The first arriving White-winged Tern, Chlidonias leucropterus, was photographed by KweeChang Ling on the 26 Sept. This was followed about a week later by a Whiskered Tern, Chlidonias hybridus, photographed by See Toh Yew Wai on 4 Oct. Both at the Serangoon Reservoir. These two “marsh terns” are winter visitors to our shores. The White-winged usually can be seen earlier in the year (July) as they also passes through Singapore on their migration south. While the rarer Whiskered Tern appeared around mid September.
Since then there were many photos of these two terns gracing the facebook pages of various groups here. Most had a hard time telling the two species apart. This is understandable as they both look very much alike in size, plumage and shape when they are in their non breeding plumage. Further plumage differences due to age added to the confusion.
The above file photo taken at the Kranji Dam showed both tern species. Try and separate them before continuing. The extreme left tern looks a little bigger, has a stouter and slightly longer bill and the black patch from the crown does not extend below the eyes. All features of a non-breeding Whiskered Tern.
Non breeding Whiskered Tern photographed at Serangoon Reservoir on 4 Oct by See Toh Yew Wai.
Now take a look at the two extreme right terns. The first thing you notice are the black round ear patches (head phones) extending below the eyes. This is a distinctive field mark of the White-winged Tern. Their bills are slightly smaller and thinner as well.
Other subtle differences are: Whiskered has slightly forked tail while the White-winged has a almost square tail. Whiskered is slightly larger than the White-winged with longer legs. Non breeding Whiskered has a greyish rump while the White-winged upper tail and rump is white.
Now that you have the different features of the two terns sorted out, try to identify the second tern from the left.
Reference: A field Guide to the Waterbirds of Asia.Wild Bird Society of Japan 1993. Field Guide to the Birds of Australia Simpson and Day 1993. A field Guide to the Birds of Thailand and South-East Asia.Craig Robson 2000. The Avifauna of Singapore. Lim Kim Seng. 2009. Edited by Francis Yap.