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Breeding behavior of the Chihuahuan Raven.

Article Excerpt
Chihuahuan Ravens (Corvus cryptoleucus) are generally restricted to arid habitats of southwestern United States and north central Mexico (Bednarz and Raitt 2002). They are common and abundant in their range, yet much of their basic life history remains unknown or poorly documented, particularly in regard to social relationships and territoriality during the breeding season.

Among the seven North American species of Corvus, two exhibit cooperative breeding: the eastern and western subspecies of the American Crow (C. brachyrhynchos) (Chamberlain-Auger et al. 1990, Verbeek and Caffrey 2002), and the Northwestern Crow (C. caurinus) (Verbeek and Butler 1981). The Common Raven (C. corax) is a solitary breeder, with no reports of colonial nesting or cooperative breeding (Goodwin 1986, Heinrich et al. 1994). The possibility of cooperative behavior has been reported for the Chihuahuan Raven, although the true status remains unclear (Heinsohn et al. 1990). Aldous (1942) observed a breeding pair and nonbreeding individuals giving alarm calls in the vicinity of a single occupied nest, and Jollie (1976) reported communal nesting, but gave few details.

We examined nesting behavior and composition of breeding groups over two breeding seasons. We hypothesized that environmental variability of desert habitats represents a major selective force on the breeding biology of Chihuahuan Ravens that may favor development of cooperative breeding (Ligon 1999, Ligon and Burt 2004). Given the variable environment, helpers might increase opportunities for adequate provisioning of young, particularly when conditions are poor. Unpredictable and harsh environmental conditions typical of Chihuahuan Raven breeding habitat may cause helpers to choose to help when the costs of rearing young outweigh the benefits (Emlen 1982).

METHODS

The study area (32[degrees] 10' N, 106[degrees] 55' W) was approximately 78 [km.sup.2] (perimeter = 37 km) on a mesa adjacent to the Rio Grande River Valley, 11-19 km southwest of Las Cruces, New Mexico. The land is managed by the Bureau of Land Management and was grazed heavily by cattle between the 1930s and 1970s, and moderately thereafter (L. J. Bevacqua, pers. comm.). The predominant plant species in this desert scrub habitat included honey mesquite (Prosopis glandulosa), soaptree yucca (Yucca elata), fourwing saltbush (Atriplex canescens), longleaf ephedra (Ephedra trifurca), snakeweed (Gutierrezia spp.), and peppergrass (Lepidiurn spp.).

We captured Chihuahuan Ravens using a cage-trap (Aldous 1936) and a rocket-net (model S200, Winn-Star, Inc., Marion, IL, USA) between February and July, 2000 and 2001. We marked ravens with federal leg bands, and one or more of the following: colored leg bands, patagial tags, bleached leathers, and radio transmitters. The patagial tags were...