In 2012, in Soham, I regularly observed a pale collared Jackdaw that wintered but remained into May and appeared paired with an un-collared western Jackdaw. So my own experience does support the idea that you can't be very confident about these birds. However, to brighten up a dull day, I do keep looking through winter corvids and do see quite a lot of these Jackdaws with neck collars. The "Nordic" Jackdaws, monedula, intergrade to the east with "Russian" Jackdaw, soemmerringii, which typically show darker bellies and throats.
The old paper from Dutch Birding 2003 is here