Did a couple of hours at Farmoor to see the Slav. Also had the interesting Lesser Scaup-type hybrid.
Slavonian Grebe (above and below)
Hybrid aythya 'Lesser Scaup-type'. (Presumed Lesser x Greater Scaup). Even has the black confined to the nail of the bill. More on this fascinating bird HERE. The American race of Greater Scaup is supposed to look mid-way between a Lesser and Eurasian Greater Scaup (something we've been keeping an eye out for on the Azores) but the back of this bird looks too dark/tightly vermiculated.