29th Annual Meeting and Symposium of the
Desert Tortoise Council, February 20-23, 2004
Abstracts

Common Raven Ecology at the Marine Corps Air Ground Combat Center (MCAGCC)

William I. Boarman and H. Douglas Chamblin
U. S. Geological Survey, Western Ecological Research Center, San Diego, California

As human communities in California deserts have grown, common ravens (Corvus corax), making use of anthropogenic resources, also have experienced increases. Ravens are a concern to resource managers at MCAGCC because they prey on juvenile desert tortoises (Gopherus agassizii) and their increase is a factor in tortoise decline. In addition, ravens have formed a large nocturnal roost on power lines, presenting a Bird Air Strike Hazard (BASH) because of the roost's proximity (3 km) to the base's air field. The broad objective of our research is to improve understanding of raven ecology to better manage the raven population. Specifically, we attempted to answer three questions: (1) are there differences in raven abundance among the anthropogenic resource sites, randomly selected features, and remote desert areas?; (2) is roosting phenology best explained by sunset, light level, or human activity?; and, (3) does raven attendance at the roost vary seasonally, with Marine training activity, or both? Results from 12 monthly surveys show that more ravens were present at attraction sites than at desert sites, suggesting that controlling raven access to subsidies may reduce their numbers on the base. Roost attendance ranged from 53 to 2100 ravens. Most ravens settled onto the roost about 30 minutes after sunset and departed from the roost about 30 minutes before sunrise. Fewer ravens attended the roost in the spring and territories at this time. Alternatively, the pattern may reflect changes in human activity on the base, as war deployments led to suspended training activities from late-winter through summer. To avoid BASH problems, Marine use of the air field should be restricted during peak periods of raven arrival and departure from the roost, especially during the fall and winter months. Results of surveys in 2004, with the resumption of normal training schedules, will determine whether roosting patterns are seasonal or are tied to Marine activities.