Mimicry By Brood Parasite Nestlings

Introduction

Motto: 'The possibility of mimicry in eggs must be treated with caution, as pure coincidence in their colouration is so general a phenomenon. . . . this state of affairs warns us to be very wary about attributing a given resemblance to mimicry.' Swynnerton (1916, p. 553). From mind-boggling similarity among various phylogenetically unrelated butter?ies to very generalized 'prey' presented by angler ?sh, mimicry has attracted human interest for a long time (Wickler, 1968; Komárek, 1998, 2003). Such resemblances result from coevolution (Janzen, 1980) or sequential evolution (Futuyma, 1998) and have been extensively studied among insects (for a review see, e.g. Wickler, 1968) and also brood parasites and their hosts (Fig. 1) (for reviews of brood parasitic systems see Rothstein & Robinson, 1998; Davies, 2000). Insect mimicry generally provides protection against predation or attracts pollinators (Wickler, 1968; Vane-Wright, 1976), whereas coevolutionary mimicry in brood parasites is a counter-adaptation against host antiparasitic response (Rothstein, 1990).

With respect to avian parasite-host coevolution attention has been focused mainly on mimicry in parasitic eggs and their rejection by hosts (e.g. Davies & Brooke, 1989; Moksnes et al., 1990). Apparent evidence for nestling mimicry in brood parasites received much less attention (for the most comprehensive review see Redondo, 1993). Is every similarity between parasitic and host propagules an example of mimicry? Various authors realized long ago that this is not the case: for example, eggs of parasite and host can be similar simply because they share the same environment, where they suffer predation from visually orientated predators. If so, then, similarity is cryptic (non-mimetic) and results from convergent evolution (Harrison, 1968; Mason & Rothstein, 1987).

In this review I will show that the issue is even more complex than was previously thought. When discussing various traits in the context of coevolutionary theories it should be clear if these traits are: (1) speci?c adaptations and counteradaptations (i.e. result of coevolution between parasites and hosts), (2) adaptations resulting from other (noncoevolutionary) selection pressures, or just (3) byproducts of some other - perhaps adaptive - traits (for discussions see Janzen, 1980; Vane-Wright, 1980; Ryan, 1990; Grim, 2002). The following analysis is based on these crucial differences and it is argued that mimicry terminology should re?ect the process that led to the given mimetic similarity. I suggest that the term 'nestling mimicry' and less so 'egg mimicry' is usually applied indiscriminately to any similarity between parasitic and host chicks or eggs, respectively (see, e.g. Jourdain, 1925; Lack, 1968; Mundy, 1973; Wyllie, 1981; Davies & Brooke, 1988; Redondo & Arias de Reyna, 1988b; Redondo, 1993; Hughes, 1997; Johnsgard, 1997; Davies, Kilner & Noble, 1998; Gill, 1998; and references in these papers).

However, the similarity could result from various proximate processes and only in some of these cases is the similarity mimetic within the generally accepted de?nition of mimicry: mimicry involves the mimic (e.g. parasitic chick) simulating signal properties of the model (e.g. host chick) which are perceived as signals of interest by a signal-receiver (e.g. fosterer), such ...