West Mojave Plan FEIR/S - Desert Managers Group

agricultural fields, were the most common concentration areas. Only three of 17 sewage pondsshowed consistent use by ravens.Based on Breeding Bird Surveys (Robbins 1986) conducted by the USFWS, BLM (1990)estimated that the number of ravens in the Mojave Desert had increased by 1,528% between1968 and 1988. Boarman (1992) felt that the increase was likely much higher in the WestMojave. Knowles et al. (1989a, 1989b) surveyed 801 linear miles of roads, 12 landfills, and 11sewage ponds in the West Mojave, every two weeks throughout 1989. They found ravensconcentrated around landfills, sewage ponds, agricultural fields, and urbanized areas. Thehighest density areas (i.e., between 250 and 1,000 ravens/100 mi 2 ) included the Victor Valley,Lancaster-Palmdale area, and around Ridgecrest (reported in Chambers Group, Inc. 1990).Increased Raven Populations in Response to Human Resources: Boarman (1992)described ravens as “…predatory animals that survive and perhaps grow in part due to food,water, or other limiting resources provided by or associated with human activities. As a result oftheir association with humans, the populations are allowed to grow well beyond the naturalcarrying capacity of the habitat.” Raven populations have likely increased due to increasedavailability of foods (e.g., landfills, sewage ponds, dumpsters, highways, cities) and water (e.g.,sewage ponds, agricultural fields, golf courses), which sustain more individuals during times oflow natural resource availability, such as winter and summer. Such artificial food sources mayfacilitate larger clutch sizes or increased frequencies of clutches and greater fledging success. Inaddition, human-made structures have increased numbers and distribution of perches and nestsites (e.g., power and telephone poles, bridges, billboards, freeway overpasses, etc.). (Boarman1992, USFWS 1994b).Prevalence of Raven Predation in the West Mojave Compared to Elsewhere: Boarman(2002) reported that the extent of raven predation could be estimated by evaluating juveniletortoise carcasses found throughout the desert. He found that Campbell (1983) had reported 136shells along the perimeter fence at the DTNA, which he attributed to raven predation. Over afour-year period in the early to mid-1980s, Woodman and Juarez (1988) found 250 tortoisecarcasses beneath one raven nest near the Kramer Hills. Mean carapace length of the carcassesbecame progressively smaller over the four years (Woodman, pers. comm.), suggesting thatravens had removed most of the relatively larger animals, and were beginning to seek outsmaller, harder to find tortoises. Krzysik (1994) reported that raven predation accounted for 4.5%of the tortoise mortality observed at Fort Irwin. During 1988 and 1990 surveys of various FortIrwin expansion area alternatives, however, Chambers Group, Inc. observed no evidence ofraven predation.Between 1998 and 2002, of the 1,033 tortoise carcasses found throughout the WMP,cause of death was determined for 104 (10%) of them (WMP, unpublished data). Of these 104carcasses, raven predation (or scavenging) was identified for 10 (9%) of them. These results aresimilar to those collected during distance sampling in the Fremont-Kramer and Superior-CroneseDWMAs in 2001 and 2002: cause of death was determined for 44 of 764 (6%) carcasses, andraven predation (or scavenging) was identified for 7% (i.e., 3 of the 40 carcasses). Thus, twoindependent data sets from the same region and time period attributed raven predation (orscavenging) to 9% (13 out of 148) of the carcasses found where the cause of death was given.Chapter 3 3-104