Interspecific Communication Signals in Parasitic Birds

Abstract

Dull plumages prevail among parasitic birds. Parasitic cuckoos are significantly less brightly marked than are nesting cuckoos. Dull parasitic birds are less readily seen and recognized by hosts, and dull plumages also appease the hosts by absence of releasers of aggressive behavior. Inconspicuousness and appeasement increase the vulnerability of hosts to the brood parasites. The dull juvenal plumages in some parasitic birds may have been incorporated into the adult communication signals by neoteny. Evidence of neoteny among brood parasites includes prolonged retention of juvenal plumage, juvenal-adult-like species pairs, and the retention of juvenal-like incomplete skull ossification throughout adult life. Polymorphism and an unusual degree of variation in plumage are adaptations in parasitic cuckoos which reduce the probability of recognition by hosts. Polymorphism in Cuculus canorus and other parasitic cuckoos is considered to be the result of apostatic selection for deviant phenotypes. Polymorphism of cuckoo eggs is mimetic, and the tendency for the eggs of Chrysococcyx caprius to be laid in nests with matching eggs is statistically significant. Aggressive mimicry which deceives host species occurs in all stages of the life cycle of parasitic birds but mainly during the period of host parental care.