Common Tern Sterna hirundo 普通燕鷗
Category I. Common passage migrant offshore in spring and autumn.
IDENTIFICATION
May 2008, Michelle and Peter Wong. Adult breeding plumage.
34-37 cm. Darker than both Black-naped and Roseate Terns on both upper- and underparts, the latter washed grey. The most numerous taxon occurring in HK is longipennis, which is distinguished by the dark, often blackish, bill and dark brown legs.
Jun. 2019, Kenneth Lam. Second calendar-year.
Although most birds have dark bill and legs, a small number have red that varies in extent from a dull wash to pure red that covers much of the bill, or entirely red legs. These birds are presumed to be either tibetana or minussensis. The brown in the wing coverts and dull primaries indicates this is a second calendar-year bird.
Apr. 2019, Michelle and Peter Wong.
In flight the underside of the secondaries lack a clear-cut trailing edge while the tips to the underside of the primaries are relatively broad.
Aug. 2021, Michelle and Peter Wong. Juvenile.
The juvenile may have a shorter bill and tail than the adult. Dark lesser coverts form a carpal bar along the leading edge of the wing visible both in flight and when perched, while the mantle and back are scaly with pale fringes and diffuse dark subterminal fringes. The outer web of the outermost tail feather is dark. The bill is initially pale before becoming dark as autumn progresses.
Sep. 2023, Paul Leader. Second calendar-year.
This second calendar-year bird has commenced moult to an adult-type winter plumage by replacing the inner primaries. The dark carpal bar is paler and more restricted than on juveniles and the bill is all dark.
VOCALISATIONS
Common Tern appears to vocalise rarely on passage through HK. The short ‘kip’ call is most common.
DISTRIBUTION & HABITAT PREFERENCE
Most records occur during observations made at sea, usually during trips dedicated to finding seabirds, during sea-watches from Po Toi or from ferry journeys.
Common Terns are regular in inshore waters but rarely appear in Deep Bay or inland sites. Most records occur in autumn, mainly September. The highest inland count is 22 over fish ponds along the Mai Po access road on 14 August 2013, while 36 longipennis were seen from the Mai Po boardwalk hide on 16 September 2014.
OCCURRENCE
Spring
The earliest record concerns one in southern waters on 21 March 2020, though in general the earliest arrivals occur in the first week of April. A count of 101 birds flying northeast past Po Toi on 23 March 2010 is very high for so early in the season. Numbers peak from the third week in April to the first week in May (Figure 1).
The highest count for HK is 2,100 seen from Cape D’Aguilar on 2 May 1999 during the passage of a tropical storm, with 742 seen there the previous day (these are excluded from Figure 1 in order to better illustrate the typical pattern of occurrence) and at least 150 elsewhere in HK. The highest spring count unaffected by a tropical storm is 330 in the Po Toi sea area on 5 May 2006. Boat-based observations in offshore waters and observations from Po Toi show much larger numbers are regularly moving through HK waters than are witnessed inland. After mid-May, Common Terns are rare. There are a small number of records in June, with the latest occurring on 28th in 2003.
Autumn
The earliest autumn record concerns 30 birds near Tap Mun on 19 July 1997 followed by one there the next day. This is an exceptionally early occurrence and at least some appeared to be first-summer birds of one of the red-legged taxa. The only other July records have occurred in the last four days of the month.
A record of approximately 200 adults at sea between Basalt Island and Tai Long Wan on 8 August 1987 is the largest flock in that month. By September most birds are in non-breeding plumage and subspecific identification is no longer possible. September also sees higher numbers passing through and tropical storms can bring substantial numbers close inshore. Sea-watches from Cape D’Aguilar recorded 360 during the passage of a tropical storm over 25-27 September 1993, while the highest autumn count is of an estimated 400 birds sheltering at Tai O, Lantau on 3 October 1995 during the passage of another storm. Since 1999 the highest autumn count is 148 in southeastern waters on 25 August 2018.
Figure 1 shows that migration peaks from the third week of August to the first week of September. The latest autumn record is of 121 during the close approach of a tropical storm on 26 October 1998.
Subspecific identification of Common Terns at sea is not straightforward given current knowledge, and most records do not attempt this. The pattern of reports so ascribed is summarised as follows.
S. h. longipennis
This is the most numerous taxon. Spring records have occurred from 25 March to 13 June, with the highest numbers occurring during 18-28 April, and the highest counts being 70 on 21 April 1996 and 60 on 18 April 1993. Reported in autumn from 12 August to 16 September.
S. h. tibetana/minussensis
Birds showing characters attributable to tibetana/minussensis are reported rarely from 1 April to 10 June, and on 19 July and 14 and 29 August. The highest count is 70 on 24 April 1993, though generally only single-figure counts are made.
BEHAVIOUR, FORAGING & DIET
Has been observed taking small fish, but no details. Loose flocks of birds may be seen on passage over the sea, and also at rest on flotsam with other terns.
RANGE & SYSTEMATICS
Most birds occurring in HK are of the taxon longipennis, which breeds in northeast Siberia and east Mongolia to northeast China. The other two easterly taxa are sometimes regarded as synonymous: minussensis breeding in central and south Siberia to Transbaikalia, south Russian Altai and south-central Mongolia, and tibetana from central Asia to Xinjiang, northwest Nei Mongol south to the Tibetan plateau. The nominate subspecies is largely extralimital, breeding in the Americas, Europe, north Africa and Asia as far east as western China.
At present, due to uncertainties over field separation, only the dark-legged and black-billed taxon longipennis is considered to have occurred in HK. While birds with obvious red on the bill and red legs are recorded, at present it is uncertain whether these refer to tibetana or minussensis.
Breeds in central and east north America and from Europe (patchily) east across Eurasia south of the Arctic Circle as far south as the Tibetan Plateau east to the Bering Sea; winters coastally in south America, Africa, India, southeast Asia and eastern Australia (Arnold et al. 2020). In China a summer visitor to the northern half of the country and a migrant through the rest (Liu and Chen 2020).
CONSERVATION STATUS
IUCN: Least Concern. Population trend unknown.
Figure 1.
Arnold, J. M., S. A. Oswald, I. C. T. Nisbet, P. Pyle, and M. A. Patten (2020). Common Tern (Sterna hirundo), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.comter.01
Liu, Y. and Y. H. Chen (eds) (2020). The CNG Field Guide to the Birds of China (in Chinese). Hunan Science and Technology Publication House, Changsha.
