Behavioral Ecology of Fledgling Brown-Headed Cowbirds and Their Hosts

Twenty-two fledgling Brown-headed Cowbirds (Molothrus ater) raised by nine host species were monitored between leaving the nest and independence. The objectives were (1) to record as much as possible about the fledgling period, especially those features that enable cowbirds to become independent, (2) to determine if fledglings behaved differently with different hosts, and (3) to determine if fledgling cowbirds possess adaptations for brood parasitism. Cowbirds usually left the nest when 10 or 11 days old, first fed themselves when 20 to 22 days old, and became independent when 25 to 39 days old. They went through three stages--inactive, active, and superactive--differentiated by how often they flew and by other behavior. General development was probably unmodified by hosts. Perching heights and home ranges of fledglings were modified in a host-specific manner. Cowbirds perched mainly at the hosts' foraging heights, and their home ranges corresponded to their hosts' territories; both are related to feeding efficiency. Cowbirds did not recognize hosts as individuals; they generally responded positively only to their host species. Hosts fed fledgling cowbirds more than they fed an equivalent weight of their own young. The loud, persistent calling of fledgling cowbirds may cause them to be fed more and is probably their main adaptation for brood parasitism. Obligate brood parasitism in Brown-headed Cowbirds (Molothrus ater) is of interest to evolutionary biologists because of the adaptations that it has elicited in the cowbirds and the counteradaptations of their hosts. Friedmann (1929, 1963, 1966, 1971) and Friedmann et al. (1977) have summarized known records of cowbird parasitism and this species' life history. Rothstein (1974, 1975, 1976a, b) studied recognition of cowbird eggs by potential hosts. Robertson and Norman (1976) studied the response of host species to adult cowbirds near their nests. Little attention has been directed toward the period between the time when young cowbirds leave the nest until they become independent of their hosts, often referred to as the fledgling period. Eastzer et al. (1980) as part of their study watched a fledgling cowbird during the day it left the nest, Friedmann (1929) mentioned a few aspects of this period, and Nice (1939) reported on a hand-reared cowbird. Other scattered references occur in the literature, but I know of no detailed study of the fledgling period. With few exceptions, this critical period has been largely ignored in life history studies of North American passerines. I investigated the fledgling period of these cowbirds, first in order to describe its events in detail, particularly the relation between hosts and their cowbird(s) and the behavioral and ecological features that enable fledglings to become independent. Second, I wished to determine if fledgling cowbirds raised by different hosts behaved differently. Lastly, I sought to determine if cowbird fledglings possess adaptations for brood parasitism other than preadaptations. they share with closely related, nonparasitic species as described by Hamilton and Orians (1965). STUDY AREAS AND METHODS This study was concentrated in four areas: (1) McKee-Beshers Wildlife Management Area and adjacent Chesapeake and Ohio National Park (Hughes Hollow), (2) Edward's Ferry, (3) Adventure, and (4) Reston. I watched other fledgling cowbirds at a bridge on Willard Rd., near River Rd., and Seneca. Reston is in Fairfax Co., Virginia; all other areas are in Montgomery Co., Maryland. Most observations were made from 27 May to 25 July 1975, but others were made in 1976. Twenty-two fledgling cowbirds raised by nine host species were followed for 348.9 h (Table 1). To determine if fledgling cowbirds' behavior was modified by their hosts, I observed cowbirds with hosts that differed greatly in size (5 g to 40 g), habitat (open fields to forests), and feeding habits (ground foragers to canopy gleaners). Each cowbird was banded with a U.S. Fish and Wildlife Service aluminum band. Owing to the small number of birds involved and the distances between study areas, color bands for individual recognition were not necessary. By banding on either the right or left leg, I could identify each individual. Unbanded fledglings