Burrowing owls (Athene cunicularia) depend on burrowing mammals for nest sites throughout most of their range in North America. On the Pawnee National Grassland in Colorado (where they are a state-listed threatened species), burrowing owls are strongly associated with a metapopulation of black-tailed prairie dogs (Cynomys ludovicianus) where periodic colony extinctions are caused by plague (Yersinia pestis). Plague is a non-native pathogen that is typically lethal for prairie dogs and indirectly affects owls through the loss of nest sites and alteration of habitat. In addition to plague, we considered precipitation (which influences vegetation and prey populations) and competition resulting from the clustering of owl nests to be the most important ecological factors affecting breeding owls in our study area.
Our goal was to determine the relative importance of these factors and their relation to nest abundance, apparent nest success, and fledging success. We monitored 310 owl pairs from 2005 – 2008 on extinct, recolonized, and active prairie dog colonies; we also collected diet samples and piloted the application of a new mark-resight productivity estimator.
Results/Conclusions
Nine prairie dog towns in our sample were extirpated by plague during this study, and each was recolonized by prairie dogs within the year. Nests occurred at higher than average density on recently recolonized towns and were closely associated with the small areas with prairie dog activity. The increase in overall productivity on these recolonized towns (compared to extinct or long active towns) was mainly due to high nest abundance and nest success; the number of fledglings per successful nest was almost unchanged. Nest and fledging success were highest in a dry year (2006) and declined the next year when mid-late summer was wet and most formerly old, large towns were smaller in size following plague outbreaks and gradual reoccupation by prairie dogs. We are currently investigating the interaction of plague, precipitation, nest density, and prey use and their effects on owl breeding success.
