Response to Common Cuckoo Cuculus canorus model egg size by a parasitized population of Rufous Bush Chat Cercotrichas galactotes

The different gentes of the Common Cuckoo Cuculus canorus are adapted to parasitize different species of passerine birds, the females of each gens laying eggs of constant type and usually similar in coloration to those of the host species (Brooke & Davies 1988, Moksnes & Rmkaft 1995). This kind of mimicry apparently results from the hosts' rejection of unlike eggs (Davies & Brooke 1988, Brown et al. 1990, Moksnes et al. 1990). Cuckoo eggs are smaller than expected in relation to body mass (Lack 1968, Payne 1974), their size being usually slightly larger than the hosts' (Baker 1942, Lack 1968, Wyllie 1981, Alvarez 1994, Moksnes & Reskafi 1995). The only attempt made to test the response of a Cuckoo host (the Reed Warbler Acrocephalus scirpaceus) to egg size showed that giant model eggs coloured like Reed Warbler type Cuckoo eggs were not preferentially accepted or rejected, although they were more likely to be rejected than model eggs similar in size and colour to the eggs of the Reed Warbler Cuckoo gens (Davies & Brooke 1988). This result was interpreted as an adaptation of the small eggs of C. canorus and other parasitic Cuculidae to deceive hosts and avoid egg rejection. The size of Cuckoo eggs, relative to the Cuckoo and to eggs of the host, suggests the action of one or several forces, including the acceptance and rejection of the Cuckoo eggs by potential hosts, and constraints in relation to incubation time (Darwin 1859, Baker 1942), Cuckoo physiology and Cuckoo chick competition. The Rufous Bush Chat Cercotrichas galactotes is regularly parasitized by the Common Cuckoo in southern Spain (27% of the nests in an agricultural area south of Seville), where it lays eggs larger than the host's (Alvarez 1994), and the parasite's egg rejection is carried out by the female of the host pair (Palomino et al. 1998). Although body size of the Iberian and northwestern African race of Common Cuckoo (C. c. bangsi) is smaller than other European races (Cramp 1985), egg size of the aforementioned population (Alvarez 1994) is not smaller than that of its northern relatives (Baker 1942, Wyllie 1981, Moksnes & Rerskaft 1995). The lower egg size provided by Johnsgard (1997) for C. c. bangsi points to the likely existence of different gentes within the subspecies, and in fact, egg volumes of Common Cuckoo and Rufous Bush Chat (Alvarez 1994) fit perfectly in the relationship found by Moksnes and Rmkaft (1 995) between egg sizes of Common Cuckoos and different host species In order to understand better the significance of the greater egg size of the Common Cuckoo, and whether i t would contribute to acceptance by the host, I have tried to isolate the Rufous Bush Chat response to model Cuckoo eggs of various sizes.