Tuesday, 19 August 2014

Sandwich Terns, Port Seton

A busy summer with a Sandwich Tern influx on the Forth coast, many ringed with 3 character coloured darvic rings - too great an opportunity to miss and I managed to stop off near high tide most days to read rings; the terns also brought a few Roseates with them which attracted skilled observers with high power scopes (mine is just 27x!) which added to the haul. On average c. 1 in 20 birds had darvics and by 19 August we had recorded 144 codes of which 114 were unique, the 30 others were re-reads [162 codes, 125 unique by 4/9]. c. 25% of the birds were juveniles, apparently resulting from a bumper breeding season on the east coast, and many of these had been ringed, just over 50% of darvics being on juvenile birds. Some details here derived from histories returned by the ringers (usually same day!), along with a selection of photos.

First batch here are the general aspect of terns on the rocks at Wrecked Craigs, on the day of the highest count - 1400 on 7 August (following a roost count of 2000 at Aberlady the previous day, by far the biggest count here for 10 years), and ring-readers in action on 8 August (with Andrew Barker, Dave Allan and Billy Barber, who found the Bridled Tern at same spot in July, photo (c) Michael Welch). Best views are on the rising tide which brings them in very close to the prom wall and on the highest tides eventually flushes them all off, whence they often commence fishing; on windy days and with lower tides they tend to remain further east at Long Craigs rocks and spread along the coast.

They flush quite often but usually resettle nearby as there is virtually no other dry ground between Musselburgh and Craigielaw Point at high tide.

Rings come in various colours (red, dark blue, green, white, yellow and light green), in first below is a light-green darvic (NCV, central) on a juvenile ringed this year in the Netherlands:

In fact four Dutch-hatched juvs were seen up to 18 August, when NA7 was present:

One adult bird from Netherlands was also present on several dates, initially causing confusion by its symmetrical dark markings; this turned out to be blue-NFA which had been dyed yellow (picric acid) to facilitate local tracking in its colony:

Overall c. 60% of the birds were originally colour-ringed at Forvie in NE Scotland with rings starting "E", in five colours, though these did include a few which had previous to that been ringed as chicks in Northumbs. Green and blue rings had been used at Forvie for 2014, just 11 of these were seen, e.g. green-ETP:

The quickest arrival of such was in three days, green-EHS present here on 8 August having been on the Ythan on the 5th.

Most juveniles were however from Northumbs colonies, 40 birds with blue or red rings starting "U", for example blue-UAV, blue-UVL, red-UCJ and red-USS below. Photos such as these are being gathered for analysis of progression of moult on birds of known age.

Northwards dispersal is typical from these colonies, as it is in NE Scotland (to the Moray Firth), an example being blue-UZS, ringed Coquet 12 July, here at Seton on 31 August but at Findhorn on 23 September. This year there were 181 and 105 chicks ringed on Farnes and Coquet, so we had here 17.1 and 8.6% of all those ringed, respectively. Calculations based on resighting rates suggest that the minimum number of birds in the area was approximately 5 times the observed totals, thus getting on for half of the Northumbs birds have perhaps come into the Forth this year. Worth comparing with Roseates too, which have also done well on Coquet, 93 juvs ringed, if they behave in a similar manner we're expect to have seen 8 of them in the Forth - birds have indeed been seen on several occasions, including up to 3 together, as per Sandwich Terns it seems likely the total number of birds involved will have been many more than those seen at any one time.

The Forvie-ringed adults have much more complex histories, with 12 (35%) now breeding in Northumbs and two seen this year in the Dutch colonies and another in the German part of the Waddensee. This means that overall 55% of birds have arrived from Northumbs colonies. For example white-EFN and white-EKN, together below, were ringed at Forvie in 2010 and bred this year on Coquet and Inner Farne, respectively:

The oldest few were ringed as chicks 13-14 yrs ago and some have been sighted at many locations over the years. White-EAP below, present on 18 August, was ringed as an adult on the Ythan in 2010, and present there in 2011 and 2012, but in between seen in Namibia on 2 March 2012, 13 March 2013 and 23 February 2014.

Initial northwards dispersal is typical of many birds from Forvie too, though clearly some come south with their young; more complex patterns can occur too, e.g. blue-EDB at Forvie on 18 June, then here on 8 August but was back round at Findhorn by 17 August; by contrast white-EZH, a 2008 chick which breeds at Forvie was at Findhorn from 27 July to 9 August but appeared here on 19 August.

In addition to the above, three 2014 juveniles from the huge colony at Blakeney Point in Norfolk were seen (blue-KF3, blue-KF4 and blue-K3D), 50 had been ringed in 2013.

What does all of this tell us? Overall there have been over 1500 resightings of the Forvie darvic ringed birds which for low cost will give good insights into the dispersal and migration strategy of this species. Though many travel huge distances over the whole course of the migration they clearly set off in different directions, and an interesting question is whether those initially heading north use a route overland, e.g. Forvie birds which go round to the Moray Firth then proceed down the Great Glen, or eventually return south down the east coast. Here in the Forth we get nocturnal migrant Sandwich Terns annually, e.g. over Longniddry from early August to late September it is common to hear an apparent south/south-west departure, often on generally damp days or during light rain; sometimes single birds but more usually ad+juv calling to each other, or sometimes small flocks. This year there have been far more than normal (11/9: 1 heard 23:10hrs; 10/9: 3+ ad, juv S 21:00hrs; recent showers, last calls well S; 6/9: 4+, at least 2 sets ad+juv SW 22:17hrs, heavy showers earlier, full moon now through clouds; 1/9: c. 10 group ads/juvs SW 21:50hrs roughly over club house; 17/8: 1+ SW 23:10hrs 17/8 some distance W, last call well S; 2/8: 2+ S 00:15hrs probably retreated; 2/7: 2+ heard 23:00hrs over Gosford saw mill). Common Terns occasionally do the same. Overland dispersal of these species has been studied between Teesmouth and Merseyside (paper), it is likely something similar happens from the Forth and probably further north. The results from Namibia show that some at least travel right to the South Atlantic, but small numbers also winter in northern France and Ireland, and indeed this year we had one in the Forth in January, so there is interest there too.

Finally it is worth mentioning the "cohort" colour-ringed birds from Forvie, a study that ran for 53 year until 2013, c/o Alistair Smith. These are simple colour band combinations, e.g. red over blue below, indicating the year of ringing. A bare minimum of 8 of these birds were logged, two just with single bands remaining, but without unique identification (short of capturing them!) no individual histories are available - they do tell us something about longevity (a lime over dark green bird recorded in 2018 was the first resighting here from the 2002 cohort!).

Other darvic birds logged in the period were a Whimbrel (yellow-A39) ringed on passage in Wales (on 24 April 2012, at Llanon between Aberystwyth and Aberaeron, one of very few sightings so far on this scheme), and this immature Great Blackback yellow-T:32E from NE Scotland:

As posted on LBN a couple of Peregrines disturbed the terns but put on a stunning display in pursuit of a Turnstone on 16 August. Many waders use the same rocks giving great views, including this Knot with fading breeding plumage:

Other birds logged for the patch were two Greenshank on the shore and an adult Little Gull W past in stormy weather on 17 August, and earlier on last day of July a patch tick Yellow Wagtail (a juvenile based on paler chest, prominent malar stripe, tips on median coverts faded to white):

First Wheatear, a juv, on Saturday 16 August:

Final shots below of terns feeding over the Forth, a misty Inchkeith in the background, and the Seton shore at dusk: