Social Status Signaling in Winter Flocking Birds: An Examination of a Current Hypothesis

-S. Rohwer recently has proposed that intraspecific plumage variability in winter flocking birds represents a polymorphism that functions to signal social status. We test several predictions of Rohwer's status signaling hypothesis for Dark-eyed Juncos (Junco hyemalis), explore some theoretical implications of our findings for juncos, and discuss the generality of social status signaling. Both darkness of hood and whiteness of tail were positively, although imperfectly, associated with social rank in captive winter flocks of first-year juncos. If plumage differences function as cues to dominance status in this species, our results suggest that their signal value derives primarily from learned associations between plumage attributes and other variables (such as sex or body size) that might influence fighting ability. Ultimate factors possibly responsible for winter plumage variability in juncos, both within and between age/sex classes, appear to be complex; selective pressure for a system of social status advertisement could (but need not) be one of these factors. Findings for some other winter flocking species reveal both similarities to, and differences from, our results for juncos. We suggest that more information on the behavior and ecology of a variety of winter flocking birds is needed before a satisfactory generalized model of social status signaling can be formulated-if such a model is possible. Received 28 February 1978, accepted 5 September 1978. FLOCKING is a widespread phenomenon among birds that winter in temperate regions. Such behavior affords many benefits; for instance, it may facilitate orientation during migration or refuging, aid in the location and efficient exploitation of food, and assist in the detection and mobbing of predators. However, flocking also may impose costs, not the least of which is the promotion of intraspecific competition for essential resources. The cost of such competition may be reduced by social systems, such as social dominance hierarchies, that function to order individual priorities at resources and to regulate the frequency, form, and intensity of aggressive behavior (discussed by Balph 1977a). Of considerable interest is a hypothesis stating that dominance hierarchies are predetermined by adaptive variation in the appearance of individuals. Rohwer (1975, 1977) observes that wintering birds differ markedly in the degree to which they vary intraspecifically in appearance and indicates that flocking species tend to exhibit polymorphism in plumage coloration or pattern, whereas non-flocking species show monomorphism. He speculates that in flocking species ". . . plumage variability has evolved to signal the approximate social status of each individual" (Rohwer 1975: 594). According to this hypothesis, position in a dominance hierarchy within variably plumaged species is signaled by the "studliness" (Rohwer 1975: 598) of individuals (defined as the degree to which each exhibits black, brightly colored, or contrastingly patterned plumage, particularly of the head or breast). Rohwer predicts such status advertisement to be especially advantageous if individuals change flocks, as it should enable both an incoming bird and established flock members to assess accurately the newcomer's proper social position without engaging in energetically costly fights. From a series of tests using Harris' Sparrows (Zonotrichia querula), he concludes that winter plumage variability does serve a status78 The Auk 96: 78-93. January 1979 This content downloaded from 207.46.13.178 on Wed, 15 Jun 2016 06:50:58 UTC All use subject to http://about.jstor.org/terms January 1979] Social Status Signaling 79 signaling function in this species. He also indicates, on the basis of qualitative observations of some other winter flocking passerines, including Dark-eyed Juncos (Junco hyemalis), that the phenomenon may be a general one. However, Shields' (1977) fine critique points out serious problems in Rohwer's (1975) paper. In the present paper we examine Rohwer's status signaling hypothesis both from an empirical and a theoretical standpoint.