There is a general concensus in North America
© Brian McCloskey
There is a general concensus in North America that Alder Flycatcher is not reliably separated from the similar Willow Flycatcher in the field. However, this is often unfounded, with good-quality digital photographs helping to confirm identification.
Ireland's first Empidonax flycatcher has been confirmed as Alder Flycatcher by DNA analysis.

The finder of the bird, Steve Millar, took to Twitter to reveal the news that a sample he'd collected after the bird's departure had been tested by the wildlife forensics lab at the University of Aberdeen.

The results showed that the bird, which spent three days on the island of Inishbofin from 9-11 October, was an Alder Flycatcher - an ID that had been all but confirmed on field views. This should now mean a straightforward acceptance by the Irish Rare Birds Committee (IRBC) and therefore addition to the Irish list.

The Inishbofin bird lends further weight to the argument that this species can be confidently identified on field views from the extremely similar Willow Flycatcher. In North America, this species pair is often simply identified as 'Traill's Flycatcher' - its name before being split into Alder and Willow - due to the perceived difficulty of confirming a field identification.

However, all seven 'Traill's Flycatchers' seen so far in Europe since the first in 2003 (in Iceland) look to have been Alder Flycatchers, with most of these being identifiable in the field thanks to high-quality digital photographs (for example, see this analysis of the 2010 Blakeney Point bird). Willow Flycatcher is yet to make an appearance in Europe.