Oystercatchers (Walney Island) (11 March 2004)

Some MRG members again travelled north on 10 March 2004 to join Morecambe Bay Wader Group in another cannon-netting attempt at the traditional site at Coastguards, Walney Island, near Barrow-in-Furness. A cold easterly wind reminded us that winter has not yet finished as we watched a nice flock of mixed waders gathering along the shoreline: Dunlin Calidris alpina, Sanderling Calidris alba, Purple Sandpiper Calidris maritima, Redshank Tringa totanus, Grey Plover Pluvialis squatarola, Ringed Plover Charadrius hiaticula, Turnstone Arenaria interpres, with the majority of the birds being Knot Calidris canutus and Oystercatchers Haematopus ostralegus.

The team of 14 people all waited patiently whilst the birds sat in front of the nets, gradually moving up the beach as the tide rose, enduring the occasional tense moments as parts of the flock took off and flew to other parts of the beach. Indeed, after appearing to be settled for an hour or more, many of the Knot headed off south and did not return. However, some birds stayed put throughout. After the tide had turned, several hundred birds were asleep in front of our net, and we took a nice mixed catch of 361 birds: 172 Oystercatchers, 167 Knot, 18 Dunlin, 7 Turnstone, 2 Sanderling and a Ringed Plover. 27 of the total already wore rings, most of them from previous year's catches at the same site. Eleven of the Knot had been caught on 21 February, 18 days previously, at Heysham, 22km away across Morecambe Bay. One of the Turnstones had originally been ringed, at Coastguards, in 1990.

Out of the 167 Knot, as many as 144 were second-year birds, hatched in 2003 (Euring age code 5). As we had found in the catch at Heysham, the adults were roosting all together in one part of the flock.

It is some time since we had taken a substantial catch of Oystercatchers, for which Morecambe Bay holds by far the largest wintering concentration in the UK. Many species are aged by looking at details of the plumage but often the colour of the bare parts can be very helpful, as I have discussed previously for Sparrowhawks, Collared Doves, Great Spotted Woodpeckers and Moorhens.

The Oystercatcher images at the left and right show the eyes and bill from (top) a second-year bird (Euring age code 5, hatched in 2003), (middle) a third-year bird (Euring age code 7, hatched in 2002), and (bottom) an 'adult' (Euring age code 8, hatched in 2001 or earlier).

The colour of the iris can be seen to change from brownish, through an orangey-red, to a deep carmine-red. The eye-ring also deepens in colour, from a dingy yellow, through yellowish-orange to a reddish-orange.

Oystercatchers' bill colour deepens from yellow to reddish-orange, especially as birds approach breeding condition.

Their legs also change in colour, starting as a greyish-blue in young birds and becoming pinkish-red in adults.

As well as the changes in bare part coloration, the shape and condition of the birds' primary feathers varied greatly with age, as shown in the images of outer primaries at the right. The top photo is from a a second-year bird (Euring age code 5, hatched in 2003), with the characteristic brownish, more pointed and abraded feathers. In the middle is shown an 'adult' (Euring age code 8, hatched in 2001 or earlier), whose primaries are blackish, more rounded and much less worn. These are the typical examples and this effect can be helpful in ageing other species as well.

The bottom image shows an odd bird, an adult that had retained its outermost primary feather. This feather shows the bleaching expected of a feather from the previous generation, and it is probably about eighteen months old. As the shape of the outermost primary is rounded, I think this means that this bird was already an adult when it grew that feather. The probable reason for this feather being retained is that the outermost primary on the other wing was broken, and there appeared to be some damage at the base of the feather, where it emerges from the wing; perhaps this had interrupted the normal moult process on its left wing, and the (normally) symmetrical moult also stopped on its right wing. The bleaching and extra wear at the tip demonstrates why birds need to moult and grow a new set of feathers annually.

Another interesting feature with Oystercatchers is the wide range of sizes and shapes of their bills. Males have shorter but deeper bills than females, and the differences can lead to their exploiting different food prey, using different feeding techniques. Those with the shorter bills are better at hammering and prising open bivalve molluscs such as mussels and cockles, while those with the longer bills specialise in probing for invertebrates. The photographs below illustrate two of the extremes; they are reproduced at about life-size, with the bill of the top bird about 110mm from base to tip:

One of the distressing aspects of getting close to Oystercatchers is discovering the high proportion that have injuries to their feet and legs, caused by their becoming entangled with sheeps' wool on their breeding grounds (mostly in the uplands of England, Scotland, the Faeroe Islands, Iceland and Norway). Sometimes ringers are able to help the birds by removing the wool, but usually it is too late. In this catch, one bird had one leg missing above the 'knee' and at least ten had one or more toes missing. The birds seem to be able to adapt to this and there are many long-lived examples.

One shorebird always has a toe 'missing' - the Sanderling has evolved to have only three toes, with the hind-toe absent. Perhaps this helps the bird with its characteristic 'light on its feet' appearance as it chases the waves up and down a beach.

Their black legs always seem to highlight the pure white appearance of the underparts of the bird, in its winter plumage at least, and this one was certainly living up to its scientific name of Calidris alba - the white shorebird:

David Norman