Swainson’s Thrush – 2015 – Skokholm

Catharus ustulatusCarfonfraithVagrant

Vagrant – one previous record, the first live bird for Britain and Ireland, logged in 1967.

For many the highlight of the 2015 season was the arrival of a first-summer Swainson’s Thrush on 2nd June (RDB, IB). It arrived on the back of a deep area of low pressure which tracked quickly across the Atlantic, delivering unseasonal gale force winds to the west coast and a Cedar Waxwing to St. Mary’s, Scilly, on the same date. Similar systems during the previous week had produced the first spring Grey-cheeked Thrush for Ireland at Termoncarragh, Mayo, on 25th May and the second spring Veery for Britain at North Ronaldsay, Orkney, on 30th May. Catharus thrushes were thus firmly on the radar in spring 2015, nevertheless it came as something of a surprise when a push of the Cottage Heligoland revealed the presence of a North American visitor, the first spring record for Britain and the second record for Wales following one on Skokholm between the 14th and 19th October 1967.

The bird was a first-summer, as indicated by the retained juvenile greater coverts, and it was clearly emaciated with no fat reserves evident, almost non-existent pectoral muscles and a weight of just 23.5g. It was quickly released into the Courtyard where it was only seen briefly during the rest of the day. Happily the bird was still alive on the morning of the 3rd and it was clearly feeding well; although it could go missing for up to an hour, it regularly emerged from the nettles at the west end of the Courtyard and sat on an Elder where it would clean its bill before descending once again into thicker vegetation.

Occasional weather windows, its predictable feeding pattern and its position away from the more fragile areas of Skokholm allowed for twitches to be organised on the 4th, 5th and 8th and for 76 visiting birders to view the bird alongside residential guests. The bird went missing on the morning of the 7th, only to be refound in the Wheelhouse Heligoland that afternoon; in the hand it was clear that the bird’s condition had improved significantly, with a tracheal pit full of fat, more rounded pectoral muscles and a weight of 34.0g (a 44.7% increase in five days). The bird was released back into the Courtyard where it again showed predictably until the evening of the 10th; regular searches from the 11th onwards failed to find the bird.

Pictures on Skokholm blog.

There are four subspecies of Swainson’s Thrush which form two genetically distinct clades, one of which summers to the west of the Rocky Mountains and the other to the east. It was hoped that mitochondrial DNA analysis of a dropped feather would allow the Skokholm thrush to be identified to subspecies, however the feather showed a cytochrome b sequence identical to birds sampled on both sides of the Rockies. Although unassigned to race, this was only the 33rd Swainson’s Thrush to be documented in Britain.

Skokholm Bird Observatory Annual Report 2015 (Richard Brown & Giselle Eagle)

More about the Swainson’s Thrush in Pembrokeshire