Variation in albifrons primaries

From cute fluffy chicks to something a little more nerdy. Little Terns have what Malling Olsen & Larsson describe as a "complex" moult; and using the word complex in relation to tern moult is really saying something! Focusing on the primary moult of adults — since that is the most obvious in the field — birds have a complete post-breeding moult including all of the primaries; it starts with the inner primaries on the breeding grounds, is suspended while the bird migrates, is resumed on the wintering grounds and finishes with the outer primaries in about December. Let's call the primaries grown during this complete moult "series one". At the same time, starting a month or two before the completion of series one, a second wave of primary moult starts again at P1 (innermost primary). Let's call this second wave "series two". A month or two after series two starts, another new wave of primary moult ("series three") starts; series one has now finished but both series two and series three are simultaneously active. Primary moult is suspended "at onset of breeding" by which time series two has usually reached about P8 and series three has reached about P3. Thus a bird arriving back at the breeding colony would have a right wing something like

(body/tertials/secondaries)3332222211

where 1 = series one and so on.

There is some limited variation in the extent of the primary moult, most noticeably in the number of black outer primaries (2–3 on most birds, sometimes 1, per Malling Olsen & Larsson exceptionally 4). These outer primaries (retained from "series one") are pretty old and worn, hence their dark appearance. The most common number of retained outer primaries on birds at Gronant appears to be two.

A bird with two retained outer primaries has a classic Littler Tern pattern; in the field, the dark primaries (and their corresponding primary coverts) give the appearance of a dark leading edge to the outer wing.

Notice how, even with a relatively good photos, it can be difficult (impossible?) to detect the moult limit between series two and series three.

Some birds in the colony showed three outer primaries.

It may only be one primary difference but notice how having three retained outer primaries transforms the appearance of the wing; the dark leading edge is now a more conspicuous dark wedge.

The number of retained outer primaries is listed as a feature for separating albifrons (Little Tern) from saundersi (Saunders's Tern breeding in Asia and East Africa), with the latter showing 3–4 retained primaries. So, how does one separate our "three retained primary" Little Terns from a Saunders's Tern. Apart from the fact we're in Wales, obviously. A Saunders's Tern should show even blacker outer primaries with a black feather shaft; you can just about make out, on the lower bird in the photos above at least, that the shaft on the outer primaries is quite pale. Saunders's Tern also shows a squarer forehead patch with the white stopping short of the eye; you can see on both of these birds that the white extends to behind the eye giving making the rear border of the white patch sharply pointed. So, good, we've established that Saunders's Tern isn't breeding in north Wales.

I also photographed one bird that shows only one retained outer primary.

An extensive primary moult like this takes us in the opposite direction from Saunders's Tern, to North American and to antillarum (Least Tern). Least Tern is reported to show on average fewer retained primaries than Littler Tern. So, how can we rule out Least Tern? The most reliable feature seems to be the rump/tail colour: concolourous with the mantle in Least Tern but contrasting white in Little Tern. This bird clearly shows a white rump/tail that contrasts with the grey mantle and thus is (unsurprisingly) a Little Tern. Perhaps the easiest way to pick out a Least Tern in a Little Tern colony is by voice — I kept an ear open for anything squeaky while we were ringing the chicks but with no success!

Finally, going full circle with the moult, a small number of adults birds at the colony had already started their post-breeding primary moult. The result: four generations of primaries in one wing!

P9-10 series one (old); P4-8 series two; P3 series three; P1-2 just starting to grow as part of (new) series one.