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The western burrowing owl (Athene cunicularia hypugea) has been declining throughout much of its range, and in some areas, quite dramatically. In the Great Plains region, these declines appear to be closely associated with declines in prairie dog numbers. Most research to date has taken place on relatively small, fragmented prairie dog colonies. The objectives of this study were to analyze burrowing owl reproductive performance (i.e., clutch size, brood size, and number fledged), and nestling body condition in relation to nest level and colony level factors in Buffalo Gap National Grassland, South Dakota. Buffalo Gap contains large prairie dog complexes and unbroken expanses of shortgrass prairie. Nest level variables included: pair arrival date on the breeding grounds, clutch initiation date, burrow length to nest, distance to the colony edge, the number of nests within 250 m of a particular nest, and nearest neighbor distance. Colony level variables measured were: colony size, number of nests, active burrows/ha, inactive burrows/ha, total burrows/ha, % active burrows, prairie dog density/ha, mean pair arrival date, mean clutch initiation date, mean burrow length to nest, mean distance to edge, mean number of nests within 250 m of a nest, mean nearest neighbor distance, and pair density. Categorical data that were analyzed consisted of burrow type, nest fate, burrow re-use the second year, egg displacement away from the clutch, and female nesting behavior.
Burrowing owl reproductive performance was affected by the same factors at the nest and colony level. At the nest level, those pairs that arrived early, initiated clutches sooner, and nested at greater distances from nearest neighbors had larger clutches, broods, and fledged more young. At the prairie dog colony scale, those colonies that had enough desirable habitat to allow for greater mean spacing of nests, resulted in early arriving pairs selecting these colonies and having greater reproductive success.
Successful nests had greater nearest neighbor distances and earlier clutch initiation dates than unsuccessful nests. Re-used burrows fledged more young and were characterized by early arriving pairs that initiated clutches early. Female burrowing owls were more aggressive in burrows that had their nests located closer to the entrance compared to nests located farther away. Late arriving pairs were more likely to have an egg displaced away from the clutch than early arriving pairs.
Body condition of nestling burrowing owls was negatively related to stress levels for both years. In 1999, which had a wet spring, body condition was negatively related to brood size and distance from nest to colony edge. Weather may influence nestling body condition since there was no relation with brood size during 2000, which was normal in terms of temperature and precipitation. In 2000, nestlings of early arriving pairs were in better body condition than those that arrived later.