There are only 44 gannet colonies occurring in the world and St Kilda is by far the largest. One fifth of the world's population of northern gannets nests on Boreray, Stac an Armin and Stac Lee in huge colonies. That's a massive 60, 428 breeding pairs (so double that for the actual number of birds!).
Why do they choose to live so close together? We think one reason is because there is a good supply of food nearby. The seas around St Kilda have a lot of fish in them!
Gannets dive for fish and squid to eat and can hit the water at nearly 100 kilometers per hour. So they have extra thick skulls, which act like crash helmets.