IDENTIFICATION

VOCALISATIONS

Displaying birds can be heard early in the breeding season, mainly from October to January. The calls are loud and slightly nasal, becoming faster when birds are in the air.

DISTRIBUTION & HABITAT PREFERENCE

Mainly recorded in eastern coastal areas, with the known breeding locations extending from north and south Lantau to HK Island and eastern sea areas from the Ninepins north to Yeung Chau (Crooked Island) in Mirs Bay (So and Lee 2010). It is likely that the longer coastline and greater number of islands is a factor in the higher numbers in eastern areas.

Away from these areas, adults and young are irregularly reported in Victoria Harbour and Deep Bay, as well as at several of the larger reservoirs and occasionally at inland sites up to 10km from the coast.

OCCURRENCE

Most reports refer to one or two adults or immatures, but occasionally four to six are seen together. These usually comprise pairs in territorial disputes or display.

So and Lee (2010) estimated 15 breeding adult pairs in 2009, an increase from 23 adults in 2003. The number of immatures and juveniles was estimated by adding the number of fledglings produced in the previous five years and calculated to be 27, making a total maximum population of 57 birds in 2009. However, the actual population was stated to be probably lower due to mortality. Chow (2014) recorded 16 breeding pairs in the 2011/12 breeding season, the highest on record in HK.

The first record in HK is a specimen in the Styan Collection at the British Museum Natural History taken some time between 1887 and 1903. Vaughan and Jones (1913) reported this species but did not find a nest site in HK. The number of breeding pairs increased from three in the 1930s and 1960s to ten in the 1980s and 1990s, most likely because of a decline in nest-robbing and disturbance by grass-cutters, which were significant problems in the past (Aylmer 1932, Taylor 1933), the eggs of this species reputedly being of value in traditional Chinese medicine (Herklots 1953).

BREEDING

Displaying adults have been reported from late September to mid-April. Nest-building has been observed from early October to early January, while incubation has been reported in the second half of December. Based on an incubation period of around 40 days, egg-laying is presumed to occur from mid-October to January as the clutch, which usually consists of two eggs, has been recorded hatching from late-November to mid-March. Both male and female share the responsibility of incubating the eggs (Chow 2014), after which the young may then remain in the nest for as long as 85 days, with fledging reported most often in March (Aylmer 1932, Taylor 1933, Dale 1965). Tsim et al. (2003) noted that an adult spent on average 46% of daylight hours incubating and 19% foraging. A nestling period of around 75 days was noted by Tsim et al. (2003), who also stated juveniles attain independent foraging ability in around 21 weeks, after when they are seldom within 2 km of the nest. Extra-pair copulation has been recorded once: a paired female was recorded copulating with an intruding male three times in a single breeding season (Tsim et al. 2003).

So and Lee (2010), as part of their investigation of the breeding ecology, reported a total of 15 territories across HK in the 2008/09 breeding season. Distances between territories ranged from 3.4 km to 18.7 km, a relatively dense distribution. Of the 15 territories, seven supported successful breeding pairs that produced eight fledglings. A total of 25 nesting sites were located from 2002/03 to 2019/2020, of which approximately half were in protected areas; the number of active breeding pairs each year ranged from six to 20, with a mean of nine (So and Yuen 2020). In the same period fledglings were produced by zero to seven pairs per year at an annual average of three pairs and four fledglings.

So and Lee (2010) noted that breeding success is variable and ranged from 14.3% of breeding pairs to 87.5%. The number of fledglings raised successfully ranged from one to ten (mean of five). Approximately 60% of the time only one of the two chicks survives. Factors cited as impacting breeding success are human disturbance, habitat quality (pairs nesting in wooded habitats produced more young birds than those in more open habitats) and weather. Cold weather was cited by So and Yuen (2020) as possibly causing territory-wide failure in four breeding seasons this century.

All nests reported by So and Lee (2010) were in natural habitats apart from a pair that nested in a cable tower at Tin Wan, Aberdeen. Most faced west, presumably due to northeast winds prevailing in the winter (Tsim et al. 2003). Most nests were in living or dead trees, with one present on a rocky ledge and another on the ground. In contrast, the typical nest site reported in the 1930s was a rocky ledge, presumably due to the paucity of mature woodland habitat at the time. Five tree species were reported as nesting sites by Tsim et al. (2003): Ficus microcarpa. F. variegata var. chlorocarpa, Acacia confusa, Machilus sp. and Cinnamomum camphora. Nests may be used for several years, and refurbishment work may cause a nest to grow into a large structure up to two metres wide and 3.5 metres deep (Aylmer 1932, Taylor 1933, Dale 1965). Some nest sites are not used annually, and several were abandoned for over five years (So and Yuen 2020). Many nests are on offshore islands, suggesting a need for undisturbed locations, while the average height above sea level (range 8-80 m) and distance from the coast (range 5-350 m) was 40 m in both (Tsim et al. 2003). Observations suggest that if a member of the pair leaves or dies, the remaining partner may hold the nesting territory in anticipation of a new mate, which may explain why some nest sites are not used every year (Chow 2014). 

BEHAVIOUR, FORAGING & DIET

Food items identified from remains found at a nest in the winter of 1959-60 were cuttlefish, crustaceans and fish such as croaker, hairtail, puffer, moray eel and garfish. Other food items noted are ‘large Macao Crab’, skate and a sting ray (Taylor 1933), sea-snakes and carrion (Herklots 1967). In a radio-tracking study of a juvenile in HK in 2002, daily flight distances were 3-15 km, with a mean of 8.8 km (Chow 2014).

SYSTEMATICS & RANGE

Monotypic. Resident in coastal areas from western India east to southeast China and south through the Philippines, southeast Asia, the Greater and Lesser Sundas and Australia (Debus and Kirwan 2020). In China resident in coastal areas of south China, including Hainan and Taiwan (Liu and Chen 2021).

CONSERVATION STATUS

IUCN: Least Concern. Population trend decreasing.