Another note from the comments under Sandwich Tern by Joseph Kennedy on 4 September, this deserves a separate discussion (Thanks, Joseph). He writes:
I have received a note Stu Wilson concerning the black tern pictures posted from the same date. I guess that I should check feet on the rest of my tern pictures. According to Sibley, young black terns should have light colored legs as would adult non-breeding birds. My picture of the young black tern has black legs
Sibley shows all plumages of white-winged terns and adult black terns as having yellow legs, if this is actually a difference, it would be an easy way to id all of the stray white-winged terns that pass through the UTC. Perhaps this is a carryover from European birds and US birds are different.
In The Sibley Guide I illustrated the adult Black Terns with dark legs, which is accurate, and the juvenile Black Tern with bright orange-red legs, which should be a darker dusky pink color, and sometimes blackish. From photos such as the one above it is clear that even juvenile Black Terns can have very dark legs, and the juveniles that have paler legs (photos in Olsen and Larsson) still show dusky pink legs.
All ages of White-winged Tern (from photos in Olsen and Larsson) show brighter orange-red or orange-pink legs – drabber on juveniles than adults, but usually (always?) brighter than on Black Tern. So this might be worth watching for in standing Black Terns. A bird with bright legs could be a White-winged. According to photos, European Black Terns are similar to American birds in leg color.
In photos I also noticed a longer-legged appearance of White-winged Tern, which is confirmed by measurements in Olsen and Larsson: tarsus averages 16mm for Black Tern vs 19mm for White-winged. This is not diagnostic, and apparent leg length depends a lot on posture, but could be a useful point to watch for on standing birds.