Common tern

20/01/03 Common tern, Sterna hirundo (prob sub sp. longipennis), at Manawatu Estuary

A ‘strange’ tern was first spotted by Sav as we scanned the birds roosting at the high tide flock on the end of the spit. At first glance the bird clearly had different plumage characteristics to the accompanying white-fronted terns. In particular the ‘tell-tale’ dark carpal area stood out. We both stood there looking at the bird for about 5 minutes trying to assess size and plumage differences. In those first few minutes neither of us could spot obvious size differences, except for maybe the fact that the ‘strange’ tern had a slightly smaller head. Strangely, in the photos below there is an obvious size difference, with the bird being smaller in overall size, and also having a smaller more ‘flat-topped’ head which tends to slope gradually to the neck (in white-fronteds this tends to be more round, with the neck being longer), and a shorter bill. The overall colour of the mantle/back and closed wings was a slightly darker grey than the white-fronteds, and the shape of the black remaining on the head was also different. A small ‘blob’ of black remained in front of the eye, which appeared to sit forward of the rest of the black on the head, so that the eye was almost bounded by white on the upper and lower edges. An area of black then extended well below the ear coverts, well below where the normal ‘straight lined’ black cap of a white-fronted tern would lie.

Other key features of this bird were the very dark (almost black) outer primaries, at least two or three of them being wholly dark. This can be clearly seen in the flight shot. This gives the bird a very dark outer wing (especially the outer primaries) when in flight. Also noticeable in the field were the almost black outer tail feathers (one on each side), again this can be seen in the photos below. When the bird was perched the secondaries can be clearly seen to be very pale edged, this is also seen in the flight shot. This is clearly a feature that none of the white-fronted terns show, and is a good feature to help with this identification.

Based on the fact that the bird was clearly not a white-fronted tern, we had several options, according to size and overall plumage similarities –

  • Arctic tern Sterna paradisaea
  • Antarctic tern Sterna vittata
  • Roseate tern Sterna dougalli
  • Common tern Sterna hirundo

Arctic tern could discounted immediately based on the length of the legs (which were similar to the accompanying white-fronted terns – Arctic terns appear as if they are almost resting on the ground), and overall shape. Arctic terns tend to have a very rounded front end with long tapering wings and small body. The legs seem to be situated well forward, and we describe the shape of the bird as an ice-cream cone on its side, with the legs being situated at the ice-cream/cone area. The extent of dark in the outer wing also effectively eliminates Arctic which should have a very pale outer wing with just a thin dark leading and trailing edge. Also the general back colouring appeared far too dark, and should have been very pale in this species. This means that pale edged secondaries would be far less contrasting than in our bird.

Antarctic tern can be eliminated due to the dark carpal area, dark primaries and dark outer tail of our bird. Although adult Antarctic terns show a red (in breeding) or red-black (non-breeding) bill, it is not really known when immature birds develop this colouration. It may be possible for first year birds to turn up on mainland New Zealand, still with a black bill. Again the dark area below the ear coverts would rule this species out, as it has a similar breeding season head pattern to white-fronted (except of course that they don’t have a white forehead).

Roseate tern should appear much smaller than the accompanying white-fronted terns. If the bird was an adult, even in non-breeding plumage, red legs would be evident. In non-breeding and all pre-adult plumages the tail is white without dark outer tail feathers, and the back colour is very pale.

So that effectively leaves common tern. Our bird clearly fits the description of a common tern, especially noting the ‘diagnostic’ primary pattern. The features described above are all features fitting this species. Interestingly, this bird also showed a strange behavioural trait that we had noticed in the bird that was present 21 Jan 2000. When standing on the ground amongst white-fronted’s the bird was constantly looking into the air, by tilting its head to either side. Not that this is a diagnostic feature, but strange that we noticed it in more than one individual.

As far as determining subspecies of our bird goes, the most likely of course is longipennis, which breeds in eastern Siberia and NE Asia and migrates through South-East Asia to Indonesia, New Guinea, and Australia in large numbers (why don’t we get more?). However, the nominate subspecies hirundo which breeds across most of Nth America, Europe and Siberia, does intergrade with longipennis over a wide area in central Siberia (some of these intergradation areas have been claimed to be of different races). Subspecies tibetana which breeds in central Asia, is not well known. So hirundo is always a possibility and has been recorded as a vagrant in Western Australia, and a possible from Queensland. Determining the subspecies is not easy, but the lack of any colour at the base of the bill suggests longipennis, as does the fact that large numbers of longipennis winter just across the Tasman.

Ageing the bird is not all that easy based on HANZAB. However, Mike Carter suggests the bird is either first or second year, (that is in its second or third calendar year) due to the fact it has active primary moult. Strong carpal bar suggests first year, and he also suggested if the bird had active body or head moult it’s a first year bird. The bird did show signs of body moult, so we suspect it is probably a first year (second calendar year) bird.
Unfortunately, the bird hasn’t been seen since the day of its discovery (this page was written on 09/02/03), despite flocks of more than 100 white-fronted terns being seen on several occasions.