Nobody knows why?
As per this thread from Birdforum a few years back. Crazy really?
www.birdforum.net/showthread.php?t=324869
...........And this is the response I received today (24/9/19) from BOU...................
"Thank you for your enquiry re. Ruddy Shelduck and Category C5.
In order for us to admit any species to any category of the British List we need to be confident of origin. Ruddy Shelduck is commonly and widely kept in captivity in the UK and abroad, of which many birds escape annually (usually after the summer moult when keepers fail to round up all birds for pinioning). So admission for a species with such a large captive population with frequent escapes is always problematic.
For Ruddy Shelduck to be admitted to Category C5 of the British List we require evidence (e.g. a ringed or other tracked individual) that a bird (or birds) from the established continental populations have occurred here. We are not saying birds from the continent cannot, or do not, come here, just that to date, we have no proof of this. Such proof is not beyond expectation as we have recently admitted Egyptian Goose to C5 based on a Dutch ringed bird occurring in Cleveland".
Kind regards
