Experimental evidence for sibling recognition in Common Terns ( Sterna hirundo )

-Young Common Terns (Sterna hirundo) did not respond preferentially to calls of siblings at 8 and 9 days of age, but did so by 12 days of age. In experiments with and without visual isolation, and with use of playback, we demonstrated a tendency to approach sibling begging calls. This differential response indicated sibling-recognition occurred, was based on experience, and involved vocal cues. Received 26 May 1987, accepted 14 October 1987. THE ability to discriminate among individuals or categories of conspecifics, to recognize mates, parents, offspring, kin, neighbors, or strangers, is fundamental to social organization. Individual recognition in birds refers to the ability of one bird to discriminate correctly among selected individuals of the same species by some combination of sensory cues, which in birds are usually visual or vocal or both. Such recognition can be observed in the field or can be demonstrated by controlled playback experiments in the field or laboratory (Falls 1982, Kroodsma 1986). Theoretically, recognition should evolve in situations where it is essential to distinguish among individuals and to respond differentially, assuming that such differentiation is not possible solely on the basis of location. Individual recognition has been demonstrated between mates (Penney 1968; Beer 1969,1970; White 1971; Brooke 1978), between parents and young (Tinbergen 1953; Hutchison et al. 1968; Penney 1968; Tschantz 1968; Beer 1969, 1970, 1979; Evans 1970a, b; Stevenson et al. 1970; Ingold 1973; Burger 1974; Busse and Busse 1977; Shugart 1977), and between territorial neighbors vs. strangers in a variety of species (Beecher 1981, Falls 1982). Parent-offspring recognition might be expected to develop early in nidifugous species where parents are required to attend mobile chicks, and where the possibility exists that young of different parents could intermingle. Such recognition could include parental recognition of young, young recognition of parents, or both. Parent recognition of young ensures that parents care for their own reproductive units, rather than caring for unrelated offspring. Young recognition of parents ensures that young seek out and beg from their own parents, increasing their chances of being fed and avoiding attacks by unrelated adults, as Evans (1980) showed in gulls. The age of recognition varies and is related to age of chick mobility (Evans 1970a). In many gulls and terns, brood size is two or three, allowing for the possibility of sibling recognition. When chicks first become mobile they could be separated from siblings, and sibling recognition by visual or vocal means would be adaptive in maintaining contact. Spatial proximity of offspring might ensure chicks are near the nest and within their parents' territory, and together the enhanced vocal stimuli might attract parents, stimulating them to provide protection and adequate food. Circumstantial evidence for sibling recognition, such as chicks attacking foreign chicks, has been described for several seabirds (Cullen and Ashmole 1963; Snow 1963; Warham 1963; Schaller 1964; Nelson 1966, 1969; Dinsmore 1972; Spurr 1975; Howell 1978). There is evidence that recognition of siblings occurs in gulls with broods of two or more young (Evans 1970a, Noseworthy and Lien 1976). When tested in the confines of a runway, colony-reared, young Ring-billed Gulls (Larus delawarensis) responded selectively to their siblings at 4 but not 3 days of age, and laboratory-reared chicks did so at 4.5 days of age (Evans 1970a). Similary, evidence for sibling recognition in nonseabirds exists (Beecher and Beecher 1983). Nonetheless, experiments with seabirds have not determined if sibling recognition is visual or vocal, or which particular calls of siblings are recognized. We examined sibling recognition in Common 142 The Auk 105: 142-148. January 1988 This content downloaded from 207.46.13.21 on Tue, 27 Sep 2016 04:23:36 UTC All use subject to http://about.jstor.org/terms January 1988] Sibling Recognition in Terns 143 Terns (Sterna hirundo) to determine whether chicks can recognize their siblings when they can see and hear them or only hear them. Further, we tested whether chicks recognized the contact or begging call. It is important to recognize that failure to respond does not necessarily indicate lack of recognition. Common Tern young can move from their nests by 3 or 4 h of age; during the first week of life, however, they usually remain in or close to the nest, huddled in nearby shelter. Thereafter they range more widely around the territory while waiting for their parents to return with food. We predicted that sibling recognition would be present at the time chicks begin to wander away from the nest (Evans 1970a), which in Common Terns occurs at the end of the first week. The two major call types of tern chicks are a begging call (directed at parents to initiate feeding) and a contact or low-intensity distress call given when isolated (Fig. 1). We hypothesized that young should be able to recognize both types of calls, because when they are separated they should find their siblings (contact-call recognition), and if their siblings are begging and being fed by their parents, chicks should go to that place to be fed as well (begging-call rec-