The Atlantic Puffin builds its nests in burrows or caves in rocky cliffs. These would preferably be on islands unihabited by people, to minimize the human disturbance to the nests and young. Two kinds of nests burrows are most common and I will describe the advantages of one over the other.
The more common burrow type is the vertical hole in a flatter cliff top. This can be easier for the birds to first create, as he has sure footing while he starts excavating. But these burrows are also much easier to access for their primary predators, the Great Black-Backed Gulls. So the eggs and young in the nest have less chance of survival.
The second type of puffin burrow is the horizontal hole in a cliff face. While this may be difficult to start building, there are many of these already excavated. These have the distinct advantage of being almost impossible to access for Gulls so the survival rate for the puffin young is much better. Gulls cannot land in the mouth of the tiny hole in the face of the cliff, so they can't reach the eggs or young. The puffin adults, who can land with a 'short runway' and are small enough to get in, have no problem feeding their young here.
Several places in the north Atlantic region are hosts to puffin nesting grounds. The one I am most familiar with is the Bird Islands, just off the north coast of Cape Breton, Nova Scotia. Here the puffin burrows are the horizontal type, and the puffin population is thriving because of the design advantage. Several islands off the coast of southeast Newfoundland are known to have puffin nests, though many are of the vertical type which leaves the puffin young at a disadvantage.
More can be learned about Atlantic Puffins on a boat tour in Cape Breton, NS, called Bird Island Boat Tours. http://www.birdisland.net/
Sheila Van Schaick