Population Biology of the Yellow‐Breasted Chat (Icteria Virens L.) in Southern Indiana

Abstract

A population of color—banded Yellow—breasted Chats was studied during the breeding seasons of 1966 through 1970 on 18 ha of upland deciduous scrub habitat near Bloomington, Monroe County, Indiana. The number of breeding males varied between five in 1970 and eight in 1966, and the number of territorial, nonbreeding males ranged from two in 1968 and 1969 to five in 1967. During the spring migratory period the capture rate of unbanded males per 100 net—hours varied between 1.40 in 1970 and 3.48 in 1969 (pooled mean = 2.57); the capture rate of females varied between 0.59 in 1970 and 3.88 in 1967 (pooled mean = 1.49). Capture rates of unbanded birds were considerably lower after the end of the spring migratory period (pooled mean capture rate of unbanded males = 0.24; unbanded females = 0.24). Approximately 50% of the males and females captured during spring migration and 75% of those captured in the summer did not settle on the study area; the rest were resident there. The sex ratio of both the breeding and the transient populations did not differ significantly from 1:1. The mean size of territories varied between 1.12 ha in 1967 and 1.58 ha in 1970 (pooled mean = 1.24 ha); territory size did not differ significantly among years. When territorial males disappeared, the vacancies in most cases were filled either by the settlement of new males or by expansion of the territorial boundaries of neighbors. New males that settled usually acquired females, which attempted to reproduce. In the absence of vacancies new males were prevented from settling by the presence or behavior of the established males. The replacements on and transients through the study area of both sexes were not necessarily birds without previous breeding experience and, in the case of males, territories in the seasons involved. The males may have abandoned territories elsewhere, or they may have wandered onto the study area from territories still held. The females had bred elsewhere; they had probably experienced nest failure and some, at least, were in the process of changing mates. Only one male was polygnous; all others formed pair—bonds with a succession of females, or remained paired to one female the entire time they were on the study area, of were for a time paired to one female and for a time unpaired. Of the males that experienced nest failure and bred again, 50% did so with new mates. No females and only 11% of the breeding males returned to the study area in the years following first capture. Nest success varied between 0.0% in 1970 and 38.5% in 1966 (pooled mean = 22.4%). Nests that were begun in late June and July succeeded in significantly greater numbers than nests begun early. Approximately 94% of the nest failures were attributed to predators. The number of young produced varied between none in 1970 and 24 in 1966. If each territory on the study area had been occupied for an entire breeding season by monogamously and polygynously mated males and unmated males in the same proportions as were observed, it is estimated that the population would have produced, on the average, 1.19 young per territory per year. This estimated mean potential productivity probably would not have produced enough young to replace estimated annual losses to the population. The area was, therefore, unfavorable habitat on which the chat population had to be maintained by immigrants in excess of emigrants. The low productivity resulted from a combination of nest predation and early cessation of reproductive activities in some years.