Scattered over several sites you'll find birds which resemble hyperboreus in some degree. But it's fun to speculate which birds could still be regarded 'argentatus at the paler end of the spectrum (apes)'. One feature that should be present in presumed hybrids is a bicoloured bill to some extent I would suggest, something not visible in the bird below, which therefore may well be an 'ape(s)'.
Pictures today at IJmuiden.
EDIT: Sorry, probably had been better added to this topic:
viewtopic.php?f=7&t=12