The common raven (Corvus corax) is a widely distributed and opportunistic scavenger and predator. In Calgary, Alberta, ravens were virtually absent in the early 1970s, but by 2000 this species was a regular breeder and common winter visitor. Numbers increased significantly during Christmas Bird Counts from 1970 to 2000. Most birds that occur in Calgary during the winter commute twice daily between a roost in the foothills and a refuse site in the city. The distance between these points is 57.5 km and results in a commute cost that does not appear to be endured by individuals remaining in the foothills. I estimated that refuse-bound birds expend 25% more energy than foothills birds when traveling to known food locations. Upon arrival at the refuse site, individuals compensated for this additional expense by spending more time walking and foraging, which are less-energy-demanding behaviours, and less time flying, which is a high-energy-demanding behavior, than birds in the foothills. The refuse site appears to act as an emergency resource when conditions in the foothills are unfavourable for meeting daily energy requirements. These conditions were mainly caused by decreasing temperature and increasing snow depth in the foothills. Decreasing day length and temperature were factors affecting roost site departure, while decreasing temperature and increasing snow depth resulted in greater numbers of birds commuting. Although the commute has a known cost, a balance is likely realized both in the short term through acquisition of food during stressful conditions and in the long term through increased overwinter survival. The commute may also be advantageous for population increases and range expansion because it exposes birds to new habitats, new food sources, and potential new nest sites along the way.
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