New Publication: Good Times, Bad Times - Tracking Endangered Parrots in Tasmania
A little while ago I started a collaboration with my good friends and colleagues at Australian National University looking at the foraging behaviour and nesting success of endangered swift parrots (Lathamus discolor). I joined Dr Dejan Stojanovic (of the Difficult Bird Research Group) in climbing some pretty huge trees and later on helped out analysing movement data. That work has now been published in the Journal of Zoology. We didn’t have the spare funds to pay the huge fees to make the paper open access but feel free to drop me a line and request a copy if you are interested.
Swift parrots are one of the few migratory parrot species in the world along with orange bellied parrots and blue winged parrots. They breed in Tasmania and migrate across the sea to mainland Australia every year. They feed on ephemeral blooms of eucalyptus trees which are highly variable both in where they occur and how many trees bloom in a given year. This gives rise to good and bad years for the parrots raising chicks and they need to be able to respond to these fluctuations. They have to weigh up the costs and benefits of raising chicks against their survival. If the costs of reproduction are too high, animals should prioritize their own survival. During bad times, mobile animals may be able to compensate for local food shortages by travelling further to provision their offspring.
We studied parental investment and breeding success in the swift parrots over two successive years at the same site where food abundance went from locally low to high. We used GPS trackers to collect fine-scale movement data, getting fixes every 10 mins. We quantified parental investment by calculating how far and fast each bird travelled to find food, how long they spent foraging, the time between visits to the nest and how long they spent on the nest.
We looked at reproductive success by measuring clutch size, brood size, number of fledglings and the body condition of the chicks. We hypothesized that in a bad year, swift parrots should adjust their parental strategy by foregoing breeding altogether, producing smaller clutches/broods or reducing provisioning investment.
Fewer swift parrots bred locally when food was scarce. In the bad year, clutch and brood sizes were smaller and nestlings were >20 g lighter (approximately 28% of mean body mass) than in the good year. Compared with the good year, fathers spent longer foraging, less time at the nest and travelled further during provisioning trips in the bad year.
Our results suggest that mobile species may attempt to mitigate the effects of a bad year on their reproductive success by rearing fewer offspring and investing more in provisioning behaviour, but this strategy may not necessarily compensate for environmental conditions. Stopping the deforestation of important breeding sites in Tasmania will cushion the impact of these resource fluctuations for the parrots and give them more options as to where and when they breed.