Arctic Tern, Ashley Rakahuri Estuary

[David Melville]

As noted in my earlier post, this bird is in the final stages of primary moult with the outermost feather still growing.

[Oscar Thomas]

Here is moult info for both species per HANZAB, key parts in bold. Perhaps someone more familiar with it than me can say if it helps the ID one way or another.

Antarctic:
Adult post-breeding (Third and subsequent pre-basic) Complete; mostly symmetrical. Begins after breeding finished; moult of individuals spans an average 6 months between Feb. and Aug. Most begin Mar. or Apr. (failed breeders sometimes start Jan. or Dec.); timing varies by up to 2.5 months between individuals. Primaries sequentially outwards; usually two, sometimes one or, rarely, three active at a time; outer primaries apparently moult at much slower rate than inner primaries.

Arctic:
Adult post-breeding (Third and subsequent pre-basic) Complete; primaries outwards. Usually delayed until in or near wintering areas, but a few (maybe failed or non-breeders) moult scattered feathers of body in late boreal summer. Of adults in s. temperate and subantarctic regions, Oct.–Dec.: two still in full breeding plumage, 11 had small amount of non-breeding plumage, four had much non-breeding plumage and five had non-breeding appearance except for traces of breeding plumage on forehead and flanks (also see Ageing). Moult of feathers of head often begins on crown, sometimes on foerhead (cf. on forehead in all other Sterna), so moulting birds often characterized by white skull-cap with black borders. Proportion of Aust. birds with much non-breeding plumages on s. migration possibly higher than in birds migrating more directly to Antarctic. Olsen & Larsson (1995) suggest that body-moult partial, with some worn breeding plumage replaced directly by subsequent breeding plumage, but confirmation lacking and several Aust. and subantarctic adults examined had complete non-breeding body-plumage. All moult of remiges occurs at edge of pack-ice of Antarctic; the two known exceptions (undated Macquarie I. specimen with primary-moult N6413121O1; and NSW beachcast on 14 Dec. with primary-moult N141V8 on left wing, V10 in the right), apparently aberrant. Earliest record of primary-moult, 15 Jan., but most must begin earlier, as PMS 38–49 by last half Jan. As there are so few data on early stages of moult of wing, published estimates of duration of primary-moult (60 days [Salomonsen 1967]; 3–3.5 months [Stresemann & Stresemann 1966]) are highly speculative. Nevertheless, there are indications that moult of wing much faster than in other terns: (1) adult with primary-moult N641311101 was also moulting s1–s9 and several tertials (Bierman & Voous 1950), thus having more remiges growing concurrently than in other moulting Sterna; (2) said to fly reluctantly and poorly when moulting remiges (e.g. Falla 1937; Bierman & Voous 1950). Pre-basic moult ends with growth of p10, before end of Mar.; many have finished by early Feb., most by late Feb. (>1000 observations in Antarctic pack-ice; D.J. James); latest record of active moult of primaries, 23 Mar. Final stages of pre-basic moult of primaries overlap much with pre-alternate moult of body and tail.

[Adam C]

Ok trying to work this one out. Here are images (all different birds) of White Fronted Terns taken at Sumner about 2 weeks ago. All birds appear to have new secondaries and (for lack of a technical term) fresh mid primaries growing, but all 3-4 outer primaries are very old and worn. Obviously the last to be replaced. Both being southern hemisphere birds shouldn't the Antarctic be at a similar stage as Oscars sequence for Antarctic above suggests? Our arctic/antarctic bird above appears to have all fresh flight feathers and outer primaries. Does this swing it back in favour of an Arctic Tern? Or is this bird just a few more weeks ahead with fresh primaries? Kind of look like that too! Or do we just lodge this one as an Artic-Tarctic Tern

[andrewcrossland]

Hi guys, hey where exactly is this bird roosting on high tides? Is on the north side of the rivermouth delta and do you accces that via the dodgy released feral pig spot at the end of the north Ashley R stopbank? Or are you forwarding the river from the south side or the estuary from the sand spit across? I sense it might be time to go an have a look at this bird?! Thanks

[benackerley]

terns are usually out from kings ave on the island in the afternoon like 4-5. And can sometimes be on the sand bar across from the island. At low tide they could be at saltwater but most likely island roost area

[Adam C]

Hi Andrew. At a hunch I'd say the Arctic/Antarctic bird has dropped in and long gone. I really hope not as another sighting and an in-flight shot might be all we need for an id. The Red-Billed Common however is likely still around. The tern colony at the Ashley is very hit and miss right now. Doesn't seem to be high tide related either but as Ben says at its peak of around 70 birds late afternoon. By evening, even right into dusk it's often down to less than 10 birds so they must be roosting elsewhere at this time of year. Same goes with early am although that makes more sense as birds will be out feeding. I also suspect the roost/rest site is getting a fair bit of attention right now and has done through late summer also, so think we need to be mindful of this too. May be a factor as to why they aren't roosting there in the evenings right now?

[David Melville]

Hi Adam,

A great shot of a White-fronted Tern halfway through primary moult. The inner 5 primaries are fully grown, new feathers; the next feather is a bit more than half-grown. There is then a 'gap' or a small just starting feather which is not visible; the outer three feathers are all old and visibly worn. This is a typical moult sequence (primary moult progressing from the inside to the outside of the wing).

If you (or others) are able to get more photos like this it would a great way to help describe moult in more detail In all species).

One thing to watch out for is the inner primaries being moulted more than once - the innermost primaries can be replaced three times in some species - see image from Bridge & Nisbet 2004. Condor 106: 336-343. It is thought that repeat replacement of the inner feathers may be an honest signal of bird quality which is beneficial come the nest breeding season (see e.g.: Bridge & Eaton 2004. Does ultraviolet reflectance accentuate a sexually selected signal in terns? Journal of Avian Biology 36: 18-21).

David

[Adam C]

Thanks David. All 3 photos are of different birds and have photos of 7 or 8 others all in exactly the same stage of moult. My argument was that an Antarctic Tern is more likely to be in a similar phase than an Arctic tern with completely opposite breeding time frame) but then we know feathers grow rapidly (up to 8mm per day I understand) so potentially if this was an Antarctic Tern it could just be a few weeks ahead in the cycle re the fresh looking outer primaries in the photos and not a clincher for the I'd. Still going in circles lol

[David Melville]

Yes, we should expect that White-fronted and Antarctic would be broadly similar in terms of primary moult - as detailed in HANZAB.

For further muddying of the waters for Common Terns, see White & Kehoe 2001. Difficulties of determining the age of Common Terns in the field. British Birds 94: 268-277 - available at
http://www.putni.lv/pdf/White_Kehoe_2001.pdf

It would be really good to encourage photographers to get more pictures of all our 'Sterna' Terns to help unravel moults.

The value of photos for studying moult will be highlighted by Mike Bell in a presentation at the New Zealand Bird conference in Nelson 1-2 June.

David

[sav]

Hi all,

I can't help thinking that everyone's attention is focused on the wrong bird here!
The little, round-headed, short legged bird is surely an Arctic Tern. The "standard" Common Tern is just that - but the red-billed Common Tern (since that is what I think it is) is really unusual.

Do we have any previous records of (presumably) ssp hirundo in NZ? It is about as monstrously unlikely as a European Black Tern being here.........!!

cheers