I'm a commercial fisherman on the West Coast of Scotland (Inner Hebrides).
During the course of my working day I normally pass rocks which Shags (and sometimes a few Cormorants too) sit on. These are not their roosting/nesting areas, but lower-lying rocks that they use to presumably rest/digest/dry their wings on.
I have noticed that there is almost always a single Great Black-backed Gull on the rock amongst the shags. I would say 95% of the time there is a gull in attendance. Always just a single gull, never more.
My question - Is there any confirmed relationship between the two species? If not, would anyone like to speculate why this occurs? Is it simply that they prefer the same spots?
TIA!