Interesting question, my gut reaction would be that this is a 'founder effect', ie within the British population when it was at its lowest ebb few white morph birds survived the era of persecution (or alternatively were not ever here in the first place) and thus the genes for pale morph Buzzzards almost disappeared from the population. The sudden resurgence in eastern England might therefore be the result of colonisation by Continental birds, witness the big Common Buzzard movement this autumn...
this from Kruger et al. (2001) Evolution 55 (6):
The common buzzard is highly variable in color, and three main morphs have been distinguished (Glutz von Blotzheim et al. 1971; Melde 1983), although the substantial variation even within these morphs led Ulfstrand (1970, 1977) to describe a continuous spectrum from dark brown to almost pure white (see also Cramp and Simmons 1980). The frequencies of the main morphs vary across Europe. Some authors have suggested that there is clinal variation in morph composition (Ulfstrand 1977), but it is now widely accepted that geographic clines are not detectable (Melde 1983; Del Hoyo et al. 1994). Nevertheless some generalizations can be made.
The dark morph is always the rarest morph, ranging from 0.1% in southern Germany (Dittrich 1985) to over 3% in central Germany (Scherner 1980), 9% in Sweden (Ulfstrand 1977), and up to 17% in another region in central Germany (Zang et al. 1989). The light morph is more common than the dark morph, ranging from 2.3% in southern Germany (Dittrich 1985) to over 10% in central Germany (Scherner 1980), 14% in Sweden (Ulfstrand 1977), and its highest frequency is 21% in central Germany (Zang et al. 1989). Indeed, using the frequencies of the two morphs given above, they are significantly correlated (r 5 0.963, P 5 0.03). The intermediate morph is always the most common, ranging from around 60% (Zang et al. 1989) to 98% (Dittrich 1985). Thus, the typical European situation consists of a polymorphic population with the majority of birds being intermediate.
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