74 



Birds 



' Our Liviiif; Resources 



Fig. 1. Juvenile desert tortoise 

 shell found beneath an aetive 

 raven nest. The hole in the shell 

 was probably pecked open by a 

 raven to eat the organs. 



populations in several parts of the country sig- 

 nificantly increased during 1965-79 (Robbins et 

 al. 1986). This increase concerns resource man- 

 agers because ravens teed on agricultural crops 

 and animal species of interest to humans. For 

 instance, in the deserts of the southwestern 

 United States, ravens prey on young desert tor- 

 toises {Gopherus agassizii'. Berry 1985; Fig. 1), 

 which in the Mojave and Colorado deseils are 

 listed as a threatened species by the USFWS 

 (Federal Register 1990). Because of high levels 

 of raven predation on tortoises, the Bureau of 

 Land Management has taken action to reduce 

 this predation (BLM 1990. 1994). We report 

 here on a 24-year trend in raven abundance 

 along roadsides in the deserts of the southwest- 

 ern United States and surrounding regions, 

 where increasing raven populations interest 

 resource management agencies (BLM 1990; 

 USFWS 1994).^ 



Our analysis of BBS 1968-92 data focuses 

 on arid lands and neighboring habitats in 

 California. Nevada. Utah, and Arizona. We used 

 data from 137 39.2-km (24.5-mi) routes within 

 the following BBS strata: Great Basin Desert; 

 mountain highlands of Arizona; Sonoran- 

 Colorado Desert; Mojave Desert; basins and 

 ranges, including portions of the northern 

 Mojave and Great Basin deserts; Central Valley; 

 and southern California grasslands, California 

 foothills (southern California routes only), and 

 Los Angeles ranges combined into one (coastal 

 southern California). 



Status and Trends 



Between 1968 and 1992, the latest year for 

 which data were available, raven populations 

 increased significantly {P < 0.01) throughout 

 the study area (Fig. 2), in spite of relatively high 

 variances among routes. Raven sightings 

 increased 76-fold in the Central Valley of 

 California, 14-fold in the Sonoran-Colorado 

 Deseit, and 10-fold in the Mojave Desert over 

 the 24-year period. Statistically significant but 

 lower increases in raven populations were expe- 



rienced in the heavily urbanized coastal south- 

 em California strata. The results for the moun- 

 tain highlands stratum are questionable because 

 of a low number of routes (;; = 7; B. Peterjohn. 

 NBS. personal communication). 



In three studies, raven numbers were highest 

 along powerlines, intermediate along highways, 

 and lowest in open desert areas (Austin 1971; 

 FaunaWest Wildlife Consultants 1989; Knight 

 and Kawashima 1993). These reports and obser- 

 vations of raven use of human-based resources 

 for food, water, and nesting substrate (Knight 

 and Call 1980; FaunaWest Wildlife Consultants 

 1989; Heinrich 1989) suggest that high raven 

 populations are a result of human subsidies 

 (Boarman 1993). 



Increased raven populations may be a con- 

 cern for threatened and endangered species if 

 increased numbers of ravens result in greater 

 predation. In California alone, there are 96 

 threatened or endangered species, some of 

 which are or may be at risk of increased raven 

 predation if raven populations continue to grow. 

 On San Clemente Island, ravens are a predator 

 of the endangered San Clemente Island logger- 

 head shrike (iMfiius htdoviciamis meamsi). and 

 along coastal California they prey on endan- 

 gered populations of the California least tern 

 {Sterna cmtllkirum bnnviii: Belluomini 1991). 

 The carcasses of II chuckwallas {Sciumnuiliis 

 obesus), a candidate species for listing as threat- 

 ened or endangered by the USFWS. were 

 recently found beneath one raven nest (personal 

 observation). This finding may be a rare occur- 

 rence, but if raven populations continue to 

 increase, more ravens may begin to prey on 

 chuckwallas. We are conducting more research 

 to understand the foraging ecology and popula- 

 tion biology of ravens and their effects on their 

 prey populations. This research will help us 

 determine how much of a threat ravens pose to 

 the region's biodiversity and learn how to 

 reduce these effects. 



Fig. 2. A 24-year trend in the average (mean) number of 

 raven sightings within each stratum studied. 



