Recognition of Parents' Voices by Young Cliff Swallows

-Cliff Swallow (Hirundo pyrrhonota) chicks were played calls of parents and unrelated (control) adults at 9 and 18 days of age. Younger chicks showed no difference in the frequency of their antiphonal begging calls to parental vs. control calls. The older, nearfledging chicks, however, responded significantly more to parental calls than to control calls: 78% of their total antiphonal calls were in response to parental playback calls. In these older chicks, the degree of preference correlated with the measured acoustic differences between the parent and control calls. The results indicate that Cliff Swallow chicks are able to recognize their parents by voice before they leave the nest. Offspring recognition of parents is discussed as it relates to the evolution of parent-offspring recognition systems in general. Received 12 September 1984, accepted 5 January 1985. SOME species live in socioecological circumstances requiring parents and dependent offspring to find one another among large numbers of conspecifics. In many colonial species, for example, a parent leaves its young in a creche while it forages at distant sites. To deliver food to its young on returning to the creche, the parent must recognize the offspring, or vice versa. In fact, parent-offspring recognition invariably has been found in such species. Theoretical discussions of this situation generally have emphasized the parents' need to recognize offspring but have ignored pressures for young to recognize parents (see Colgan 1983 and references therein). Indeed, it is sometimes argued that, far from being selective, offspring should actively seek parental care from nonparents. This emphasis derives from the straightforward argument that parents benefit only from care directed toward offspring, while young benefit from any parental care, whether received from parent or nonparent. In this view, misdirected parental care is a problem for parents but not for offspring. The argument so far leads to the prediction that parents will recognize offspring but not vice versa. Field observations, however, have not supported this prediction. For example, when an Adelie Penguin (Pygoscelis adeliae) parent returns to the creche, only its own chicks respond to its calls and solicit feedings (Penney 1968, Thompson and Emlen 1968, Spurr 1975, Thompson 1981). This example is particularly instructive, since the creche often is portrayed as the ideal situation for low-cost freeloading by young. What, then, is the shortcoming of the argument? It omits a key feature of the recognition process: parents can recognize offspring only if the latter provide individually distinctive cues ("signature" cues). Comparative evidence suggests that signature systems have been one of the key adaptations in the evolution of recognition (Beecher 1981, 1982; Jouventin 1982). Offspring signatures, however, have an additional consequence: not only do they enable the parents of the young to recognize them as offspring, but they allow other adults to recognize them as alien. Therefore, soliciting parental care from unrelated adults is not likely to be successful. Moreover, it will entail effort and perhaps some element of risk, such as attacks by unrelated adults. We thus arrive at the conclusion that the same variables that favor parents recognizing offspring must favor offspring recognizing parents. This natural-selection argument suggests that offspring recognition of parents will coevolve with parental recognition of offspring. We examine a corollary of this argument in the present paper: Where parental recognition of offspring has been discovered in a species, we should expect to find offspring recognition of parents as well. Surprisingly, convincing demonstrations of both processes exist for only a few species (see Discussion). The present study examines whether young Cliff Swallows (Hirundo pyrrhonota) recognize their parents. We have shown in a previous study that Cliff Swal600 The Auk 102: 600-605. July 1985 July 1985] Recognition in Cliff Swallows 601 low parents recognize their young by voice (Stoddard and Beecher 1983). Because antiphonal calling between parents and chicks is so conspicuous both at the nest and after fledging, we chose to examine chicks' recognition of parents using a playback method. We recorded the calls of parents at the nest and later tested chicks on the calls of their parents vs. the calls of parents from another nest. We measured recognition by comparing the number of antiphonal calls chicks gave to parent calls vs.