White-winged Tern Chlidonias leucopterus 白翅浮鷗
Category I. Passage migrant, common in spring and scarce in autumn; mostly seen over fish ponds or freshwater marsh in the Deep Bay area but also occasionally in coastal waters. Numbers have possibly declined since the 1990s.
IDENTIFICATION
Apr. 2005, John and Jemi Holmes. Adult breeding plumage.
20-24 cm. Smaller than Whiskered Tern with a more rounded head and finer bill.
Adults in breeding plumage are very distinctive due to the combination of pale grey upperwings, darkest near body and on outer primaries, pale rest of upperwing (almost white on leading edge), white rump and tail and black head and bill.
Apr. 2020, John and Jemi Holmes. Adult breeding plumage.
From below the black underwing coverts, body and head contrast with the pale grey underside to the flight feathers.
Oct. 2021, Sarawak, Malaysia. Dave Bakewell. First calendar-year.
This bird is moulting from juvenile to first-winter plumage. Note the black headphone patches behind the eyes, white base to the rump and remnants of the contrast between the dark mantle and wings (which gradually disappears with moult).
VOCALISATIONS
Typical flight calls vary somewhat in structure but are noticeably higher-pitched and less rasping than Whiskered Tern.
DISTRIBUTION & HABITAT PREFERENCE
White-winged Tern is most frequently seen in Deep Bay and over the fish ponds and gei wai in the northwest New Territories, but it may also be seen on migration over coastal waters.
OCCURRENCE
White-winged Tern is a passage migrant, common in spring but scarce in autumn. In spring it usually appears slightly later than Whiskered Tern and records before the middle of April are unusual. Previously, however, records before the end of April were considered unusual, but since 2012 White-winged Tern has regularly appeared in the second half of the month (Figure 1).
The earliest concerns one off Po Toi on 1 April 2012. Spring passage typically begins around 23 April and can be rather sudden in onset with three-figure counts possible before the end of the month. Main passage occurs in the first three weeks of May, typically peaking in the second week (Figure 1).
After these flocks of adults have moved through, there is often a secondary arrival consisting mostly of birds in non-breeding plumage from about 20 May to the end of the first week in June. The highest count at this time is 387. The latest record of a sizeable migrating group occurred on 14 June 2017 when 63 were seen at Mai Po NR. These late birds are presumably first-summer birds and the few breeding plumage birds which accompany them are possibly second-summer birds.
Prior to 1985 spring counts rarely exceeded 100 but from then to the late 1990s flocks of over 500 were sometimes seen, with groups over 100 regularly recorded. The total number of White-winged Terns in HK has exceeded 1,000 on at least three occasions, including 12 May 1986 when record numbers were present and the highest single count was an estimated 3,000 moving through Starling Inlet between 1115h and 1123h, while at Tsim Bei Tsui on the same day 300 were seen. A similar large movement occurred on 10 May 1987 when 1,530 passed Cheung Chau, and on 18 May 1978 when at least 1,000 birds were present in the Deep Bay area. In contrast, the highest spring count since 1999 is 750 over southern waters on 5 May 2007.
Although there are records from the second half of June to early August, these are unusual. Since 1999 the earliest returning birds were a notably large group of 65 in the Tolo Channel on 17 August 2015, which is the highest autumn count since 1999. Numbers on southward passage never match those in spring, but large numbers may appear during autumn tropical storms; the highest such count is 300 at Long Valley and 30 at Cape D’Aguilar on 3 October 1995. Small numbers are occasionally reported in October, the latest on record being nine at Hebe Haven on 31 October 1987.
As with Whiskered Tern, records of large autumn flocks of White-winged Terns in non-breeding plumage seen at sea to the south of HK Island and in Victoria Harbour during the 1960s and early 1970s lack substantiation, but available evidence suggests at least some error in identification.
BEHAVIOUR, FORAGING & DIET
Flies back and forth over water surface before swooping or dropping down to take prey from at or just below surface of water. More than Whiskered Tern it may hover above water surface. Highly gregarious, flocks in spring generally pass through rather quickly. Habitually roosts on wires over ponds.
RANGE & SYSTEMATICS
Monotypic. Breeds from east Europe east through southwest Russia and north Kazakhstan to west Mongolia, and in discrete areas of south and northeast Siberia and northeast China and adjacent parts of east Mongolia and Ussuriland; winters in Africa away from the Sahara and in southeast Asia and Indonesia (Gochfeld et al. 2020), though probably not in coastal south China as indicated. In China breeds in the northwest and northeast and is a migrant through areas to the south (Liu and Chen 2020).
CONSERVATION STATUS
IUCN: Least Concern. Population trend stable.
Figure 1.
Gochfeld, M., J. Burger, D. A. Christie, G. M. Kirwan, and E. F. J. Garcia (2020). White-winged Tern (Chlidonias leucopterus), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.whwter.01
Liu, Y. and Chen, Y. H. (eds) (2020). The CNG Field Guide to the Birds of China (in Chinese). Hunan Science and Technology Publication House, Changsha.
