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You are lucky - they are almost ripe, ours are always taken green by the squirrels.
Yeah they are just falling out of the husks? now. There are plenty of red squirrels locally and the greys are making inroads in recent years but there is so much hazel itd take a lot of squirrels to create competition for nuts between them and me! Its time I had a bag or 2 gathered and put away to sweeten. Its a pity your squirrels hit them so hard, that would be annoying.
There is just one native hazel. Two other species occur infrequently, mostly as cultivated plants. Fide Stace, The Turkish hazel (Corylus colurna) has bracts twice as long as the nut; the filbert (Corylus maxima has the bracts forming a cylindrical epicalyx, rather than the bell-shaped one of the other two species.
So, while I wouldn't have worried about confounding species, this is Corylus avellana.
Fide Wikipedia there are 14 or more species worldwide. Some of these are likely harder to distinguish.
Hillier's manual mentions several of them, so you might find them in large collections.
Chances are then I will only be seeing Corylus avellana on my rambles. I gathered a few nuts today. There didnt seem to be as many nuts as in previous years when I went looking for them. I watched a red squirrel vigorously ripping nuts from the high branches today, I was on a high bank and the squirrel was at eye level, didnt know I was there, the photo wasnt possible through the vegetation.
Lat/Lng: 55.0531, -5.9706
OS grid ref: NW465803