Vigilance in the White-Breasted Nuthatch: Effects of Dominance and Sociality

-Using captive pairs of White-breasted Nuthatches (Sitta carolinensis), I tested the nonexclusive hypotheses that the vigilance of social foragers has two components, (1) vigilance for predators and (2) vigilance for dominant conspecifics, and thus is mediated by the size of the foraging group as well as by an individual's dominance status. Evidence from pairs of nuthatches, tested while solitary and while together, revealed that males increased their vigilance when solitary, that females were subordinate to and more vigilant than males when both sexes were housed together in the aviary, and that the advantage of foraging socially in terms of a reduction in vigilance was greater for males. My results demonstrate that vigilance in White-breasted Nuthatches was sex specific and that male social dominance was the responsible mechanism. I suggest that subordinate social foragers have the additional constraint on their foraging time of keeping higher-ranking flock mates under surveillance. Received 27 June 1986, accepted 26 January 1987. THE two commonly hypothesized advantages of foraging socially rather than solitarily are improved predator avoidance and enhanced foraging efficiency (reviewed by Wilson 1975, Moriarity 1976, Rubenstein 1978, Morse 1980, Krebs and Davies 1981). There is a growing realization that these two benefits are not independent. In fact, the size of a foraging group influences how individuals apportion their time budgets among such incompatible activities as scanning for predators and searching for food (e.g. Pulliam 1973; Powell 1974; Caraco 1979; Barnard 1980; Bertram 1980; Caraco et al. 1980a, b; Elgar and Catterall 1981; Lendrem 1983; Studd et al. 1983; Elgar et al. 1984). Several studies have demonstrated that as flock size increases, individual birds can allocate more time to foraging activities while the per capita vigilance level is maintained (e.g. Powell 1974, Siegfried and Underhill 1975, Caraco 1979, Jennings and Evans 1980, Sullivan 1984a). A common shortcoming of these studies was their failure to consider that the costs and benefits associated with social foraging may be sex specific, dominance specific, or both [but cf. Moore 1972 (in Caraco 1979), Caraco 1979, Ekman and Askenmo 1984, Waite 1986]. Based partly on evidence that birds increased their vigilance in areas of higher predation risk (Barnard 1980, Caraco et al. 1980a) or when a predator was present (Caraco et al. 1980b), previous workers assumed that the principal function of vigilance was predator detection. Thus, these studies overlooked the possibility that the time devoted to foraging was more constrained for subordinates because higher-ranking conspecifics interfered with their foraging (but cf. Robinson 1981, Knight and Knight 1986). I tested experimentally the possibility that females of a heterosexual pair are more vigilant than males because they must keep their dominant mate under surveillance while also maintaining a certain level of vigilance for predators. I used captive mated pairs of White-breasted Nuthatches (Sitta carolinensis) to examine the nonexclusive hypotheses that the vigilance of animals that forage in social groups is comprised of both vigilance for predators and vigilance toward other members of the foraging group, and, hence, is influenced by two proximate factors, sociality and dominance status. One prediction follows from each of these hypotheses, respectively. First, a male nuthatch should be more vigilant when alone than when with his mate. Second, a female nuthatch should be more vigilant than her (dominant) mate when they forage together. No prediction regarding the vigilance levels of solitary vs. social females is made because females, owing to their need to keep their dominant mates under surveillance, actually may be more vigilant when social than when solitary.