Egg shape mimicry in parasitic cuckoos

Abstract

Parasitic cuckoos lay their eggs in nests of host species. Rejection of cuckoo eggs by hosts has led to the evolution of egg mimicry by cuckoos, whereby their eggs mimic the colour and pattern of their host eggs to avoid egg recognition and rejection. There is also evidence of mimicry in egg size in some cuckoo-host systems, but currently it is unknown whether cuckoos can also mimic the egg shape of their hosts. In this study we test whether there is evidence of mimicry in egg form (shape and size) in three species of Australian cuckoos: the fan-tailed cuckoo Cacomantis flabelliformis, brush cuckoo Cacomantis variolosus and pallid cuckoo Cuculus pallidus. We found evidence of size mimicry, and for the first time evidence of egg shape mimicry in two Australian cuckoo species (pallid cuckoo and brush cuckoo). Moreover, cuckoo-host similarity was higher for hosts with open nests than for hosts with closed nests. This finding fits well with theory, since it has been suggested that hosts with closed nests have more difficulty recognising parasitic eggs than open nests, have lower rejection rates, and thus exert lower selection for mimicry in cuckoos. This is the first evidence of mimicry in egg shape in a cuckoo-host system, suggesting that mimicry at different levels (size, shape, colour pattern) is evolving in concert. We also confirm the existence of egg size mimicry in cuckoo-host systems.