When birds go bad: circumstantial evidence for infanticide in the communal South-American Guira Cuckoo R.H.F. MACEDO and C.A. BIANCHI Departamento de Zoologia, Universidade de Brasília - UnB, 70910-900 Brasília, D.F., Brasil Received 11 November 1995, accepted 7 November 1996 The Guira Cuckoo (Guira guira) is a communal breeder (joint-nester) that experiences high rates of egg loss due to tossing behaviour of individuals in the group, as well as exceptionally high mortality among nestlings. Here, we report events leading to the complete loss of broods in nine nests, resulting in the death of 48 nestlings. Circumstantial evidence surrounding each episode is suggestive of infanticidal behaviour by adult group members. No evidence of predation was noted, and the occurrence of dead nestlings in the nest, or on the ground, sug- gests that group members may adopt infanticide as a reproductive tactic. Many groups renest at least twice during the extended rainy season, with renestings observed 18 times during 68 group-years. Hence, individuals may have repeated opportunities for breeding successfully. We predict that infanticidal behaviour should be executed by those individuals excluded from reproduction within their groups. A new nesting episode, initiated within a shorter time interval, would presumably benefit the excluded individual(s) by providing a new breeding opportunity. Direct evidence is needed to verify the infanticide hypothesis. KEY WORDS: infanticide, parental effort, Guira Cuckoo, Guira guira, communal breeding, Brazil. Introduction . . . . . . . . . . . . . . . . . 45 Study area and methods . . . . . . . . . . . . . . 47 Results . . . . . . . . . . . . . . . . . . 48 Discussion . . . . . . . . . . . . . . . . . 51 Acknowledgements . . . . . . . . . . . . . . . 52 References . . . . . . . . . . . . . . . . . 52 INTRODUCTION The conventional definition of infanticide is any “behaviour that makes a direct and significant contribution to the immediate death of an embryo or newly hatched (or born) member of the performer’s own species” (MOCK 1984). Infanti- Ethology Ecology & Evolution 9: 45-54, 1997