CONICET | Buscador de Institutos y Recursos Humanos

The strange-tailed tyrant, Alectrurus risora, is a tyrant flycatcher that inhabits savannas, wet grasslands and marshes in southern Paraguay and north-east Argentina. During the last century, its original range has contracted by 90% in association with conversion of natural grasslands to agriculture. At present, it is classified as vulnerable and further decline in range and number is expected because suitable habitat is subject to current extensive agricultural intensification and afforestation. Knowledge of the factors that affect the reproductive success of threatened species is necessary for accurate assessments of the extinction risk of their populations. We studied the breeding biology and analyzed factors that affect nest survival of strange-tailed tyrants in a protected area of the humid Chaco, Formosa Province, Argentina. Our study comprised eight consecutive breeding seasons and the monitoring of 199 nests. This species has a social polygynous mating system, with males owning contiguous territories with up to three females. Females bred in the same area over several years, but males rarely remained in the area in consecutive years. Modal clutch size was three eggs and decreased with time of breeding. Hatching rate and chick survival were 0.86 and 0.89, respectively. Nest survival (modelled with MARK) was 0.2 and decreased with age of the nest and time of breeding. Females had on average 2.3 nesting attempts per season and in most cases renesting occurred after a failed attempt. Considering nest survival, number of nesting attempts and number of chicks that fledged in successful nests, each female fledged approximately 0.9 chicks per breeding season. Turnover of males across breeding seasons, renesting attempts by females and low variance in number of chicks fledged in successful nests may reduce the effects of polygyny and high nest failure on variation in reproductive success of males and females.

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