THE OCCURRENCE AND SIGNIFICANCE OF ANOMALOUS REPRODUCTIVE ACTIVITIES IN TWO NORTH AMERICAN NON-PARASITIC CUCKOOS COCCYZUS SPP.

Among the unusual breeding habits of the non-parasitic Yellow-billed and Black-billed Cuckoos of North America are great variability in clutch size and rate of laying, initiation of incubation long before the clutch is complete, occasional laying in nests of other species, annual irregularity in the timing of the breeding season, and semi-nomadic post-migratory movements into breeding areas where food is abundant. These facts, in addition to their peculiar diet and the very large size of their eggs, suggest that cuckoos have extraordinary problems in obtaining adequate energy for reproduction. At Bloomington, Indiana (U.S.A.), during a 15-year period, anomalies in the reproductive activities of cuckoos were concentrated into two years in which food was abundant. This was particularly true of one of these years, when there was a vast emergence of periodical cicadas: the Yellow-billed Cuckoo advanced its normal schedule and bred during peak cicada abundance, laid unusually large clutches, and parasitized Black-billed Cuckoo nests. Some females may have resumed laying in nests in which, having already deposited clutches of normal size, they had been incubating for long periods; the alternative possibility is that there was intraspecific brood parasitism. The erratic egg-laying behaviour of these cuckoos is attributed to the evolution of mechanisms permitting very quick exploitation of a favourable feeding situation. It is suggested that reproductive behaviour has become so responsive to an abundance of food that normal ordering and integration of the stages of breeding have been lost in some females. Such a loss could be responsible for the laying of eggs in alien nests, and it may have been the antecedent of obligate brood parasitism in parasitic cuckoo species.