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Nest site selection in birds is widely believed to have evolved mainly as a defense against nest predators (Martin, 1993). The logic in this argument is simple: nest predators account for most nest failures (Ricklefs, 1969; Martin and Li, 1992; Morton et al., 1993), and in many species the probability of predation varies with nesting habitat (e.g., Blancher and Robertson, 1984; Conner et al., 1986) or nest site features (e.g., Murphy, 1983a; Kelly, 1993; Norment, 1993; Martin, 1996). In particular, vegetation density in the habitat or in the immediate area around the nest have often been implicated as important influences on the probability of predators destroying nests (for review, see Martin, 1992).
However; a number of studies have reported no relationship between cover and success (e.g., Gottfried and Thompson, 1978; Holway, 1991; Filliater et al., 1994; Howlett and Stutchbury, 1996), possibly for the following reasons. First, nest cover is likely to be an important defense against visually hunting predators, but if predators locate nests using other sensory cues or through random search then cover may be unimportant (Zimmerman, 1984; Howlett and Stutchbury, 1996). The importance of nest placement as a deterrent to nest predation might also vary tremendously over time if the abundance of nest predators and nest success exhibit high temporal variability. A clear picture of the relationship between nest placement and success is not possible if nest predation rates are atypical during the period of study. In addition, most studies have limited the spatial scale of measurement to the immediate nest site, but habitat, patch and landscape level variation may significantly influence the probability of success (Kelly, 1993; Norment, 1993; Suhonen et al., 1994). Finally, parental behavior and nest placement have only rarely been viewed as a functional system. Comparative studies suggest that nest conspicuousness and intensity of parental defense may be positively related (Ricklefs, 1977). If parental behaviors (e.g., vigilance, attack) affect the probability of nest predators finding and/or destroying nests, then we should not necessarily expect exposed nests to be prone to high rates of nest predation. Indeed, dense habitats may hide searching nest predators and protect them from attacks by parents.
Additional data are needed to better understand the relationships between parental behavior, nest placement, habitat use and nest success of birds (see Martin, 1992). To this end, we compared the nesting biology of cedar waxwings (Bombycilla cedrorum) and eastern kingbirds (Tyrannus tyrannus). We chose to study these two species because they breed in the same general habitats yet respond to nest predators differently. Waxwings are inconspicuous near their nests and often desert a nesting attempt if they are disturbed early in the nest cycle (Crouch, 1936; Lea, 1942; Putnam, 1949). Kingbirds, on the other hand, aggressively defend their nests and only rarely desert (Davis, 1941; Bent, 1942; Blancher and Robertson, 1982; Murphy, 1996a). Our goals were to (a) document nesting success; (b) quantify, the extent to which nesting habitats of waxwings and kingbirds overlapped, and (c) determine if nesting habitat or nest site influence the probability of nest success for either species. Given that both species nested in the same locations, we assumed that they faced the same predator community and that any differences in nest success not attributable to nesting habitat or site would be the result of interspecific differences in predator defense behavior.
Both species are socially monogamous and build open-cup nests in trees 2-6 m aboveground (Davis, 1941; Lea, 1942). There are no significant differences in either maximum nest length (t = 0.90, P [greater than] 0.37) or breadth (t = 1.62, P [greater than] 0.11, df = 34 nests). Kingbirds (40 g) are insectivorous during the breeding season, but they often supplement their diet with fruit. Waxwings (35 g) are mainly frugivores, but capture insects using kingbird-like aerial hawking (Bent, 1950, pers. observe.). Kingbirds migrate to South America for the northern winter, but waxwings remain in North America and maintain a vagrant existence, moving to locations of high fruit availability (Bent, 1950). Egg-laying peaks in early June for kingbirds, and ca. 2-wk later for waxwings; Putnam, 1949; Leck and Cantor, 1979; Murphy, 1983a; Blancher and Robertson, 1985a. The greatest differences between the two species are in their social systems and aggressive behaviors. Kingbirds are highly territorial and defend their nests intensely against con- and heterospecifics (Davis, 1941; Smith, 1966; Blancher and Robertson, 1982), whereas waxwings may nest either as single pairs or in loose colonies, and rarely attack predators or other animals that approach their nests (Crouch, 1936; Lea, 1942; Putnam, 1949).
STUDY AREA AND METHODS
Field method. - Our main study site was located in the Charlotte Valley near Hartwick College's Robert R. Smith Environmental Field Station (42 [degrees] 26[minutes]N, 74 [degrees] 57[minutes]W), Delaware County, N.Y. A …