Recent evidence suggests that birds on the Atlantic coast of southwest Europe are distinct from all three, and may be an as-yet undescribed subspecies.

It is distinguished from the great cormorant by its smaller size, lighter build, thinner bill, and, in breeding adults, by the crest and metallic green-tinged sheen on the feathers.

[12] European shags are preponderantly benthic zone feeders, i.e. they find their prey on the sea bottom.

Their chicks hatch without down and so they rely totally on their parents for warmth, often for a period of two months before they can fly.

Fledging may occur at any time from early June to late August, exceptionally to mid-October.

[13][14][15][16] Evidence collected at one colony, the Isle of May, Scotland, between 1985 and 2014, suggests that shag chick diet composition in this population has diversified in response to ocean warming.

[17] Shags also feed on fewer sandeel on windy days, presumably due to the strong effect of wind on flight in this species.

[18] The year-round diet of full-grown shags at this colony has also changed over the past 3 decades, from sandeel specialists to an increasingly diverse prey base.