Swift Parrot decline in Hobart substantiates gloomy predictions for the species

September 16th, 2020 | by David Waugh
Swift Parrot decline in Hobart substantiates gloomy predictions for the species
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You must really like Swift Parrots (Lathamus discolor) if throughout sixteen years you make a grand total of 20,155 observations of them. This accolade goes to Dr. Andrew Hingston of the University of Tasmania, who observed the species in the suburbs of Hobart in south Tasmania from 2002 to 2017, and has recently published1 his analysis and interpretations of the observations, in the context of what is happening to the Swift Parrot in Australia in general.

Swift Parrot adult feeding in Tasmanian blue gum.

Overall, the wild population of the migratory and nomadic Swift Parrot is in a nose-dive. In the year 2000 this species was listed as “Endangered” in the IUCN (International Union for the Conservation of Nature) Red List, with the major threat considered to be habitat loss, fragmentation and alteration happening in its wintering habitats of mainland south-east Australia and breeding habitats in Tasmania. A much more detailed appraisal of the situation was indicated, and in 2010 the Loro Parque Fundación commenced its support to an in-depth project to investigate the breeding biology and migratory behaviour of the Swift Parrot, led by Professor Robert Heinsohn of the Fenner School of Environment and Society, Australian National University.

Eucalyptus forest close to Hobart.

The findings of Prof. Heinsohn and his team confirm the forest destruction, in particular the unsustainable forestry and loss of suitable trees for breeding in Tasmania (even though the Tasmanian blue gum was proclaimed as the floral emblem of Tasmania in 1962). They also discovered and documented the major negative impact of nest-predation on chicks and adult female Swift Parrots by Sugar Gliders (Petaurus breviceps) introduced to Tasmania from mainland Australia. On the recommendation of the researchers, the Swift Parrot has now been given “Critically Endangered” status, because modelling predicts that its population is declining by more than 80% within three generations (12-18 years).

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Sugar glider.

Verification of the prediction by monitoring the entire population across its breeding range is difficult because of the nomadic behaviour of the species, and also that Swift Parrots are difficult to count because they are small, green, and forage high in the dense canopies of Eucalyptus trees. However, Dr. Hingston surmised that if the forecast decline in population size is so large, it could be possible to detect a similar rate of decrease in a relatively small area (8 km2) of the breeding distribution, the suburbs of Hobart, over the length of time on which the predicted reductions are based. His ample observations not only address the prediction of population decline, but also add useful knowledge to the existing information on the biology of this species.

Graph showing the variation in foraging between different months.

In all years, the first observation of a Swift Parrot occurred by 23rd September, and the breeding season ran from September to December. Observation of the largest flock each year occurred during the breeding season in 13 of the 16 years (median date of 4th October), and three individuals over-wintered in 2010. There were 10,208 instances where Dr. Hingston observed Swift Parrots foraging, and over 99% of those observations occurred in three species of Eucalyptus trees, the Swamp, Tasmanian Blue and Manna Gums (and their hybrids), with their flowers accounting for 91.2% of all foraging observations.

Graph showing the decline in the average size of the largest flock between 2002 and 2017.

Swamp Gum and Tasmanian Blue Gum were the major food sources between July and December, and in the latter part of the breeding season the parrots supplemented the diet by gleaning leaves of Manna Gum. Gleaning of leaves and bark appeared to be associated with infestations of the plant louse (psyllid) Cardiaspina sp., a tiny, sap-sucking insect which exudes honeydew. The plant louse larva produces a protective cover of crystallized honeydew known as a lerp. From the end of the breeding season the foraging activities changed markedly as the parrots used of a wider array of food sources.

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To examine population trend, Dr. Hingston used two measures of abundance during the breeding season: the average (mean) size of the largest flock, and the grand mean size of all flocks, observed per month from September to December. The mean largest flock size in the 2002 breeding season was 93 individuals, in the breeding seasons of 2004, 2005, and 2014 between 27 and 45 individuals, and for all other years less than 15 individuals. The result was a significant negative exponential relationship between year and mean largest flock per month in the breeding season, equivalent to an 87.3% decline in abundance over 15 years.

Graph showing decline in average flock size between 2002 and 2017, adjusted for yearly differences in flowering intensity of Eucalyptus trees.

Because the numbers of Swift Parrots present within the study site each year might have been influenced by variations in local food availability, the flowering intensities of the main food sources, the Tasmanian Blue Gum and Swamp Gum, were scored in every breeding season. The scores of flowering intensity were then used to adjust the values for abundance of the parrots. Even with such adjustment, the same downward trend could be seen. Furthermore, a similar pattern of decline was evident by using the grand mean of the size of all flocks.

This concurrence between Dr. Hingston’s local observations in the Hobart suburbs and the Heinsohn team’s modelling of the entire population substantiates the findings of the modelling, reinforcing that urgent action is required to prevent extinction of the Swift Parrot.

1 Hingston, A. B. (2019) Documenting demise? Sixteen years observing the Swift Parrot ‘Lathamus discolor’ in suburban Hobart, Tasmania. Australian Field Ornithology, Vol. 36: doi: http://dx.doi.org/10.20938/afo36097108.

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Author: David Waugh, Correspondent, Loro Parque Fundación

Photos credit: title , 1 – D. Stojanovic; 2 – Adavyd/Wiki; 3 – J.C. Boone-Wiki; 4-7 – Hingston, 2019

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