Introduction

This junco of the Transition and Boreal Zones of mountains in the southwestern United States, Mexico, and parts of Central America is resident throughout much of its range. In southern Arizona and southwestern New Mexico, pairs maintain all-purpose breeding territories and then migrate to lower elevations and form winter flocks with Dark-eyed Juncos (Junco hyemalis). These relatively tame birds forage on the ground and are easily seen, often frequenting campsites and picnic areas. In s. Arizona, pairs remain together throughout the breeding season, producing up to 3 successful broods per season. During the breeding season, males are often heard singing their multipart, warblerlike song from high atop conifers.

Yellow-eyed Juncos are locally abundant, sedentary, and philopatric, and they adapt well to captivity. These traits make this an exceptional species for behavioral studies, and its behavior and life history are reasonably well understood. Songs of the Yellow-eyed Junco, with their 2 to 3 parts and multiple trills, are highly variable within populations. This species was featured in classic studies on individual variation in song (Marler and Isaac 1961) and song development (Konishi 1964). A series of studies on the effects of flock size on behavior and individual trade-offs among foraging, vigilance, and aggression were based on winter flocks of Yellow-eyed Juncos in southern Arizona (Caraco 1979a, 1979b, Caraco et al. 1980a, 1980b, Pulliam et al. 1982). Growth, energy expenditure, and time allocation are well understood in this species from studies conducted over the annual cycle in southern Arizona (Weathers and Sullivan 1989a, 1989b, 1991a, 1991b, 1993, Sullivan 1990, Sullivan and Weathers 1992). In most passerine species, young birds are difficult to find once they leave the nest, and consequently little is known about fledgling and juvenile life history stages. Studies on mortality patterns, acquisition of foraging skills, energy expenditure, and time allocation among immature Yellow-eyed Juncos have made a valuable contribution to our understanding of fledging and juvenile passerines (Sullivan 1988a, 1988b, 1989, 1990, Weathers and Sullivan 1989a, 1991a, 1991b). Since most studies to date on this species have been conducted at the northern exent of its range, it would be worthwhile to investigate the behavior and population dynamics of other populations of Yellow-eyed Juncos.