Pair-Formation Displays of the Great Egret

Abstract

Many displays of the Great Egret (Casmerodius albus) have been described recently by McCrimmon (1974), Wiese (1976), and Tomlinson (1976). Yet much remains to be understood about the displays' functions and how these are integrated to serve the egret's communication needs. In this paper I shall present further information about this species' displays with an emphasis on description of form, variability, and contexts. I shall then attempt to interpret signal function and evolutionary derivation. This paper is intended to complement a study of social behavior of the Great Blue Heron (Ardea herodias; Mock 1976). The communication systems of these two species are compared in detail elsewhere (Mock, unpubl.). Each display will be treated separately with the following format: name, description of a typical performance, variations in form, social contexts, and a discussion of signal design and evolution. I use a modification of Meyerriecks' (1960) display terminology (see Mock 1976, Wiese 1976) with display names capitalized (Moynihan 1955). Synonyms in heron display terminology are reviewed in Mock (1976). On the basis of form, context, and probable functions I distinguish between reactive displays and displays. Reactive displays are characterized by relatively fixed orientation toward the signal-receiver and are apparently performed in response to the receiver's actions. These signals serve primarily agonistic functions (especially as distanceincreasing threats). Reactive Great Egret displays include Upright, Fluffed Neck, Forward, Stab-Crouch, Bill Duel, Supplanting, and Bill Clappering. Spontaneous displays are much less rigid in their orientation, are not obviously elicited by the actions of other birds, and may even be performed in the absence of signalreceivers. Typically, spontaneous displays are given in long sequences by unpaired males and seem to function primarily as femaleattractors. They include Stretch, Snap, Wing Preen, Bow, Twig Shake, and Circle Flight. Only one display, the Extended Neck Flight, does not fit well into either category: it combines properties of both and probably conveys both kinds of information in its various con-