Nest-Site Selection and Reproductive Success in Common Ravens

Abstract

Studies of habitat selection by animals can be categorized in two ways: behavioral and evolutionary (Krebs 1994). Most studies of habitat selection consider the behavioral perspective whereby biotic and/ or abiotic components of the environment that appear to influence habitat use are identified, generally by comparing used sites with random or available sites (e.g. Mosher et al. 1986, Seamans and Gutierrez 1995). The evolutionary approach examines the effects of selecting particular habitats on an index of fitness, e.g. survivorship or reproduction (see Martin and Roper 1988, Petit and Petit 1996). If nest-site selection is a heritable trait, then natural selection should favor individuals that choose nest sites that confer greater reproductive success. Common Ravens (Corvus corax) are suitable subjects for examination of reproduction in relation to habitat characteristics owing to their variable clutch size, which ranges from three to seven eggs (Dunk et al. unpubl. data). Thus, in any one year, a relatively large range in number of young fledged is possible within a population, and it may be possible to detect a cline in reproductive success relative to nest-site characteristics. Common Ravens are widely distributed throughout North America and Europe. In western North America, ravens have been characterized as pests that easily adapt to human-modified landscapes (Butchko 1990, Boarman 1993, Marzluff et al. 1994). During the past 25 years, raven numbers have increased in many areas of the western United States (Boarman 1993, Dunk et al. 1994, Marzluff et al. 1994). Despite their ubiquitous nature, large numbers, and broad geographic range, little is known about many aspects of the breeding biology of ravens. In particular, very little has been published on raven nest-site selection. Ravens nest on many substrates, including cliffs (Ratcliffe 1962, White and Cade 1971, Hooper 1977, Skarphedinsson et al. 1990), highway overpasses and billboards (White and Tanner-White 1988), churches (Heinrich 1989), power poles (Knight and Kawashima 1993), and trees (Dorn 1972, this study), but nest-site characteristics have been quantified only for cliff nests (White and Cade 1971, Hooper 1977).