Field identification of Gull- - billed Tern

K. E. Vinicombe Illustrated by Alan Harris n the ten-year period 1977-86, 44% of all records of Gull-billed Tern I Gelochelidon nilotica submitted to the British Birds Rarities Committee were rejected. It is probably true to say that many of these records came from inexperienced observers, who, having obtained brief views of a fly-by tern, submitted a description which provided little detail other than a thick, gull-like bill. This high rejection rate, however, belies the fact that the Gull-billed Tern is actually not difficult to identify. The similarly sized Sandwich Tern S. sandvicensis is the confusion species, but many observers do not appreciate that juvenile Sandwich Terns have shorter, thicker- looking bills than adults, have rounder wing-tips, and also call quite differently. This paper summarises, therefore, the main differences between the two species.

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{Brii. Birds 82: 3-13, January 1989] 3 4 Field identification of Gull-billed Tern

2. First-winter Gull-billed Tern Gelochelidon nilotica, Israel, November 1981 (Killian Mullamey)

It must be remembered that the Gull-billed Tern is now a very rare bird in Britain and Ireland, currently averaging about four records a year, mainly on the English south and east coasts. Most occur in May, with fewer in summer and autumn. Anyone faced with a possible Gull-billed Tern is advised to exercise caution and to claim the sighting only if the majority of the following points have been noted.

Structure and flight An awareness of the structural differences between the two species is an essential first step to identification. The Sandwich Tern is a large bird,

3. Gull-billed Tern Gelochelidon nilotica, Bulgaria, June 1977 (/. Lawton Roberts) Field identification of Gull-bilkd Tern 5

4. Gull-billed Tern Gelochelidon nibtica, Greece, June 1982 (T. A. Box) similar in size to Kittiwake Rissa tridactyla, and it lacks the long tail streamers of Common S. hirundo, Arctic S. paradisaea and Roseate Terns S. dougallii. Compared with Gull-billed, it is noticeably slim and rakish- looking, with long, narrow, often rather angled wings. The head and neck protrude noticeably and the bill is long and slender with a yellow tip. Gull-billed is similar in size and also lacks long tail streamers. Compared with Sandwich, however, it is sturdier and more thick-set with, of course, a relatively short, thick, black bill (which can look bulbous), and a rather short, thick neck. This bull-necked appearance is emphasised by the wings being set farther forward than on Sandwich. The wings are

5. Gull-billed Tern Gelochelidon nibtica, Florida, USA, May 1982 (R. Ingkston) 6 Field identification of Gull-billed Tern

6. Gull-billed Tern GefocheUdon mlotka, Louisiana, USA, January 1982 (David Tomlinsan)

broadly based and long, not so sharply pointed as those of Sandwich, more swept back, rather than angled, and with straighter and more parallel edges. The general effect may, therefore, recall a small gull, and the slow, easy, graceful, rather languid flight suggests Common Tern. Indeed, the whole effect in flight may recall a juvenile or winter-plumaged Common Tern, rather than a Sandwich. At rest, Gull-billed again looks shorter-necked than Sandwich, it has a rather more rounded breast, the closed primaries are deeper-based and, most significantly, it is long-legged: Gull-billed's tarsus is about 25% longer than that of Sandwich (BWP). Gull-billed looks sleek and round- headed when at ease, but, although lacking the crest of Sandwich, the head can look rather square when the bird is alert. Apart from the bill differences, the most significant difference between summer adult Gull- billed and Sandwich is this lack of a crest, which is always obvious on adult summer Sandwich. The net effect of these structural differences is that, on the ground, Gull-billed again looks more gull-like; in winter and juvenile plumages (see below), it suggests an elongated winter adult or second-winter Mediterranean Gull Lams melanocephalus.

Calls The calls of the adults are totally different. Sandwich has a well-known shrill, guttural, rasping 'kerr-ick', whereas Gull-billed has a low, deep 'ger- erk' or 'ger-vik': quite unmistakable. Confusion arises, however, with the calls of juveniles, which are quite different from those of the adults. Juvenile Sandwich has a high-pitched, squeaky and rather throaty 'pee- up' or 'peep' and juvenile Gull-billed a similar high, but soft 'pe-eep' or a quick 'pe-pe-eep'. Field identification of Gull-billed Tern 7 Plumage Adult summer The most significant difference between the two species is the colour of the rump and tail. On Sandwich, both are white but, on Gull-billed, the rump and tail are almost wholly whitish-grey and, therefore, continuous with, though paler than, the mantle and wings. When assessing this character, however, the strength of the light has to be taken into account, since strong light may make this area look whiter. The outer tail feathers are white. The other significant difference is that of wing-tip pattern. Gull-billed usually shows a noticeable dark trailing edge to both the upper and the under primaries, that on the underwing recalling Common Tern. Sandwich lacks the dark trailing edge to the upper primaries and, although it also has dark tips to the under primaries, these are, in comparison, greyer, less clear-cut and less obvious.

Fig. 1. Gull-billed Gelochelidon nilotica, Forster's Sternaforsteri, Roseate S. dougallii, and Sandwich Terns S. sandvicensis (Alan Harris)* *Figs. 1 & 2 are taken from the forthcoming 77K Macmillan Field Guide to Bird Identification by Alan Harris, Laurel Tucker and Keith Vinicombe, and are reproduced here by kind permission of the publishers, Macmillan Press. 8 Field identification of Gull-billed Tern

Fig. 2. Gull-billed Gelochelidon nilotica and Sandwich Terns Sterna sandvicensis {Alan Harris) Both species undergo a complete post-breeding moult. So far as the primaries are concerned, this starts with the inners in late summer and finishes with the outers in early to mid winter, when the terns reach their winter quarters. In late winter, they moult their inner primaries a second time, prior to their spring migration (although, according to BWP, about one-third of European Gull-billed Terns do not change their inner primaries a second time in late winter). The result of this moult sequence is that, while in Europe, both species have old outer primaries and new inner primaries. The significance of this is that, when new, the primaries Field identification of Gull-billed Tern 9

7. Gull-billed Tern Gebchelidon nilotica, Turkey, May 1979 (P. A. Doherty)

have a pale grey bloom (radii) which is steadily lost with wear, revealing progressively more of the blackish base colour (rami). This produces a contrast between the old dark outer primaries and the new grey inners, so that both species often have noticeable dark outer primary 'wedges'. In Europe, these wedges are most pronounced when the outer primaries are oldest: in late summer and autumn. It seems, however, that, on average, Gull-billed Tern wears less dark than Sandwich and usually shows less obvious primary wedges. Indeed, even as late as early September, some adults still look uniformly pale whitish-grey across the entire upperwing. Such individuals look very uniform and concolorous at a distance, showing little contrast between upperwing and underwing. Other plumage differences are less significant. First, the upperparts of Sandwich always look pearly grey and relatively pale, but, in some lights, those of Gull-billed can appear as a quite dark, leaden-grey. Secondly, the black extends right down the nape on Gull-billed whereas, on Sandwich, it is confined to a cap, and the whole nape is white. This difference is especially noticeable on perched individuals when the head is raised. The line of demarcation between the black on the rear crown and the white nape is rounded on Gull-billed, but pointed on Sandwich.

Adult winter Adult Sandwich starts to lose the black lores, forehead and crown from mid June onwards, so that, by late August or September, it has a 10 Field identification of Gull-billed Tern

8. Gull-billed Tern Gelochelidon nihtka, Algeria, July 1982 (Arnoud B. van den Berg) prominent white forehead and a black 'shawl' from the eye back over the nape. Individuals in full winter plumage by late July are probably non- breeders (BWP). Adult Gull-billed starts its moult later than Sandwich, from late July to mid August, although some still have a full black crown in early September. The significant difference between moulting adults is that Gull-billed does not gradually acquire a white forehead. Instead, the whole cap is moulted at once, so that transitional individuals have the entire cap mottled with white. When the moult is completed, Gull-billed has a completely different head pattern from Sandwich. Instead of a black shawl across the nape, it has a white head with a discrete black patch immediately in front of and behind the eye, strongly reminiscent of a first-year Mediterranean Gull. Some have very fine black streaking on the crown and nape (difficult to see in the field), while others have a grey wash to the rear crown, but Gull- billed does not show a black shawl. When moulting back into summer plumage in late winter, Gull-billed has black mottling right across the crown.

Juvenile In this plumage, the two species are easily separated. Juvenile Sandwich has a completely black crown (including the forehead), but a body moult soon after fledging means that, by the autumn, juveniles acquire a white forecrown and a black shawl, just like those of winter adults. Juvenile Sandwich has the upperparts strongly patterned with dark brown and white. Note in particular that this patterning occurs along the leading wing-coverts (cf. Gull-billed Tern below). During the late-summer post- juvenile moult, however, much of this upperpart patterning is lost, so that, by autumn, the mantle and scapulars are plain grey and traces of immaturity are confined to the wing-coverts, tertials and tail. Perhaps the most important point to remember about juvenile Sandwich Tern is that the bill is often markedly shorter than the adult's and lacks a yellow tip. It looks stubbier, therefore, and, to the inexperienced eye, can suggest Gull- billed; for this reason, attention to plumage detail is essential. Another Field identification of Gull-billed Tern 11

9. Gull-billed Tern Gelochelidon nilotica, Gambia, December 1983 (M.J. Tarrant)

significant point is that juvenile terns can fly before their outer primaries are fully grown, so that recently fledged individuals have noticeably shorter, more rounded wings than adults. On Sandwich, this, in combination with the shorter bill, can suggest something unusual. Juvenile Gull-billed is totally different from juvenile Sandwich. First, it lacks a black crown or shawl, but, instead, shows a small black ear-covert patch like that of a winter adult. Very close inspection may reveal brown shaft streaks on the crown and nape feathers, not readily apparent in the field. Instead of being strongly patterned on the upperparts, the mantle and scapulars are relatively uniformly ginger, but, by September, this colour fades to cream and is soon replaced by grey first-winter feathering. Note in particular that juvenile Gull-billed has relatively plain wing- coverts, lacking Sandwich Tern's dark feathering along the leading coverts. The general effect is that, from a distance, juvenile Gull-billed looks very plain above, and flying juveniles can be difficult to separate from winter adults. Other differences include the tertials, which are ginger with a brown feather centre, whereas juvenile Sandwich has the tertials also strongly patterned with dark brown. The tail feathers, too, are different: black on Sandwich, thickly edged with white, whereas on Gull- billed they are grey with a pale tip and a dark subterminal patch. Note that, like adults, juvenile Gull-billed also shows a prominent dark trailing edge to the under primaries. 12 Field identification of Gull-billed Tern First-year In early winter, towards the end of the post-juvenile moult, both species resemble the respective winter adult, but are easily aged as they retain juvenile tertial and tail feathers. First-winter Sandwich also shows a dark secondary bar, which is absent on winter adult. By late winter, even these feathers are lost, and both resemble winter adults. First-winter Sandwich begins to moult its juvenile inner primaries from early December to January, and the moult is completed by May to July, when the inner primaries start to moult again. First-winter Gull-billed starts its inner primary moult between December and late March, and it is often suspended in early summer and resumed from July to September. It then starts to replace its inner primaries again (BWP). In their first- summer, both species remain in their winter quarters and resemble winter adults, but some Gull-billed show broad black drops to the feather centres of the crown and nape (BWP), Second-summer Subsequently, they resemble adults, but second-summer Sandwich does not attain full summer plumage, showing white flecking on the forehead, lores and crown, while second-summer Gull-billed may show narrow white fringes to the black feathers of the cap.

Habitat and behaviour The Gull-billed Tern is not a bird of the open sea, but occurs on coastal marshes, lagoons and estuaries, and may even hawk insects over dry land. It also catches larger prey, such as frogs, and individual English vagrants have developed a liking for the eggs and chicks of Little Tern S. alMfrons. Unlike Sandwich Tem, Gull-billed does not habitually plunge-dive. Its easy, relaxed-looking, sinuous swoop to take food items from the surface of mud or water is distinctive.

Other confusion species Forster's Tern Winter and juvenile Gull-billed may be confused with winter and juvenile Forster's Terns S. forsteri, which also show a black ear-covert patch. This species has been virtually annual in Britain and Ireland since 1980, and occurs mainly in winter. It is easily separated from Gull-billed by the following features: (1) the bill is relatively long and thin, (2) body size is much smaller (slightly larger than Common Tern), (3) the structure is also more similar to Common (in particular, it has a longer tail), (4) in first-winter plumage especially, it has whiter inner primaries, and (5) it has pale legs (dull orange on first-winter and bright scarlet on adult).

Common Tern Beware also of winter-plumaged Common Terns, which can occur from July onwards. These lack long tail-streamers, show an all-dark bill, and have a grey tint to the rump and tail. They are, however, easily separated by their size and shape, dark carpal bar and red legs. First-year Common Field identification of Gull-billed Tern 13

10. Forster's Tern Sterna forsteri, USA, January 1983 (KUlian Mullamey) Terns are similar to winter adults, but often show great contrast between old, dark outer primaries and inner secondaries and new, grey inner primaries and outer secondaries. Such birds may be encountered throughout the summer.

Roseate Tern Note also the similarity between the juvenile plumages of Roseate Tern and Sandwich Tern. Juvenile Roseate is easily separated, however, by size and shape, which are more similar to those of Common and Arctic Terns. Acknowledgments We are very grateful to Peter Colston and the British Museum (Nat. Hist.) at Tring for allowing access to skins.

K. E. Vinicombe, 11 Kemdngton Avenue, Bishopston, Bristol BS7 9EU Alan Harris, 117 Spring Hills, Harlow, Essex CM20 1TA