We measured ventilation, oxygen consumption and blood gases in burrowing owls (Athene cunicularia) breathing moderate and extreme hypoxic gas mixtures to determine their hypoxic ventilatory threshold (HVT) and to assess if they, like other birds and mammals, exhibit a relationship between HVT and hemoglobin O2 affinity (P50) of their blood. An earlier report of an attenuated ventilatory responsiveness of this species to hypoxia was enigmatic given the low O2 affinity (high P50) of burrowing owl hemoglobin. In the current study, burrowing owls breathing 11% and 9% O2 showed a significantly elevated total ventilation. The arterial partial pressure of oxygen (PaO2) at which ventilation is elevated above normoxic values in burrowing owls was 58 mm Hg. This threshold value conforms well to expectations based on the high P50 of their hemoglobin and the HVT vs. P50 relationship for birds developed in this study. Correcting for phylogenetic relatedness in the multi-species analysis had no effect on the HVT vs. P50 relationship. Also, because burrowing owls in this study did not show a hypometabolic response at any level of hypoxia (even at 9% O2); HVT described in terms of percent change in oxygen convection requirement is identical to that based on ventilation alone.

In 2004, I initiated a year-long study to investigate the food habits of burrowing owls (Athene cunicularia). Burrowing owls have been found in a variety of human-altered landscapes; however, little is known about burrowing owl food habits in urban landscapes. Burrowing owl food habits during the non-breeding season are also largely undocumented, despite increasing concern over the survival of overwintering burrowing owls. Differences in prey consumption between reproductive and non-reproductive owls during the breeding season have not yet been examined. I collected pellets over a 12 month period at four study sites affected by different levels of human alteration in the southern San Joaquin Valley of California. Data was collected at four study sites representing natural (Wind Wolves), semi-natural (Allensworth Ecological Reserve), agricultural (Friant Kern Canal), and urban (Bakersfield) landscapes. Invertebrates, primarily ground dwelling insects, were the most commonly consumed prey type, found in 96% of all pellets examined. Among vertebrates, mammals were the most commonly consumed (18.5% of all pellets). Shannon-Weiner diversity indices identified differences in prey diversity consumed between seasons within each site and between sites within seasons, except during the breeding season where diversity was the same at all sites. The diversity indices at Wind Wolves (natural site) and Bakersfield (urban site) were the same, while the diversity indices at Allensworth Ecological Reserve (semi-natural site) and Friant Kern Canal (agricultural site) were the same. Binary logistic regression was used to determine if consumption of individual prey types varied by site, season, and a site/season interaction. Mammals were consumed in greater proportions during the breeding season at most sites compared to other seasons. The proportion of pellets containing mammals during the breeding season decreased as the level of human-alteration increased, with mammal consumption highest at Wind Wolves (60.0%) and lowest at Bakersfield (13.1%). Consumption of several insect categories differed by site and/or season (Coleoptera, Dermaptera, and Orthoptera), but overall consumption of insects was not different by either factor. To assess differences in prey consumption between reproductive and non-reproductive owls, pellets collected during the 2005 breeding season were classified as having come from a nest burrow or a non-nest burrow based on positive identification of reproduction. Shannon-Weiner diversity indices and binary logistic regression were calculated for this data set. No differences were detected in overall diversity or in the proportional consumption of individual prey categories. The results of this study indicate that burrowing owls have a highly variable diet and may have sufficient ecological plasticity to allow them to adjust their food habits to the prey species available in human-altered landscapes. However, the implications of altered food habits on burrowing owl fitness in heavily disturbed landscapes, particularly urban landscapes, needs further study.