IDENTIFICATION

VOCALISATIONS

The usual call is a harsh, loud and grating ‘graaarr’ uttered in flight. Similar calls are given when perched.

Xeno-canto does not allow free access to vocalisations of this species.

DISTRIBUTION & HABITAT PREFERENCE

The core range is around Central, HK Island, especially in and close to HK Park and the Zoological and Botanical Gardens (ZBG). Yellow-crested Cockatoos are also frequently seen elsewhere in the western half of HK Island east to Braemar Hill and south to Aberdeen, and in Kowloon, especially Kowloon Park. Observations in the New Territories are unusual and it is not clear whether these refer to individuals wandering from the established HK Island population or to ex-captive birds.

Yellow-crested Cockatoo was reported from 1.5% of squares in the 1993-96 breeding atlas and 0.7% of squares in the 2016-19 breeding survey, suggesting a contraction in range between the two periods. Winter atlas data (1.3% of squares in 2001-05 and 0.8% in 2016-19), shows a similar, albeit smaller, decline. The main change, notable especially in the breeding season surveys, was a reduction in the number of birds seen and no evidence of breeding in Kowloon, but there is no evidence to suggest that this indicates a decline in the HK population as a whole.

In keeping with its largely urban range, Yellow-crested Cockatoo is most often encountered in parks and roadside trees or seen flying through the urban area where it makes frequent use of man-made structures such as buildings and lamp standards. It is also in found on the edges of Country Parks on HK Island, though it makes limited use of natural secondary woodland in these areas, probably because, as yet these support relatively few trees large or old enough to have developed cavities suitable as potential nest sites (Andersson 2021).

OCCURRENCE 

As one of the most conspicuous, attractive and exotic extalimital introduced species and given its global rarity, the origin of the HK population of Yellow-crested Cockatoos has attracted considerable interest in recent years. Accounts in the popular press have stated that the population is descended from birds released at Flagstaff House (formerly the residence of the commander of the British armed forces, now the Museum of Teaware), Central immediately prior to the Japanese invasion in 1941. The first published reference to this origin story appears to be Graham (1976) who stated that “These birds originally resided as pets in Flagstaff House, Victoria Barracks. In 1942, at the time of the Japanese Occupation, the British released them, unsure of the birds’ fate. Remarkably, the birds have lived in the lower Mid-Levels area ever since.” Subsequently, Chalmers (1986) stated that the population possibly originated from this source, albeit supplemented by more recent escapees; this suggestion was followed by both Viney et al. (1994) and Carey et al. (2001).

Sadly, perhaps, this rather engaging story is wholly without foundation: in fact the true origin of the population of Yellow-crested Cockatoo is much more prosaic and is well-documented in the HKBR 1961 - 1983.  There were no reports of cockatoos Cacatua sp. in HK until two birds were reported on HK Island in 1961 and 1962. In both years the observers noted that one bird was larger than the other and only the larger bird bird had a yellow crest. Since the smallest all-white cockatoo, Tanimbar Corella C. goffiniana, is of a similar size to Yellow-crested Cockatoo, it does seem possible that the larger bird was in fact a Sulphur-crested Cockatoo C. galerita. However, four Yellow-crested Cockatoos were reported in the then Victoria Barracks (now Hong Kong Park) in 1963. Though its status was initially somewhat obscured by a report that five birds were temporarily released on a daily basis by their owner (Webb 1967), small, but slowly increasing numbers were reported annually in the same general area for the remainder of the decade, with a high count of ten birds in 1969.

Numbers increased slowly in the 1970s, with a minimum of 21 birds reported in 1972, though this total was not exceeded until 1979 when 30 were recorded. There was also some geographical spread, notably to the former Stonecutters Island where up to six were reported and breeding was suspected from 1974. During this decade, it became apparent that in addition to yellow-crested birds, orange-crested birds, then treated as subspecies C. s. citronocristata of Yellow-crested Cockatoo, but now considered a distinct species, Citron-crested Cockatoo C. citronocristata, were present. HKBR reports indicate that this taxon was first seen in 1972, but it was stated by Webster and Phillipps (1976) that orange-crested and yellow-crested birds were equally common. If this was indeed so then, this situation lasted for only a brief period; in 1983 the maximum count of cockatoos was of 31 birds at Victoria Barracks and observations elsewhere suggested that the population had reached around 50 individuals, but there was only a single record of citronocristata, and this taxon does not seem to have been reported since the 1990s.

Unfortunately, in 1984-89 records of this (and most other) species were not published in the HKBR; however, Chalmers (1986) suggested that the population comprised about 50 individuals on the north side of HK Island, with smaller numbers elsewhere, largely on Stonecutters Island and Kowloon. Carey et al. (2001) described a similar distribution, but suggested that the population was in the order of 60 to 100 birds, with the highest count being of 53 birds in Victoria Barracks on 23 September 1987. Subsequently, Leven and Corlett (2004) estimated that the population was about 150 birds and was slowly increasing; however, simultaneous roost counts conducted in 2011 produced a maximum count of 108 birds (Mok et al. 2012). Both Gibson and Young (2017) and Andersson (2021) also suggested that the population was around 150 individuals, with the latter estimating that the HK Island population was about 120 birds (Stonecutters Island, where Mok et al. (2012) counted a roost utilised by up to about ten individuals, was excluded from Andersson’s study due to access difficulties). These estimates indicate that the population has been broadly stable in the present century; Andersson (2021) considered that it was still slowly increasing; however, further surveys to monitor population trends would be desirable.

BEHAVIOUR, FORAGING & DIET 

Yellow-crested Cockatoos are usually encountered as single birds, pairs or as parties of up to about ten birds, in flight (when they can be very vocal) or foraging in large trees, especially Cotton Tree Bombax ceiba, likely ecologically the single most important component species in its environment, providing nest sites, food and roost sites.

Whilst birds usually forage in pairs or as small parties, roost sites are communal and are used thoughout the year except by breeding pairs. Much the largest roost site, utilised by up to  88% of the HK population, is located in or near Hong Kong Park, with smaller roost sites noted in the ZBG, University of Kong Kong, Stonecutters Island and perhaps Kowloon Park (Mok et al. 2012). Birds assemble at pre-roost locations in the late afternoon, before flying to roost sites; usually all birds roost at the same location, but sometimes the pre-roosting flock splits between two or three sites. Roost sites are usually in trees, especially Bombax ceiba, but lamp standards and the roof of a building have also been utilised. Roosts may occupy one or more trees, and exact roost locations vary frequently, as noted by Wang and Chu (2021), who also showed that roost site switching was seasonal, with spring roosts in greener areas, summer roosts in areas with higher artificial light levels (thought to be associated with predator avoidance, especially, for recently-fledged and more vulnerable young), and winter roosts in locations with higher microhabitat temperatures (perhaps an adaptation to the relatively cold HK winters by a species with a natural range in the uniformly warm tropics).

Yellow-crested Cockatoo has a vegetarian diet, feeding on buds, young shoots, flowers and nectar and both fleshy fruit and dry seeds (Juniper & Parr 1998). In HK, Wang et al. (2018) showed that flowers and fruit are preferred to other plant material but considered that in their diet they are non-specialist and flexible and noted foraging on 11 plant species, most of which are non-native. However, over 75% of the diet was provided by four tree species: Broussonetia papyrifera, Ficus variegata, Liquidamber formosana and, especially, Cotton Tree Bombax ceiba, which was utilised throughout the year at different stages in its phenological cycle.

BREEDING 

According to the HKBR, breeding was first suspected in HK on Stonecutters Island in 1974 and 1975 and proven at the then Victoria Barracks in 1976, albeit Graham (1976), who referred to what was presumably the same breeding pair, stated that she was shown one young bird and noted that the finder of the nest told her that the same nest site had been used successfully in 1975.

Yellow-crested Cockatoo is an obligate secondary cavity-nester (i.e., it does not excavate a cavity de novo but relies on an existing cavity formed either by natural damage or decay or one which has previously been excavated by another species, such as a barbet or woodpecker). All nests found in HK have been in natural cavities formed by decay or damage in large trees (the HK range of Yellow-crested Cockatoo does not now overlap with any barbets or woodpeckers, though it may have overlapped with Great Barbet in the 1960s), though there are anecdotal accounts of birds prospecting in, and being deterred from utilising, cavity-like sites in buildings (Andersson 2021).

Andersson (2021) found active nest sites in the trunks of eight tree species (plus one dead tree of unknown species): the exotic Bombax ceiba, Ceiba pentandra, Parkia javanica, Spathodea campanulata and Terminalia mantaly, and the native Cinnamomum camphora, Litsea monopelata and Ficus virens. Of 16 nest holes, five were in Bombax ceiba and two in the structurally similar Ceiba pentandra. Interestingly, no nest holes were found in Chinese Banyan F. microcarpa, despite large specimens of this species being abundant in the study area (M. R. Leven pers. obs.). All nest sites were in large trees (defined by Andersson as trees with a diameter at breast height (DBH) of greater than 50cm), with a mean of 81cm DBH. As such, the distribution of large trees may currently be a factor limiting the range of Yellow-crested Cockatoo on HK Island (Andersson 2021), albeit, as she stated, secondary woodland there is now approaching an age (c. 80 years) where frequency of large trees may soon increase.

Andersson (2021) showed that Yellow-crested Cockatoo is usually dominant over other species at potential nest sites, suggesting that it is not threatened by exclusion from suitable nest sites by competitors; however, the supply of large old trees with suitable hollows in its core range in urban parks might be a limiting factor, especially as park management practices usually favour the removal of dead limbs or trees for arboricultural or safety reasons. Further, at least in the past, hollows in some city parks were deliberately blocked to prevent access by fauna.

There is little information on the breeding cycle in HK and its timing. The first nest found contained a chick thought to be about three weeks old on 24 June, whilst a young bird considered to be newly-fledged was found at the same site on 25 May in a subsequent year.

RANGE & SYSTEMATICS 

There are five subspecies all of which are confined to islands in Wallacea (Indonesia and East Timor): nominate sulphurea, abbotti, djampeana, occidentalis and parvula. In addition to HK there is another established population outside its native range, in Singapore. However, the Singapore population, which has been present since around 1988, comprises fewer than ten individuals and has shown no signs of increasing, perhaps because competition for nest sites with the introduced Tanimbar Corella and the reintroduced Oriental Pied Hornbill Anthracoceros albirostris (Andersson 2021).

It is not currently known which taxon or taxa are the source of the HK population. The race abbotti is distinctly larger than the other taxa (Juniper & Parr 1998) and can probably be excluded, but the other races may not be seperable in the field on current knowledge.  The closely-related Citron-crested Cockatoo Cacatua citronocristata, formerly treated as a race of Yellow-crested Cockatoo, co-occurred with Yellow-crested Cockatoo in HK in the 1970s – 1990s but has now disappeared; it is not known if the two species interbred.

CONSERVATION STATUS 

IUCN: CR. Population trend decreasing. World population is estimated at 1200 – 2000 individuals (BirdLife International 2021).

With a population of 100 – 150 individuals (c. 10% of the global population), HK has an important role in the conservation of Yellow-crested Cockatoo, its rarest breeding species in a global context. This responsibility is recognised by HKSAR Government, academic institutions and non-governmental conservation organisations, as is evinced by the recent HK studies used in the preparation of this account. Looking forward, information gaps and conservation priorities within HK include, but are not limited to: ongoing population monitoring (as is underway (Mok et al. 2012)); genetic analysis to establish which taxon or taxa are present; enforcement of protection (trapping, albeit perhaps low-level, still occurs (e.g. Andersson 2021)), and park management protocols to retain and replace nest trees and potential nest trees, including retention of dead and moribund trees where this is practical and consistent with public safety.