Egg signalling: the use of visual, auditory, and chemical stimuli
The fundamental biological function of avian eggshells is to enclose an incubation environment in which an embryo can develop (Roberts and Brackpool 1994). However, birds have evolved a number of eggshell traits that are capable of transmitting information to a variety of receivers, from incubating parents through to predators and brood parasites. Communication of information consists of three parts: the sender, the signal itself and the receiver. Not all features will have a signalling function but may still be used as a cue by a receiver (e.g. a predator or a brood parasite). A signal is an act, or structure, which has evolved to indicate the quality of a signaller and it functions to change the behaviour of the intended receiver. A cue is a feature of the external world used by a receiver to decide on its future actions (Hasson 1994; Maynard Smith and Harper 1995). A cue can be a signal if it is deliberately communicating that information to the receiver.
Overwhelmingly, research into egg signalling has concentrated on the appearance of the eggshell, namely its colouration and pigmentation (Figure 11.1). Although this chapter will largely focus on this subject, we will also discuss other ways in which eggs may be acting as signalling cues. The first section of this chapter will focus on the acquisition of colouration and will explain the main hypotheses for its functional significance. The second section will discuss how eggs can be used as signals by the reproductive female (and possibly the developing embryo), the nature of signals that eggs are able to convey, and the identity of receivers who interpret these signals.