IDENTIFICATION

VOCALISATIONS

When disturbed, sometimes utters a harsh, dry ‘kek’, often repeated, in flight.

The song is a repeated rather low-pitched ‘whoo’.

DISTRIBUTION & HABITAT PREFERENCE

During the 1993-96 breeding atlas, Yellow Bittern was recorded in 2.5% of 1km squares and was concentrated around Inner Deep Bay from Tsim Bei Tsui to Lok Ma Chau, with scattered records elsewhere in lowland areas of the northeast and east New Territories, Hong Kong Island, Po Toi and Lantau (although it seems unlikely that breeding occurred away from Deep Bay). In the 2016-19 survey, this figure had reduced to 0.3%, comprising three squares around Deep Bay, with the highest numbers at Mai Po NR.

Away from the Deep Bay area, there are regular records of migrants at wetland sites throughout HK, apart from HK Island itself from where there are only about ten records, some of which occurred urban parks.

OCCURRENCE

Yellow Bittern occurs throughout the year, though numbers in winter are low with usually only one or two birds noted at individual sites, the highest being five at Mai Po NR on 21 December 2019.

By the last week of March, possibly earlier, passage is underway. Main spring passage now occurs in the second half of May, whereas previously it was rather more prolonged and included the last week of April and first half of May (Figure 1). The highest counts are 50 at the Lok Ma Chau MTRC Ecological Enhancement Area on 21 May 2008 and at San Tin fish pond area on 17 October 2002. Away from Deep Bay the highest count is 44 at Po Toi, also on 21 May 2008, on which day a total of 107 birds were recorded in Hong Kong during a fall of small bitterns.

The highest count considered to involve breeding birds was of 30-50 pairs at Mai Po on 15-16 June 1974, while Chalmers (1986) estimated up to 50 pairs breeding in the Deep Bay marshes as a whole. Up to 50 were recorded at Mai Po in spring from the last week of April to the third week of May up to 1982. However, the highest counts since then are 30 on 29 May 1988 and 15 on 23 May 2008 indicating a substantially reduced breeding population.

The autumn passage period is difficult to define accurately due to the presence of post-breeding birds. However, it would seem to occur from at least the last week of August, when there is a rise in mean weekly aggregate counts, until the middle of November, as nocturnal migrants have been recorded over Pak Sha O, Sai Kung West CP from 22 August to 14 November.

The peak autumn count is 100 on 31 August 1969 and 20 August 1972; however, the highest count in that season since 1978 is 50 on 5 September 1992 and 17 October 2002. In recent years there have been no double-figure counts after the last week of September, though in earlier years 30 were recorded as late as 12 October 1975. Figure 2 illustrates the peak count per year since 1958 and clearly shows the decline in numbers that occurred after the mid-1970s and a further decline after 1990.

Figure 1 compares the pattern of occurrence in the 41 years up to 1998 with that in the subsequent 21 years (the latter period saw higher observer activity and reporting). Three relative trends are apparent:

• Lower number of passage migrants in April and the first half of May.
• Smaller numbers of breeding birds during the summer.

Vaughan and Jones (1913) stated Yellow Bittern was ‘a summer visitor and a breeding species, ...very common in suitable places’, and that it ‘often makes its nest in the big river-reeds’. Dove and Goodhart (1955) also found it to be a common summer visitor, breeding in the Deep Bay mangroves. Herklots (1967) stated that it bred ‘in the Deep Bay marshes, especially in the mangroves where in 1958 it was estimated that thirty-five pairs were nesting’.

BEHAVIOUR, FORAGING & DIET

Generally difficult to observe unless in flight low above wetland areas or foraging at the edge. Quite vocal when flushed or on migratory flight.

BREEDING

Male display has been noted on 30 April, while nests have been recorded on 12^th and 22 May and 10 July (latter with three eggs). Dependent young have been noted from 29 June to 29 August, while recently-fledged juveniles have been seen from 31 May to 21 August. In recent years the only nesting substrate recorded is reeds, although Herklots (1967) stated that it bred ‘in the Deep Bay marshes, especially in the mangroves.’

RANGE & SYSTEMATICS

Monotypic. Breeds from Pakistan east through the Himalayan foothills, in China southeast of a line from southeast Tibet to Heilongjiang, and in southern Ussuriland, the Korean peninsula and parts of central Japan; occurs year-round in coastal provinces of southeast China, Indochina, southeast Asia and much of Indonesia, and in the non-breeding season in eastern Indonesia and New Guinea (Martínez-Vilalta 2020, Liu and Chen 2020, Eaton et al. 2021).

CONSERVATION STATUS

IUCN: Least Concern. Population trend unknown.