AbstractWhilst passerine birds of Australia's temperate regions, like those of the North Temperate region, breed predominantly during the warm spring and summer months, those of the Australian monsoon tropics, like those of the New World tropics, show diverse breeding seasons, in response to year-round warm conditions (Noske & Franklin 1999). Given this disparity in breeding seasons, it might be expected that tropical and temperate Australian birds differ in other aspects of reproduction. The present study compared the breeding characteristics of two Gerygone (warbler) species found in tropical mangals (mangrove communities) with those of their temperate congeners, and evaluated the roles of nest predation and food availability in shaping the breeding characteristics of the tropical species. The study took place in Darwin, where 90% of the annual rain (1600 mm) falls in the wet season, November-April. Mehods included field observations of colour-banded birds, and experiments using artificial nests, eggs and model birds.
The large, conspicuous nests of Large-billed Gerygones Gerijgone magnirostris (Pardalotidae) were suspended over, or close to, tidal creeks within mangal forests, whereas the smaller nests of Mangrove Gerygones G. levigaster were built in small mangroves on or around barren saltflats, often in isolated plants. Breeding was uniquely biannual in both species, with peaks in the wet-dry and dry-wet transition periods, and reproductive activity was minimal during the wettest months (December-February). Large-billed Gerygones had a longer breeding season and a longer incubation period (19 days) than Mangrove Gerygones (16 days), but both had clutches of 2-3 eggs and nestling periods of 15 days.
Nest success was very low for both species (17-18%), the result of high rates of nest predation and brood parasitism, and flooding by high tides. Nests of Large-billed Gerygones closest to tidal channels suffered higher losses, partly due to tidal flooding, while those built beside old nests were more successful than solitary nests, possibly because old nests signify safe locations. Both species suffered high levels of brood parasitism (34-41%) by the Little Bronze-Cuckoo Clzrijsococcijx minutillus, a gerygone specialist. In addition, cuckoos at least occasionally killed gerygone nestlings. Several nests with 2 or 3 cuckoo eggs suggested female cuckoos competed for host nests. Artificial "cuckoo" eggs were rejected by some Mangrove Gerygones, but not by Large-billed Gerygones, suggesting differential discrimination between the two species, yet both species attacked a taxidermic mounted specimen of the cuckoo. Newly-hatched cuckoos possessed down feathers and closely resembled the hatchlings of the Large-billed Gerygone, but were less similar to those of the Mangrove Gerygone. Whether the former represents nestling mimicry or anti-predator crypsis requires testing.
Dietary data from the literature suggest that the Large-billed Gerygone uses its large bill to capture larger, hard-bodied insects (such as beetles) than does the Mangrove Grygone, which eats mostly smaller, softer prey (such as bugs). Insects in the mangals peaked in abundance and biomass during the late dry-early wet season, corresponding to the second breeding pulse of the gerygones. Small flying insects showed another peak, coinciding with the first breeding pulse. Breeding of Mangrove Gerygones was correlated with the seasonality of small flying insects, consistent with their putative diet. Insects were more abundant, but fluctuated more, on saltflats than along tidal creeks.
Consistent with the tropical-temperate life history paradigm, these tropical gerygones showed much longer breeding seasons, slightly smaller clutches, lower nest attentiveness, and higher nest predation rates than their temperate counterparts, but their incubation periods and nestling provisioning rates were shorter and higher, respectively, than expected. Nest attentiveness (percentage of time on the nest) of the Large-billed and Mangrove Gerygones was exceptionally low (38-40%), but incubation behaviour suggests that the risk of nest predation by diurnal predators was less important than other considerations (e.g. energetic value of food or stable nest temperatures).
Several lines of evidence confirmed the importance of nest predation in the life history of the study species. For Mangrove Gerygones, nests in isolated plants were more successful than those on the edge of saltflats or inside the mangal forest, and the distance of the nest plant from the nearest plant was the most important variable explaining nest success. The results of artificial nest experiments suggest that nest predation was lower on saltflats than along tidal creeks, providing an explanation for the popularity of saltflats as breeding grounds for both insectivorous and non-insectivorous bird species. Nest predation of artificial nests was lowest during the early dry season, coinciding with the first breeding pulse of gerygones, and lower in urban than more natural (non-urban) sites, possibly due to fewer predators occurring in the former sites.
Generalised linear models suggested that rates of nest predation and brood parasitism were the most important variables accounting for the timing of breeding of the Large-billed Gerygone, while abundance of small insects was possibly more important for the Mangrove Gerygone. These and other differences (such as in incubation period and nest characteristics) between the two species may relate to their proposed divergent phylogenetic history, in which the Large-billed Gerygone probably evolved in rainforests while the Mangrove Gerygone may have been derived relatively recently from an arid-adapted ancestor. Differences with their temperate counterparts are more easily explained by adaptive radiation within the continent, than by reference to a global tropical versus temperate life history dichotomy. Further study would be required to determine whether adult survival and extent of parental care in these tropical species are as high as, or higher than, other tropical and Australian birds.
|Date of Award||Aug 2004|
|Supervisor||Richard Noske (Supervisor)|