as our sampling frame. This shapefile includes polygons that represent black-tailed prairie dog colonies with digitized boundaries, created using imagery collected in 2019 by the National Agriculture Imagery Program (NAIP). We used a spatially balanced sampling design to select potential plots and selected new samples for each survey year (2022 and 2023) to maximize sample size and spatial coverage of the large study region. From early May through early August, we conducted four surveys on 175 plots in eastern Colorado, counting all burrowing owls seen, with two visits occurring prior to juvenile emergence and two occurring after. We estimated occupancy using a static multistate occupancy estimation model with two states: 'occupied' and 'occupied with successful reproduction'. We estimated density and abundance using distance sampling methods. We estimated productivity using a zero-inflated beta generalized linear model. We used occupancy data from Tipton et al. (2008) and dynamic occupancy models to evaluate burrowing owl local colonization and local extinction between 2005 and 2022 - 2023. Our analyses indicate that burrowing owl occupancy and density are highest in southern Colorado and lowest in northern Colorado. Colonies with higher prairie dog activity level had higher probability of reproduction and higher densities of adult burrowing owls. Vegetation height was the main driver of juvenile density such that colonies with taller vegetation supported lower densities of juvenile owls. We estimated burrowing owl occupancy to be 0.84 (95% CI [0.62, 0.95]) and probability of successful reproduction on occupied plots to be 0.86 (95% CI [0.70, 0.94]). We estimated an average density of 3.47 (95% CI [2.79, 4.15]) adult owls/km2 prior to juvenile emergence, 8.20 (95% CI [6.39, 10.00]) adult owls/km2 after juvenile emergence, and 18 juveniles/km2 (95% CI [13.86-23.66]). We expanded our density estimates to our sampling frame and estimated that there were 4,913 (95% CI [3,948-5,875]) adult owls prior to juvenile emergence, 11,613 (95% CI [5,333-17,893]) adult owls after juvenile emergence, and 26,580 (95% CI [19,623-33,537]) juvenile burrowing owls on black-tailed prairie dog colonies in eastern Colorado. We found that prairie dog activity had a positive effect on burrowing owl density, successful reproduction, and productivity regardless of prairie dog colony size. This indicates that burrowing owls are effectively utilizing and nesting on small prairie dog colonies in eastern Colorado, which could make them more resilient to breeding season habit loss, fragmentation, or degradation. In addition, we found that northern Colorado had lower burrowing owl occupancy and adult density, but had a similar probability of successful reproduction and juvenile density compared to south and central Colorado. If northern Colorado can sustain stable burrowing owl densities, burrowing owl populations may have enough successful reproduction to maintain stable populations. The covariates we investigated in this study did not adequately explain this spatial pattern. However, it is likely that differences in climate, prairie dog population dynamics, land use, or some other factor could cause differences in local habitat and breeding conditions across Colorado. The previous burrowing owl population assessment in eastern Colorado estimated burrowing owl occupancy to be 0.80 (95% CI [0.66-0.89]), density to be 3.04 adult owls/km2 (95% CI [2.15, 5.13]), and adult abundance to be 3,554 (95% CI [3,928-8,445]) owls in eastern Colorado. This suggests that overall, burrowing owl populations in eastern Colorado are relatively stable and are likely to remain stable if efforts continue to preserve the prairie dog colonies that are vital for burrowing owls during the breeding season. The burrowing owl is a state-threatened species in Colorado at the time of this thesis and thus we recommend future burrowing owl surveys to track population changes through time. Future monitoring efforts can help identify the drivers of burrowing owl population change and clarify the spatial patterns we found. These future efforts should occur more frequently than the ~17 year time period between this population assessment and the last assessment in 2005. We recommend conducting burrowing owl surveys every 5 years because it exceeds the time lag between black-tailed prairie dog colony local extinction and cessation of burrowing owl nesting. In addition, a 5 year time interval coincides with the timing of the Colorado Natural Heritage Program's black-tailed prairie dog mapping efforts in eastern Colorado, from which we constructed our sampling frame. Using updated mapping efforts is vital for monitoring efforts because it may decrease the probability that a plot selected from the sampling frame contains a prairie dog colony that has gone locally extinct. We recommend future efforts should select new plots to survey for burrowing owls in addition to resurveying a subset of the plots from this study. Revisiting sites from this study would be helpful in determining burrowing owl population trends through time, while selecting new plots can increase the spatial coverage of surveys. This 2-year study provides an updated status assessment of burrowing owl populations across the black-tailed prairie dog range in Colorado that will help calibrate burrowing owl population models incorporating prairie dog colony extent, inform future monitoring plans, and help guide conservation of keystone species and their communities.