Identifying Marsh Terns in Singapore

September marks the arrival of Whiskered Terns to our shores, followed closely behind by White-winged Terns. However, these two species are among the most mis-identified in Singapore, with juvenile Whiskered Terns being easily confused with White-winged Terns.

Jared Tan During each migration season, the arrival of Whiskered Tern Chlidonias hybrida and White-winged Tern C. leucopterus brings about much added life to our coastal waters and freshwater bodies. Whiskered Tern are usually seen in Singapore from September to October, while White-winged Tern are usually encountered from September to January and April to May and often occur in larger numbers. While adult Whiskered and White-winged Tern are straightforward to identify in both breeding and non-breeding plumage, immature birds are often misidentified. Here we shed some light on the various characteristics which can be used to differentiate between these two similar-looking species and some other tern species as well. Feeding behaviour Whiskered Tern and White-winged Tern are known as marsh terns, which typically feed by skimming and snatching food from the water surface. This is in contrast with the feeding behaviour of our resident Little Tern Sternula albifrons, which usually plunge-dives for food. While marsh terns usually engage in skimming, nevertheless it is still possible for them to sometimes engage in steeper dives to catch prey when the opportunity arises. Immature plumage Comparison of immature Whiskered (left) and White-winged Tern (right) Whiskered Tern are larger (24-28 cm), and have a heavier-built appearance than White-winged Tern (20-24 cm). Whiskered also have a longer, heavier bill compared to that of White-winged. While wing length may be difficult to discern in the field, the wings of Whiskered Tern are slightly longer and have a more uniform width throughout the wing length than that of White-winged. In addition, the tail of Whiskered Tern is generally slightly more notched than in White-winged. It should be noted that these differences may be minute at times, and may not be the most reliable of features to depend on for identification purposes. It is often said that the presence of ‘headphones’ equates to a bird being a White-winged Tern. However, immature Whiskered Tern also gives off the impression of having ‘headphones’, thus the mere use of the word ‘headphones’ as a distinguishing feature can be rather misleading. In Whiskered Tern, the dark patch on the ear coverts does not extend as far down as in White-winged, and the white patch behind the ear coverts also does not extend as far up onto the crown and inwards towards the eye. While this may be one of the most talked about features, there are also other important distinguishing features to look at especially when the head pattern does not give a conclusive ID. Immature birds of both species display a brown mantle and scapulars, which can be very useful in determining the identity of the bird. The scapulars of Whiskered show prominent contrasting dark crescents, in comparison to the generally more uniformly brown coloured scapulars of White-winged. Another important feature to look at is the collar of the bird. In Whiskered, the black cap touches, or almost touches the mantle, leading to the lack of a white collar or just a narrow one. On the other hand, the black cap of White-winged does not touch the mantle, giving it a wide, obvious white collar. The rump of the bird is also a useful indicator of the species. In Whiskered, the rump is a similar pale grey colour to the tail and upperwings, while it is white in White-winged. It is important to note that this feature only applies to immature birds, as the rump eventually becomes pale grey in adult White-winged Tern. Overexposed photos may also lead to the pale grey rump of Whiskered Tern appearing white, so care should be taken to ensure that the colours turn out accurately. Lastly, the marginal coverts also differ. Whiskered generally show marginal coverts similar in colour to the rest of the upperwing, while White-winged usually have darker marginal coverts which are more contrasting compared to the rest of the upperwing. The upperwing of Whiskered is also slightly paler compared to that of White-winged. Immature Whiskered Tern Immature White-winged Tern Immature Whiskered Tern Immature White-winged Tern Adult plumage (non-breeding) Adult Whiskered Tern in non-breeding plumage do not have ‘headphones’, but instead display a streaked greyish cap, with the dark areas behind the eye not extending downwards onto the ear coverts like in immature birds. On the other hand, the ‘headphones’ of Adult White-winged Tern in non-breeding plumage look similar to that of immature birds. Given that the head patterns of both species look very different in adult non-breeding plumage, there should not be much confusion with regards to their identification. Both species lack the brown mantle and scapulars found in immature birds and display a pale grey rump. Thus, unlike immature birds, the scapulars and rump can no longer be used to distinguish between adults of the two species. Adult Whiskered Tern in non-breeding plumage Adult White-winged Tern in non-breeding plumage Adult plumage (breeding)...