Persistence, size, trends and productivity in populations of two Critically Endangered Indonesian cockatoos

Reuleaux, Anna (2023) Persistence, size, trends and productivity in populations of two Critically Endangered Indonesian cockatoos. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The aim of this thesis was to understand patterns of persistence, size, trends and productivity in populations of two Critically Endangered and heavily traded Indonesian cockatoos: the Yellow-crested Cockatoo Cacatua sulphurea and Citron-crested Cockatoo Cacatua citrinocristata. The Yellow-crested Cockatoo was formerly common and widespread on many islands of Wallacea. Its former subspecies on Sumba, the Citron-crested Cockatoo, was recently elevated to species level. After dramatic declines information on the remnant populations of both species is essential to guide future conservation management. A challenge with species that have disappeared from most of their range is to identify correlates of local persistence. The Yellowcrested Cockatoo and Citron-crested Cockatoo have undergone catastrophic declines due to habitat loss and especially trapping. The former is now extinct in much of its range across Indonesia while the latter has lost substantial numbers and some of its old sites on Sumba. Of 144 sites supporting cockatoos in 1950 only 76 did so in 2015. I compared socio-ecological conditions between the occupied and unoccupied sites, using 'random forests'. Tree cover, sparse human densities and low road densities promoted cockatoo survival but site-specific conditions (e.g. sacred groves, NGO activities) were also important. These local influences offer exciting possibilities for low-cost conservation prescriptions tailored to individual sites. One of the few Yellow-crested Cockatoo populations still numbering >100 individuals survives on Komodo Island. Distance sampling was combined with density surface modelling (DSM) to predict local densities and estimate total population size for this island. The population estimate of 1,113 (95% CI: 587–2,109) individuals on Komodo was considerably larger than previous conservative estimates. Coincidence between the DSM and a set of independent cockatoo observations was high (93%). Standardised annual counts by national park staff showed increases in cockatoo records from <400 in 2011 to ~650 in 2017. Taken together, the results indicate that Komodo National Park, alongside and indeed because of preserving its iconic Komodo Dragons Varanus komodoensis, is succeeding in protecting a significant population of Indonesia's rarest cockatoo species. This study's findings highlight the potential of DSM for locating abundance hotspots and estimating global population size in a range of threatened taxa. Although the importance of long-term monitoring is widely recognised, very few tropical bird species have been monitored over the span of 25 years. A multi-species distance sampling survey from 1992 was replicated in 2017, and present data on five parrot species and a hornbill, with three threatened island endemics, Citron-crested Cockatoo Cacatua citrinocristata (CR), Sumba Eclectus Eclectus cornelia (EN) and Sumba Hornbill Rhyticeros everetti (EN), two restricted-range species, Great-billed Parrot Tanygnathus megalorynchos and Marigold Lorikeet Trichoglossus capistratus, and one Wallacean-Papuan species, Red-cheeked Parrot Geoffroyus geoffroyi. Densities of the three large parrots and the hornbill in 2017 were similar to 1992 but densities of the smaller Red-cheeked Parrot and Marigold Lorikeet declined significantly in the same time span. Quantity of forest on Sumba is assessed for both years, presence data and local density estimates to gauge island-wide population sizes. The Marigold Lorikeet may need a re-assessment of its global Red List status. The uncertainty in trends for the threatened cockatoo, eclectus and hornbill is of concern as they show no definite sign of sustainable recovery from the extensive trapping of past decades. Knowledge of breeding success and its limiting factors is crucial in assessing species' conservation needs. As hole-nesters, parrots are particularly influenced by the availability of suitable cavities and low breeding output. On Sumba, the Citron-crested Cockatoo has to compete with an unusually rich hole-nesting bird community affected by forest loss. Ninety-five nesting cavities of cockatoos were monitored including competitors and potential nest-predators, over one to four breeding seasons, using a combination of camera-traps, direct checks on nest contents, and observations from the ground. Competition for suitable cavities was intense among three large parrot species, two owls and a hornbill. The Endangered Sumba Hornbill dominated observed direct confrontations and was the most frequent visitor to active parrot nests, suggesting a further role as a potential nest-predator. Cockatoos prospected many cavities but rarely then attempted to nest. At the few cavities where cockatoos did breed, predation pressure was likely low, and observed success rate high (10 successful of 15 nests). Intense competition for cavities suggests a shortage of suitable nest-sites, the need to preserve old hole-bearing trees and a role for nestboxes. Both studied cockatoo species would benefit from targeted local awareness-raising and law enforcement, with the whole endeavour backed up by longer-term forest restoration. The recent split of the Citron-crested Cockatoo leaves the resultant two species at higher risk of extinction than when they were assessed combined. The population on Sumba remains under pressure from illegal trapping, habitat loss, nest site competition and appears to have low productivity. Only two of the six Yellow-crested Cockatoo subspecies retain wild populations > 300 individuals but for each subspecies recovery is still possible, if conservation management tailored to each location is implemented, including public awareness programmes, provision of nestboxes, logistical and capacity building support, habitat protection and law enforcement.