Rare vagrant birds can be a challenge to identify correctly. In many migratory bird groups (e.g. waders, terns, and petrels), several species look very similar to each other. There are further complications with species that look very different depending on their age and breeding status (e.g. juvenile plumage versus adult non-breeding plumage, or adult breeding plumage).
When a previously unrecognised vagrant species reaches New Zealand, it is even more challenging, as it will not be featured in New Zealand field guides and websites. Unless bird-watchers are thinking globally, a previously unrecorded species may be overlooked if it is misidentified as a species that is already on the New Zealand list. Curator Vertebrates Colin Miskelly describes how this was the initial fate of New Zealand’s first black tern.
The black tern is spectacular when in full breeding plumage, but juveniles and non-breeding birds look completely different and are very similar to several other tern species.
The first sighting
Elizabeth Taylor was the first person to notice New Zealand’s first (and so far only) black tern. On 14 January 2022, she photographed a huge flock of several thousand white-fronted terns at Waikanae estuary, north of Wellington.
With so many birds present, she decided to spend time looking for other rarer tern species among the flock. Elizabeth eventually found and photographed a different-looking tern, which was much smaller and with darker plumage.
Elizabeth did not know what the different tern was, and posted an image of it on a Facebook bird identification page on 20 January. Initial suggestions from knowledgeable birders were that it could be an Arctic tern or common tern, before several suggested that it was a white-winged black tern, a rare migratory species of which a few birds reach New Zealand every year.
The second sighting
Dunedin-based birder Oscar Thomas and his partner Ela Hunt were visiting Wellington at the time, and headed to Waikanae estuary on 21 January, hoping to photograph the ‘white-winged black tern’.
Fortunately, the bird was still present. Oscar managed to capture some superb photographs of it and posted them on the BirdingNZ.net web-forum later that day. It was Ela who first raised doubts about the identity of the bird when they were reviewing the images before they were uploaded. She was curious why its legs were darker than other ‘white-winged black terns’.
After the images were circulated to other expert birders, John Graff from Western Australia and expat Canadian Russell Cannings suggested to check for dark shoulder tabs (diagnostic of black terns), and Ian ‘Sav’ Saville also suggested that the bird could be a black tern – which was not previously known from New Zealand.
Distinguishing one good tern from another
Black terns and white-winged black terns are closely related and look very similar to each other at all life-history stages. The Waikanae bird was in immature plumage, with the key distinguishing features from an immature white-winged black tern being the grey patch of feathers on the shoulder (in front of the folded wing), a longer bill, and shorter, darker legs.
A long way from home
The black tern is a very unlikely species to reach New Zealand. There are two separate populations, with one breeding in northern Europe, western Russia, and Kazakhstan, and migrating to western Africa, and the other breeding in Canada and northern USA and migrating to Central America and northern South America.
Three black tern records have been accepted from Australia, all apparently of the American form, and the most recent in 1968. Based on photographs taken by Oscar Thomas and others, it is thought that the Waikanae bird is of the Eurasian subspecies, meaning that it was at least 12,000 km from where it should be.
A New Zealand ‘twitch’
As soon as the New Zealand birding community realised the likely identity of this rare tern, many rearranged their plans to get to Waikanae as quickly as possible. By good fortune, I was heading over to Kāpiti Island as part of a bird counting team on 22 January.
By leaving Wellington an hour early, we were able to ‘twitch’ the tern before boarding the boat from only 2 km away. There were three other birders present by the time we left Waikanae estuary at 8 am, and several dozen other birders saw the tern that day (i.e. the day after it was identified).
The bird then disappeared, before resurfacing at Plimmerton (26 km to the southwest) on 26 January. Many more birders saw the black tern at Plimmerton through to 10 February, before a final sighting at Pukerua Bay (between the two other sites) on 15 February.
We don’t know how many birders and/or photographers managed to see (and recognise) the New Zealand black tern, but it was likely to have been between 50 and 100 people. This pales in comparison to the United Kingdom and USA, where thousands of people may attempt to ‘twitch’ a rare bird in a single day.
Adding black tern to the New Zealand list
Oscar Thomas submitted his images and description of the bird as an Unusual Bird Report to be considered by the Birds New Zealand Records Appraisal Committee (three of whom had successfully ‘twitched’ the tern themselves).
The five RAC members have to unanimously agree on the identification of a new bird species before it is added to the New Zealand list. This high bar was reached in early May 2022, meaning that black tern could be officially added to the New Zealand list.
Congratulations to Elizabeth Taylor, Oscar Thomas, Ela Hunt, John Graff, Russell Cannings, and Sav Saville for the part they played in finding and identifying this rare vagrant from distant lands.
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