Gonadal and hormonal phenology in free-living male rufous whistlers, Pachycephala rufiventris (Passeriformes : Pachycephalidae)

Abstract

The hormonal and gonadal profiles of the few Australian old endemic passerine species studied to date have exhibited less annual variation in gonad size, reduced amplitude in testosterone (T) and luteinising hormone (LH) levels in comparison with those exhibited by many predominantly migratory species from the north temperate zone. Because none of the Australian endemics studied to date were migratory species, we studied a migratory population of rufous whistlers in central western New South Wales to determine whether differences in life history might be associated with different gonadal and hormonal profiles. Breeding stages were classified into five functional categories: territory/pair establishment, nest building, incubation, feeding nestlings/fledglings and post-breeding. We captured adult males at all breeding stages, examined their gonadal status through unilateral laparotomy, collected a blood sample and assessed this for plasma T and LH content using radioimmunoassay. Both hormones peaked early in the breeding season: LH levels during territory/pair establishment at 2.8 ng/mL 1.7 (s.d.) and T levels during nest building at 1.8 ng/mL 1.6 (s.d.). Following incubation LH and T (in particular) levels decreased substantially with the onset of parental duties – whilst feeding nestlings/fledglings T was 0.1 ng/mL 0.1 (s.d.), and LH was 1.6 ng/mL 1.2 (s.d.). This temporal variation in hormone secretion and an observed synchronous peak in gonad size at nest-building is reminiscent of migratory species from the north temperate zone displaying biparental care. Peak concentrations of both plasma T and LH in whistlers were intermediate between those reported for passerines from the north temperate zone with a similar life history and sedentary Australian old endemic passerines examined to date. This possibly reflects differences in life-history traits, the functional use of T in this species and/or possible phyletic effects.