IDENTIFICATION

VOCALISATIONS

The typical flight call is a deep ‘rok’ or ‘ohk’ uttered when flushed, flying around wetland areas prior to migration or on nocturnal migratory flight.

DISTRIBUTION & HABITAT PREFERENCE

Most reports are from the Deep Bay area; a few records have also occurred at Shuen Wan, Long Valley and Pui O.

The presence of suitable emergent vegetation, either wet Phragmites reeds or Schoenoplectus club-rushes, appears to be an essential component of the habitat conditions required by Eurasian Bittern in HK for prolonged stay. Such conditions are required for roosting, possibly because of the reduced risk of predation by land-based predators, particularly mammals, and for foraging (Stanton et al. 2015).

OCCURRENCE

Figure 1 illustrates the pattern of records since 1999. Rarely reported before the middle of October, the earliest date on record is 12 September 1987. Birds generally appear from the third week of October. It seems certain that at least some of the birds recorded from mid-September to mid-November are passage migrants. Apparent passage to mid-December may at least partly reflect more obvious behaviour by newly-arrived winter visitors. Nocturnal migrants have been recorded over Pak Sha O, Sai Kung West CP in autumn from 23 October to 11 November.

It seems that some winter visitors are established by at least the third week of November. The wintering population seems to decline in late January, and numbers increase from early February indicating passage, which peaks in the second half of March. In March Eurasian Bitterns can be highly visible at Mai Po NR as birds circle above the reed marsh at dusk sometimes prior to flying north. Leader (1999) suggested that both migrant and wintering birds occur, a conclusion supported by Stanton et al. (2015). Birds apparently departing on northward migration have been recorded from 7 March to at least 20 April.

Since 1958 most records away from areas with substantial reed marsh in the Deep Bay area, and therefore presumed to relate to migrants, have occurred during the period 23 October to 7 December and 15 March to 19 April. The latest date on record is 24 May 2002.

Vaughan and Jones (1913) stated a few Great Bitterns occurred every winter, and Macfarlane and Macdonald (1966) noted that it was flushed occasionally in the Deep Bay marshes, usually singly though once in a party of five.

BEHAVIOUR, FORAGING & DIET

Appears to be a diurnal forager, with birds returning to a wet reedbed to roost overnight (Stanton et al. 2015). Presumed to feed on fish and possibly shrimps, as these are present in the favoured ponds at Mai Po NR. Has been observed at Mai Po taking a 15 cm Nile Tilapia Oreochromis niloticus from the water’s edge on a reed-fringed bank, which was carried back onto dry land before consumption.

The behaviour of Eurasian Bittern in HK is relatively well-studied and was reported in Leader (1997) and Stanton et al. (2015). Among the behaviour discussed was that of visible migration, which was observed on three dates (19-21 March) when birds departed minutes after sunset on calm, warm and clear evenings; this was presaged by flocks calling in flight, though not all birds necessarily departed. Groups of 3-17 birds were observed leaving Mai Po NR; in addition, a flock of birds was recorded on nocturnal migration over Pak Sha O, Sai Kung West Country Park on 10 November.

Stanton et al. (2015) point out that although traditionally regarded as an essentially solitary species, Eurasian Bitterns, at least in HK, exhibit social interactions outside the breeding season and display a certain amount of tolerance of other individuals in relatively close proximity.

RANGE & SYSTEMATICS

Breeds from western Europe east through Eurasia to Sakhalin and northern Hokkaido between approximately 40^oN and 60^oN; winters in disjunct areas in sub-Saharan Africa, the Middle East, the Indian subcontinent, southeast China and Japan (Martínez-Vilalta et al. 2020). In China breeds in the northwest and northeast and winters southeast of a line from Yunnan to Beijing (Liu and Chen 2020). The nominate subspecies occurs from Europe to east Asia, including HK, while B.s. capensis occurs in south Africa.

CONSERVATION STATUS

IUCN: Least Concern. Population trend decreasing.