Population ecology and habitat requirements of Burrowing Owls in eastern Washington
Burrowing Owl populations have declined in many portions of their breeding range and a candidate species for state listing in Washington. Availability of nesting burrows is one factor thought to limit populations, but birds often disappear from or fail to occur in what appears to be suitable habitat with burrows available. Effective management and conservation of Burrowing Owl populations requires better understanding of the proximate and ultimate factors that influence burrow occupancy, reproductive success, natal recruitment, and annual return rates. We have been working on Burrowing Owls in eastern Washington for the past 4 years. The objectives of this project are to examine proximate and ultimate factors that influence burrow occupancy, nesting productivity, burrow fidelity, natal recruitment, and annual survival of Burrowing Owls in eastern Washington. We are using standardized roadside call-broadcast surveys (Conway and Simon 2003) to locate nests within two large study areas in eastern Washington. One study area is in southern Washington encompassing the towns of Kennewick and Pasco, and the other study area is in central Washington encompassing the towns of Moses Lake and Othello). We are visiting nests weekly to document stage of the nesting cycle and to document behaviors that might offer cues to status of the nesting attempt. We use an infrared probe to look into the nest chamber on alternate visits to each nest so that we can count eggs or nestlings. We also measure a suite of vegetation and landscape features surrounding each nest to determine which environmental features correlate with important fitness parameters. We monitor approximately 200 nests each year and we try to put unique color bands on all adults and juveniles at each nest. We have put radio transmitters on approximately 180 juvenile owls over the past 4 years to determine causes of juvenile mortality at our Moses Lake study site. We also conduct winter surveys to estimate the proportion of owls that overwinter. Owls that overwinter on the study site initiate nests earlier and have higher reproductive success.One of my graduate students (Matthew Smith) is finishing up his thesis research on the Pasco study site examining the adaptive function of a very odd behavior used by nesting Burrowing Owls. Burrowing Owls often bring horse or cow manure to their nest burrow, shred the manure into fine pieces, and then spread the shredded manure in and around the burrow entrance, tunnel, and nest chamber. This strange behavior was thought to have evolved as a adaptation to reduce nest depredation via olfactory concealment of the eggs or nestlings. However, Matt’s experiments show support for another hypothesis whereby owls appear to be using manure to attract insects (a principle prey item) to their nest burrows. Another of my graduate students (Vicki Garcia) is finishing up her thesis research on the Moses Lake study site examining the factors that influence age of natal dispersal in Burrowing Owls. We have noticed large variation among nests in the age at which the juveniles disperse from their natal burrow. Vicki has conducted a series of manipulative experiments to evaluate the influence of food abundance and ectoparasite load on age of natal dispersal. Her results suggest that high food abundance in the area surrounding the nest burrow causes juvenile owls to delay natal dispersal whereas high parasite load (a cost of being reared in a burrow 3m underground) causes juveniles to dispersal at a younger age.
- Smith, M. D., and C. J. Conway. 2007. Use of mammal manure by nesting burrowing owls: A test of four functional hypotheses. Animal Behaviour 73:in press.
- Conway, C. J., V. Garcia, M. D. Smith, L. A. Ellis, and J. Whitney. 2006. Comparative demography of burrowing owls within agricultural and urban landscapes in southeastern Washington. Journal of Field Ornithology 77:280-290.
