The breeding biology of the Mangrove Gerygone, Gerygone laevigaster, in the Darwin region, with notes on brood parasitism by the Little Bronze-cuckoo, Chrysococcyx minutillus

Abstract

Gerygones comprise 19 species of small (mostly 6–7 g) foliage-gleaning insectivores distributed from south-east Asia to New Zealand. I collected breeding data on the Mangrove Gerygone, Gerygone laevigaster, from several populations in and around Darwin, Northern Territory. The egg-laying season extended from March to October, coinciding with the late wet season and entire dry season, with peaks in April and October (n = 45). All populations showed repeat nesting after failure; one colour-banded pair made four attempts over one season. The species is apparently multi-brooded, one pair attempting a late brood after two previous successful broods. Most complete clutches (74%) were of two eggs, and the remainder, three. Most nests (84%) were built in Grey Mangroves, Avicennia marina, on the edge of saltflats, and 91% were in the top half of the nest plant. Incubation and nestling periods were 16–17 and 14–17 days, respectively. Of 31 nests whose fate was known, only three (10%) produced fledglings. Five broods (16%) were parasitised by Little Bronze-Cuckoos, Chrysococcyx minutillus; the incubation period for one cuckoo egg was about 15 days, while two nestling periods were estimated to be 18 and 24 days. Several nests were inundated by high spring tides, but predation was probably the major cause of nest failure: seven (23%) nests exhibited holes, six (19%) were destroyed, while eight (26%) showed no damage. The long, mainly dry-season, breeding of this species may be an adaption to greater insect availability at this time of the year, or avoidance of wet-season predators (such as reptiles), or both.