Common or Mealy (flammea) overlaps in appearance with Arctic (exilipes), Icelandic (islandica) and Lesser (cabaret).
Cabaret overlaps with Greenland (rostrata) and flammea.
Islandica overlaps with rostrata, flammea, exilipes and Horneman's (hornemanni).
In a vagrant context the two birds (above), one from the Greenland population and one from the Lesser Redpoll population would be inseparable.
Again, these two birds together (above) in the UK would probably be passed off as two Arctic Redpoll- the bird on the right is Icelandic (islandica).
Two specimens (above) that show the overlap between Mealy and Arctic (exilipes)- assuming correct identification by the collectors.
Overlapping features between Mealy and Lesser are well known (above)
A not so typical collection of Redpolls. The Mealy is smaller and darker than the Lesser and would probably be identified as such, the Arctic (exilipes), Greenland and Icelandic could maybe all be passed off as a flock of Mealys and the Horneman's is on the small size and might even fall within the range of a large exilipes.
How the field guides would like things to be (above) and indeed sometimes they are. There are fairly diagnostic types within the variation of the Redpoll complex, for example:
a) A small brown Lesser Redpoll is pretty safe
b) A large dark Greenland Redpoll is pretty safe
c) A large chamois faced Horneman's is a safe bet.
d) A small, striking pale Arctic (exilipes) is another safe one.
Mealy and Icelandic- well there appears to be complete overlap. In the context of vagrants there will naturally be a lot of uncertainty unless we are dealing with various peaks in the variation landscape- various ends of various spectres.
The icelandic population perhaps throws a spanner in the works for some of the species concepts which are currently accepted for the Redpolls. The status of exilipes, hornemanni, rostrata and flammea in Iceland is presumably a fundamental consideration when trying to work out what the Icelandic populations represent in order to establish identification criteria for islandica. If these Icelandic populations are migratory and occur in Britain- applying identification criteria to vagrants in certain settings is very problematic indeed. All images copyright of the Natural History Museum, London.