Skull was found on Kilchoan Beach on Colonsay
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I wondered why you think this is Black Guillemot? I would think Common Guillemot is much more likely. This strongly resembles my Common Guillemot skulls but I don't have the benefit of a Black Guillemot skull for comparison. Try www.shearwater.nl or www.skullsite.com.
Lat/Lng: 51.586, -0.077
OS grid ref: TQ333892