Fluctuación anual de la masa corporal, tamaño de gónadas y muda de Guira guira (Aves Cuculiformes: Cuculidae)

The relationships between body mass, gonadal size and postnuptial molt of Guira Cuckoo Guira guira are described based on 92 specimens (53 males and 39 females), collected in Magdalena district (Buenos Aires province) during an annual period from 1991 to 1997. Considering the yearly average, no significant differences were found between body mass of males and females. Two periods in variation in the size of the gonads for each sex were determined: one of increased activity (September-January) and the other of sexual inactivity (April-August). Males showed an earlier gonadal development than females. The first egg laying began in October. In mid-December, a decline in the size of the gonads in relation to the onset of postnuptial molt began and peaked in March. The molt was asymmetric; 85,8 % of the studied specimens were in molt. The process was extended through May.