Welcome To Sumba’s Avian Housing Crisis

Finding suitable housing for raising a family is not only a problem faced by people: parrots are dealing with this too

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The citron-crested cockatoo, Cacatua (sulphurea) citrinocristata, is in serious trouble. Due to growing poaching for the pet trade since the mid-1970s, these cockatoos’ numbers plunged by 80% over the last 43 years. Additionally, this parrot is entangled in an ongoing battle with a variety of other wild bird species for access to increasingly scarce nest cavities to raise their families.

This lovely parrot is a popular — and increasingly expensive — pet and thus, its wild population is besieged by trappers and poachers, who learn where they nest and proceed to steal all their babies year after year, which suppresses overall productivity and recruitment of young birds into the breeding population. Thus, the citron-crested cockatoo is officially listed as Critically Endangered.

The citron-crested cockatoo is a medium-sized white cockatoo with an orange crest, pale orange ear patches, and pale yellow undersides on its wings and tail. The sexes are alike, although the sex of the adult cockatoos can be identified by eye color: females have a coppery eye color whilst males have a black eye color. Originally classified as the smallest subspecies in the yellow-crested cockatoo complex, this parrot was recently elevated to full species status.

The citron-crested cockatoo is endemic to the eastern Indonesian island of Sumba. This island is home to a number of mammals but it is especially rich in avifauna, with nearly 200 bird species known to live there. Seven bird species, including the citron-crested cockatoo, are found only on Sumba and on a few nearby islands, but nowhere else in the world. For this reason, Sumba is recognized as an important center of endemism and biodiversity in the region. But a growing number of its unique bird species are threatened by human activities.

Sumba has an ancient geological history. It is thought to be a 20 million year old fragment of the Australian continental crust that strayed north from the mainland (Figure 1), thereby isolating its unique biota. Sumba’s landscape is mostly comprised of low limestone hills that experience a dry season from May to November and a rainy season from December to April. Originally, most of the island was covered in deciduous monsoon forest while its south-facing slopes, which remain moist during the dry season, were evergreen rainforest. The western side of the island is more fertile and more heavily populated than the east. Sumba’s human residents have one of Indonesia’s lowest per capita incomes so they are primarily subsistence farmers, keeping large numbers of livestock, as well as growing maize, rice, cassava and other crops, which served as the impetus for clearing of most of the original pristine forest.

In recognition of growing threats to its endemic cockatoos from trapping, Sumba finally made it illegal in 1992 to trap them. Despite this protective measure decades ago, the Critically Endangered citron-crested cockatoo population has not recovered. Why? Was their productivity always low? Or is the effect of having a diminishing number of suitable nest cavities more severe than expected?

To answer these questions, a recent study by an international collaboration of scientists from Manchester Metropolitan University, BirdLife International, Burung Indonesia and Bogor Agricultural University (ref), documented interactions between citron-crested cockatoos and their nest competitors as well as all potential nest predators at 95 nest cavities. These interactions were captured using camera traps (Figure 3), and took place over one to as many as four breeding seasons for a total of 266 monitored cavity-seasons. The research team also relied on physical inspections of the nest cavities and observations from the ground.

Camera trap data were particularly informative. These data allowed the scientists to investigate occupancy rates and outcomes of potential cockatoo nest cavities, to identify seasonal cavity use across the community of large hole-nesting birds, and to establish visitation rates to nest cavities by potential competitors and predators. (It is interesting to note that Sumba hornbills were the most frequent visitors to active parrot nests, suggesting they may be a potential nest predator.)

Of the 95 nest cavities monitored, the research team focused their observations on 36 cavities with repeated cockatoo activity, but only 12 of those cavities were ultimately occupied by cockatoos (Figure 4). Although the cockatoos did inspect and prepare many cavities, they rarely attempted to nest in them, and ultimately, those cavities ended up being occupied by other cavity-nesting birds, or by bees.

As expected, visitation rates were higher at unoccupied cavities than at active nests, which were guarded fiercely — and loudly — by the occupants. Competition for cavities was intense and included a number of species, although most interactions ended up as screaming contests that took place in the treetops instead of direct violence (Figure 5).

Although rare, 30 direct confrontations between cavity nesting birds were recorded by the camera traps during the study period. The majority of these confrontations were won by Sumba hornbills, Rhyticeros everetti, an Endangered species, and by owls. But the most common confrontations occurred between citron-crested cockatoos and great-billed parrots, Tanygnathus megalorynchos. In these contests, the cockatoos almost always prevailed: winning 14 out of 16 observed conflicts.

Although the research team devoted approximately 300 person-days seeking nest cavities during four breeding seasons, they found it very difficult to detect whether the cockatoos were actively nesting. It is deeply troubling that the researchers ultimately found just 10 successful cockatoo nests and these nests fledged only 12 young in total during the entire study period. Similar patterns of low productivity have been reported in previous studies of these cockatoos during the past 15 years, which is an extreme reproductive imbalance even for a very long-lived species like a cockatoo.

The intense competition for tree cavities amongst Sumba’s cavity nesting birds found by this study indicates that suitable tree hollows are in short supply. This demonstrates that large old trees with cavities must be protected from logging, and reforestation with large cavity-forming tree species must be a conservation priority. Further, to ease the avian housing shortage in the short-term, the study authors suggest providing safe artificial nestboxes for the cockatoos. Artificial nestboxes also provide opportunities to better assess the scale of the nest cavity shortage, to allow for cameras to be installed in the nestboxes to monitor nesting progress and, of course, to prevent illegal trapping.

Considering that illegal trapping is still an ongoing problem, the scientists suggest that improved law enforcement targeting middlemen combined with educational programs amongst local communities to raise their awareness of the plight of these unique cockatoos could be most effective in protecting them.


Anna Reuleaux, Benny A. Siregar, Nigel J. Collar, Ani Mardiastuti, and Stuart J. Marsden (2022). Productivity constraints on Citron-crested Cockatoos in a rich community of large hole-nesting birds, Avian Research 13 | doi:10.1016/j.avrs.2022.100015


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