Not long out of the egg
Arctic Terns are colonial nesting birds. I’m sure of that fact. I’ve spent 18 years exploring Westray and seeing their colonies, large and small, on the coast. Whether it’s on a sheltered bay, on shattered rocks at the most exposed northern tip of the island, or on the grassy slopes, Arctic Terns always nest in colonies.
Walking back from enjoying one of their colonies I decide to check out a small pool and suddenly I’m being dive-bombed by Arctic Terns. They should be just giving me the occasional warning ‘chip’ as they pass by as I’m over a mile from the colony. This is a full-on attack. There are only four or five birds and they are complaining in the most extreme way.
There can’t be nests here, can there?
I look down

It appears that all you need is two nests to be a colony.
I leave them in peace.