A year ago a Sandwich Tern was found on Chicago's lakefront in the United States. Observers of this bird soon realized that it may be of European origin, and I set out to investigate.
I've now completed my research and published my findings at North American Birding (the blog, not the ABA magazine). To cut to the chase, I believe there can be little doubt that this bird was Thalasseus sandvicensis sandvicensis, and represents the first record of a live bird in North America (there is a specimen that was picked up dead on the east coast some years back).
Here's a photo of the bird:
A big part of this identification rests on the shape of the outer primary feathers. Garner et al compare (Identification of American Sandwich Tern, Dutch Birding Vol:29:5) the tips of the outer primaries of (Adult winter):
What I've discovered is that the worn primaries are just as distinctive as the fresh, and that the white fringe, when it wears away, leaves the primary tips (particularly noticeable on P6-8) with a distinctive hook shape.A major difference acuflavida and sandvicensis is the width of the white fringe to the inner web of the outer primaries. On acuflavida, the white fringe is very narrow (1-1.5 mm) whereas, on sandvicensis, the fringe is wider (2-4 mm), each primary also having 3-5 mm of white at the feather-tip (lacking in acuflavida) (Olsen & Lasson 1995).
I am very interested in the opinions of European birders who have experience with T. s. sandvicensis, and would love to hear opinion ether on this forum or in the comments at the blog post.
I would also be very interested in seeing any photos of European Sandwich Terns that either corroborate or contradict my findings.
The NAB article is at: http://www.nabirding.com/2011/09/25/sandwich-or-cabots/