Seasonal habitat requirements and use by the western burrowing owl (Athene cunicularia hypugaea) in the Northern Sacramento Valley, California

The western burrowing owl (Athene cunicularia hypugaea) is a small fossorial owl found in the western United States, Canada and northern Mexico (Shuford and Gardali 2008). Populations in northern California have suffered recent declines, and a better understanding of their habitat requirements is needed in order to make better management decisions (Wellicome and Holroyd 2001). For this study I described internal and external characteristics found in owl burrow sites in the northern Sacramento Valley, California in the fall and winter months. I measured and recorded a variety of characteristics at 17 locations confirmed to be utilized by burrowing owls. I used iButton® data loggers to record internal humidity and temperature throughout the day for multiple years. I calculated means by season and semidiurnal period and compared them to ambient means. The physical attributes I recorded included burrow entrance direction, burrow type, distance to water, type of water source, whether the area was grazed, and whether burrowing mammals were present. Using a 1-m2 quadrat, I recorded vegetation height and density three meters in front of, behind and to each side of each burrow entrance and calculated the means for each site. I also utilized student research assistants to conduct visual occupancy surveys to determine the season(s) of usage for each burrow. Observations were summarized for burrow types. All sites were on or near land grazed by cattle in the winter. Natural burrows were all located in cut-banks of seasonal drainages. Seventy-one percent of all burrows were located within 6.55 m of water in the winter. Natural burrow internal temperature means were consistently higher in the mornings and lower in the evenings than ambient conditions for every season, except for winter, which was slightly higher. Artificial burrows and rock piles displayed a similar pattern but were consistent across seasons. Culvert temperature means were higher than ambient means in all seasons and semidiurnal periods except for spring which showed lower internal means than the ambient means. The greatest difference between internal and ambient temperature means was 10.35° C (SD=2.83) for artificial burrows (n=676). Internal relative humidity [rH] means tended to be higher than ambient means for all natural and culvert sites. For artificial burrows, internal rH means were lower in the A.M. and higher in the P.M. than ambient means for every season. Internal humidity means for rock pile burrows were higher than ambient means in the P.M. The greatest difference between internal and ambient relative humidity means was 55.17% (SD=26.15) for natural burrows (n=763).