A GENUS OF CUCKOO CUCULUS-CANORUS PARASITIZING RUFOUS BUSH CHAT CERCOTRICHAS-GALACTOTES

The incidence of Cuckoo Cuculus canoprus parasitism among complete clutches of Rufous Bush Chat Cercotrichas galactotes in an area of southwestern Spain was 27.6% (N = 29) and 27.3% (N = 44) in two successive years. Breeding Rufous Bush Chats mobbed adult Cuckoos only if the latter tried to approach their nests. They deserted seven (46.7%) out of 15 parasitized nests found at the egg stage. Cuckoo eggs were mimetic in size and colour with respect to Rufous Bush Chat eggs, but could be distinguished by their greater size and mass and by their lighter colour. Cuckoos do not successfully parasitize other bird species breeding in the area and their eggs are not similar to those laid by any of them. Out of the 15 Cuckoo eggs found, 40% hatched and 33% reached the fledgling stage.