Knot (Heysham) - Part II (21 February 2004)
With around three-quarters of the population of Calidris canutus islandica spending every winter on a small number of British and Irish estuaries, (Red) Knot is clearly a species of great conservation importance that merits continued study whenever possible, so a number of MRG members went to Heysham to join Morecambe Bay Wader Group in another cannon-netting attempt. Many of the day's events followed a similar course to that of the previous year. The settled weather and the highly productive mudflats of the Bay had obviously allowed the Knot and Oystercatchers Haematopus ostralegus to feed very well, as they moved to the roost site three hours before high tide, long before their feeding grounds were covered by the rising tide. However, two Peregrines Falco peregrinus had not yet fed, and made their customary attacks on the waders, scaring away probably as many as 80% of the Knot flock. The remaining birds soon settled, on the grass not far away from our nets. As Jack Sheldon and I took our places in the firing position, in his van, we were amazed to see a Peregrine eating a Knot, on the ground next to the sea wall, some 20 metres away from the edge of the remaining flock of Knot. This Peregrine appeared to be a small, brownish bird, presumably a first-year male. He remained there for at least 15 minutes, tearing and eating the unfortunate prey:
Most of the waders were asleep and apparently unconcerned at the raptor's presence, possibly because they knew it was already feeding, or maybe because they only respond to Peregrines on the attack, in the air. In these circumstances, perhaps it was not surprising that the birds chose to roost close to us rather than their predator:
even though we could see that some of them already wore rings (like the one in the centre of the following photo) and should have remembered why we were there:
After the Peregrine flew off, some gentle pressure from Phil Holland, along with the rising tide, persuaded birds to leave the sea wall and congregate on the edge of the grass, and John Wilson finished the job by driving his car to push the flock towards the nets. The photograph below shows the position during this 'twinkling' exercise (cannon-netters' jargon for moving birds towards the catching area). The two nets can be seen, in a line going away from us on the right hand side, just over the top of the light - remember that this is a helicopter landing area used in case of an emergency on the offshore Irish Sea gas-rigs. The nets are set to fire towards the sea, with their front edge measured to land about one metre from the sea wall on the left hand side. In this photograph, the birds - the grey mass in the centre - are still outside the catching area, on our side of the nets.
One final move by John and many of the birds were nicely in the catching area ... you can imagine the terse conversation between Jack and myself ... about 2,000 ... let's take them ... fire! ... and we had a perfect catch. The rest of the 30-strong team, who had been watching from just outside the helipad, ran to join us at the net and covered the birds with rolls of light cloth, to keep them calm, whilst they were extracted from under the net.
During extraction, it was very noticeable that the flock seemed to be segregated into blocks, with parts predominantly second-year birds (hatched in 2003) whilst others were mainly adults. Since we can only ever catch a small proportion of the flock, this effect would jeopardise any attempt to gauge the success of a breeding season by the age ratio amongst the birds caught. It might be thought that the older, more experienced birds would take positions in the centre of the flock, leaving the more exposed outside to the younger birds, but in fact on 1 February 2003 we caught only the edge of the flock and had 97% adults and 3% second-years, implying that, on that occasion at least, the older birds were on the outside.
This time, our total recorded catch of 1,996 birds - at least 50 escaped from the nets or the keeping cages - was almost equally split in ages, with the 1,950 newly-ringed birds comprising 977 second-years and 973 adults. It was pleasing that 46 of the birds were already ringed. Twenty of them had been ringed at Heysham in previous years (16 on 14 February 1998, two on 27 January 2001 and two on 1 February 2003). There were 22 British-ringed birds from elsewhere (outside Morecambe Bay), no surprise as it is well known that Knot tend to arrive in Britain in the autumn on the east coast estuaries, especially the Wash, and spend a couple of months there whilst the adults moult, then they spread out and move to other estuaries for the winter. Many movements of Knot are listed on the webpages of the Wash Wader Ringing Group and analysed, with maps, in The Migration Atlas: movements of the birds of Britain and Ireland.
Three of the birds in our catch had rings within 34 numbers of each other - all of them age 5 (second calendar year) - and we have learned from the Ringing Unit at the BTO that they were ringed on 8 November 2003 on the Solway Estuary, the next major estuary about 100km to the north of Morecambe Bay. Many birds are faithful to a wintering site but records like these demonstrate the way that some can move around between estuaries, and emphasise the importance of conservation of the whole network of British estuaries that are important for waterfowl (wildfowl and waders). Thankfully, almost all of the British estuaries have several levels of conservation designation - national (Sites of Special Scientific Interest (SSSIs)), European (Special Protection Areas under the EU Wild Birds Directive), and international (Ramsar sites under the Convention on Wetlands of International Importance). Some, including Morecambe Bay, are also Special Areas of Conservation under the European Union Habitats Directive.
The final four birds in our catch had been ringed overseas, three in the Netherlands, and one in Germany. Two of the Dutch-ringed birds also wore colour-rings and leg flags so that ornithologists could identify them individually in the field without having to catch them again:
We know that all of these birds are from the population that breeds in arctic Canada and northern Greenland, the race Calidris canutus islandica - the only other race that normally occurs in Europe is the nominate C. c canutus breeding in Siberia and wintering in Africa - but unfortunately there was no direct proof in this catch of their Nearctic origins from a 'Washington'-ringed (make that 'banded'?) bird. There have, however, previously been Knot exchanged between Morecambe Bay and researchers at Alert on Ellesmere Island, at a latitude of 82º30'N, less than 500 miles from the North Pole. (The above link is to webpages that contain a full description of the long-term shorebird monitoring at Alert, including a charming photograph of a Knot on its nest with a chick.)
The birds in our catch seemed to show an unusually wide range of variation in plumage. One bird (Euring age 5, North American SY (second calendar year)) was a very immature-looking individual that had replaced very few feathers in its post-juvenile moult last autumn. The photo below shows the characteristic feathers (coverts and tertials) with their dark subterminal band, generally brownish colour and frayed tips. On the more exposed feathers the lighter-coloured buffish edge has been worn away, but it is still present on those feathers that have been more hidden.
It also showed a pinkish breast and I wrongly thought, at first, this was a really bizarre immature bird starting to show a 'Red' Knot's summer plumage, but John Wilson pointed out that in fact these pale pink feathers were also the remains of its juvenile plumage, not often seen in Britain:
We spent some time looking at the range of variation in the Knots' tertial feathers. One of the most worn, that had clearly not replaced any feathers in its postjuvenile moult ('first prebasic molt') last autumn, is:
On the other hand, most of the second-year birds (Euring age 5, North American SY (second calendar year)) had moulted one or more tertial and scapular feathers, such as this bird:
Following on from this, I commented on the 2003 catch that some of the 'adult' birds were apparently showing two generations of feathers in the wing, especially coverts and tertials, and that they might be birds of Euring age '7' (third-year in North American banding codes). This time we had more firm evidence from a bird already wearing a ring from our 2003 catch, and thus known to be at least one-and-a-half years old. The photograph below shows a close-up of the inner part of the bird's right wing, concentrating on the region of the scapulars (at the left of the image), tertials (in the centre) and the coverts above them.
The longest tertial feather appears to be a dull brownish-grey, rather than the glossy, clear grey colour of the adult feathers, and is also quite abraded. I feel that it is unlikely to be a juvenile feather - such an exposed feather would surely have been much more worn that that - and I suspect it to be one of the feathers that the bird replaced in its first post-juvenile moult. But much more striking are the small coverts between the scapulars and tertials (and some of the rather dishevelled median coverts), which clearly are retained juvenile feathers, more brownish-grey and showing the characteristic dark subterminal band and the buffish edging. Several of these feathers are normally quite sheltered - some of the overlying feathers having been moved aside for this photograph - and they seem to be similar to those that are often retained for two years in Sparrowhawks Accipiter nisus, thus allowing them to be aged as '7' (third calendar year). As well as examining feathers, we also looked at the coloration of the Knots' bare parts, especially their legs, but they varied from odd ones that were almost black, to dull green to bright olive-green, with so much individual variation that seemed to be unrelated to age. Since most Knot do not breed until they are two years of age, it might not be surprising if some birds below breeding age were distinguishable by plumage.
Any comments on the possibility of ageing some Knot as '7' (TY) would be appreciated.
Finally, as can be seen from the photographs of the flock, the birds roost very close to each other. On our first catch on this site, on 14 February 1998, we caught over 1,750 Knot, all within an area of less than 100m2: this is a packing density of around 20 birds per square metre. Pause for a few seconds to visualise an area of one square metre, then imagine 20 Knot in it; and another 20 in the next such area; and another; and so on ... At this density, it is no surprise that catching with cannon-nets can be such an efficient means of studying them. We plan to re-visit the Heysham flock in future years.