IDENTIFICATION

VOCALISATIONS

The classic call is an interrogative ‘kwok’ or variants of this uttered when leaving or returning to the roost or on nocturnal migration.

DISTRIBUTION & HABITAT PREFERENCE

Occurs throughout HK in a variety of wetland or wetland-edge habitats. Highest numbers occur in the Deep Bay area, with the largest roosts occurring in the mangroves; smaller roosts occur in undisturbed trees and bushes around inactive fish ponds. Forages in freshwater marsh, fresh and brackish water fish ponds, calm coastal waters and natural and man-made watercourses. Large roosts also occur in other mangrove areas such as those on Lantau.

Black-crowned Night Heron forages in both open wetland habitats such as fish ponds, drainage channels and both man-made and natural rocky shores, as well as in more vegetated habitats such as the edge of mangrove and in freshwater marsh. Appears to shy away from narrow watercourses in shrubland or forest except at the edge.

OCCURRENCE

Although Swinhoe (1861) described Black-crowned Night Herons as nesting in some numbers at Guangzhou, and Kershaw (1904) and Vaughan and Jones (1913) described it as a common resident along the Guangdong coast, the first mention of its occurrence in HK is that of Herklots (1941) who stated that while it certainly bred, it was ‘apparently much commoner in winter’. In addition, he quoted records from Cheung Chau as indicating it was also a passage migrant. Macfarlane and Macdonald (1960) also stated that the species was a passage migrant but that there were also breeding records. However, at approximately one per year, the number of records submitted to the HKBWS during the 1960s was somewhat lower than might be expected, and the peak count at this time was 70 in Deep Bay on 26 December 1967. Most other records were of just one or two birds, and there were no breeding reports.

Numbers increased significantly during the course of the 1970s, such that in 1976 a peak count of 300 was made at Mai Po during 5-6 April and at Mong Tseng on 24 October, with 241 at Mai Po on 8 October 1978. Subsequently, the number present has generally been at least as high, and generally higher, though quite how much of this is due to increased observer activity and reporting is unclear.

During the ten years up to 1978 there was a paucity of winter records; however, by 1 January 1980 the number of wintering Night Herons had increased to 200 birds in the Mai Po mangroves. The number of records continued to increase during the period 1978-1987, with most in spring (33%) and autumn (33%), and fewer in summer (15%) and winter (19%). Peak counts in spring and autumn were 250 at Yim Tso Ha on 28 May 1983 and 400 at Mai Po mangroves on 9 October 1982.

In the period since 1988 the records submitted occurred throughout the year. The highest spring count away from Starling Inlet (where many breed) is 150 at Tsim Bei Tsui in March 1995, while that in autumn is 500 at Mai Po in November 1992. The winter high was of a count of 2,500 at Tsim Bei Tsui on 21 January 1996, which remains the highest so far recorded.

The winter waterbird counts (Figure 1) generally under-estimate the number of Night Herons as they are nocturnal and roost diurnally in dense vegetation in undisturbed areas. In addition, the pattern during the course of the year is influenced by the greater visibility of birds during daytime in the breeding season.

During monitoring of nocturnal autumn migration using a static audio recorder at Pak Sha O, Sai Kung from 2017 to 2021, this was the most numerous species, passing through between mid-August and mid-November.

BREEDING

Herklots (1953) reported that Night Herons bred at the Hang Ha Po egretry, but by the mid-1960s the colony has been abandoned (Macdonald and Macfarlane 1966, Herklots 1967). The next breeding records were not until the 1970s when birds were observed at the Yim Tso Ha Egretry, Starling Inlet in 1971 and at the Mai Po Egretry in 1977. Subsequently, breeding activity was recorded at Yim Tso Ha, the outer mangroves at Mai Po, Mai Po Village, Hebe Haven, Ho Chung and Shuen Wan, and the number of breeding pairs grew. The largest summer population was found around Starling Inlet, with a peak of 300 to 500 birds in 1983. Figure 2 illustrates the number of breeding pairs surveyed annually since 1998 and indicates that most birds breed away from Deep Bay (at Starling Inlet) and highest numbers were present during 1999-2002.

The nesting period appears to be earlier than other ardeids, with recently fledged young reported from the sheltered breeding site in Kowloon Park on 27 April.

BEHAVIOUR, FORAGING & DIET

Generally, forages solitarily. Outside the breeding season most birds leave the diurnal roost from around 15 minutes before sunset to up to 15 minutes after; return after foraging is much more extended and can occur well before sunrise. Usually seen flying to foraging areas at dusk or when returning at dawn; migrant flocks can be obvious early morning at Mai Po NR as they search for roost sites. Seen more often in daytime during the breeding season. Frequently calls in flight throughout the night, though much less so in the daytime. Appears to be the commonest nocturnal migrant ardeid passing through HK.

RANGE & SYSTEMATICS

The most widespread heron globally, breeding on every continent except Australia and Antarctica. Across Eurasia it is generally not found north of 55^oN in Europe, southwest and central Asia and China (away from desert and plateau areas). Northerly populations are migratory, and both resident/year-round and wintering populations occur in sub-Saharan Africa, across the Indian subcontinent, Indochina and south China south to Indonesia (Hothem et al. 2020).

Four subspecies are recognised of which the nominate occurs throughout Eurasia south to Indonesia and Africa, including HK. The remaining three subspecies occur in the Americas, including Hawaii.

CONSERVATION STATUS

IUCN: Least Concern. Population trend decreasing.