When I was in San Jose last week my good buddy Jack suggested going to Redwood Shores (Redwood City), a site where large numbers of shorebirds gather close to the road at a high-tide roost before then flying in groups over observers’ heads as the adjacent tidal-flats are uncovered by receding water. Amongst large numbers of American Avocets and Marbled Godwits were, Jack told me, hundreds of Willets – a species that in its non-breeding plumage looks little more than a rather nondescript grey shorebird until it reveals its startlingly-patterned wings on taking flight. I’ve been wanting an opportunity to take photos of these wonderful birds in flight for some time, and for an hour I was transfixed trying to get them both in the frame and in focus: I managed to take a few photos, some of which I’ve posted below. As well as posting the images I thought it may be interesting to discuss the feather groupings that make up the striped upper- and under-wings the Willet is so well-known for…
…to this. “Western” Willets Catoptrophorus semipalmatus. Dec 2007.
Wing patterns like these – which incidentally are similar to patterns found in eg many chalidrids and charadrius plovers like Killdeers – are the result of two things: multi-toned/multi-coloured flight feathers, and a very precise arrangement of all the feathers covering the wing. The arrangement – common to all birds – consists of layers of small protective, insulating feathers called coverts and the longer flight feathers they lay over (the flight feathers – the primaries and secondaries – are folded like a fan and covered when the bird is not flying which is why the striking pattern can’t be seen).
In the case of the Willet, the striking black-and-white pattern of the outerwing results because the primaries are white basally and black distally – ie are white towards the end where the feather fits into the wing and then black towards the tip. The primary coverts – as the name suggests – lie over the bases of the primaries: the bi-coloured long primary feathers stick out beyond them. These overlapping layers create the total effect.
These outerwing feathers contrast strongly with the marginal, lesser and median coverts of the innerwing which are essentially brown (and which are mainly the feathers seen on the closed wing of a shorebird when the bird is feeding or roosting etc). In the Willet‘s case the greater coverts are well-patterned and they make up the bar in the centre of the spread wing.
To some extent the upperwing pattern is also shown on the underwing – but here it’s more diffuse and not quite so contrasting. The upper and under surfaces of both primary and secondary feathers are often different – which means that the same feather looks different depending on which surface you’re looking at (eg whether the bird has its wings raised or lowered). The coverts on the underwing – which are NOT the same feathers as those on the upperwing – often create a totally new and distinctive pattern (think of a wigeon’s gleaming white underwing, for example, which is created by very white underwing coverts). In the Willet’s case the main difference between upper- and underwings come about because the primaries are neither as deep black distally and are more extensively white basally when seen from below, and the secondaries are mostly lacking the dark centres that are seen on the spread wing’s upper surface. The more extensive dark underwing coverts and the dark axillaries (the group of feathers found in the “armpit” which are hidden when the wing is closed) also combine to form a large area of dark feathering which is not found on the upperwing.
A question that might be asked is why do Willets (and many other birds) have such well-patterned wings? The answer probably lies in an amalgam of evolutionary advantages, but certainly migratory birds in general are able to follow individuals of their own species by locking onto the wing patterns of other flock members. Showing such a striking flash of black-and-white when an individual is disturbed into flight also probably momentarily startles and distracts a predator whilst at the same time acting as a warning signal to other members of the flock (especially important in eg very windy conditions when they may not be able to hear auditory signals like alarm calls).
From a birder’s point of view – or mine at least – such an (almost) incongruous pattern being shown by such an otherwise unremarkable bird is just one of the elements that makes birding so enjoyable. Hopefully understanding how these patterns are made up might help other birders who are perhaps less convinced at least look on the humble Willet with a bit more awe and wonder…
Shorebird roost, Redwood Shores
Willet, Bolsa Chica, Nov 2007
All photographs taken California, Nov/Dec 2007. Copyright Charlie Moores