Black throated Diver  Garvia arctica.  Status:   scarce in summer, more frequent in winter and on  passage.

The last confirmed breeding record was in 1972.
 Black throated diver April 2018 Calgary

Black throated Diver in Calgary Bay May 2016 photo by Paul Thomas. Visitor from Sheffield
Photos  below were taken on Loch an Dorb, near Granton on Spey by Peter Lewis

This pair birds spent summer on a loch on Mull in 2011, but nothing came of it regarding breeding

winter plumaged bird photo by Michael Booth 2014

Black throated Diver in summer plumage.

Mainly a winter visitor to Mull's shores and sea lochs, with occasional birds staying throughout the summer. It's always worth scanning the lochs with a scope to look out for this magnificent bird.