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Guillemots (also known as the Common Murre), like Puffins and Razorbills, are members of the Auk family.

Just like the Razorbill, Guillemots are descended from a bird called the great auk which is now extinct. The great auk was considered to be the northern hemispheres equivalent of a penguin, which is very apparent in their appearance. They have a chocolate brown back, with a white under belly, and a thin, sharp, black beak.

However, unlike penguins Guillemots are capable of flight, although (just like the other Auks) their wing spans are very small in comparison with the size of their bodies, which gives them a very inefficient high energy wing beat. But their wing size and shape enables them to be incredible swimmers and divers, propelling themselves deep underwater in pursuit of prey (small fish like sprats, sand eels, and crustaceans).

They are among the deepest diving birds in the world, typically diving beyond 30m, but capable of reaching depths up to 180m, and staying underwater for a couple of minutes.

There are huge numbers of Guillemots on Skomer, as well as smaller populations on both Skokholm and Grassholm. They only come to land to breed, spending the rest of their time out at sea, where they feel most comfortable.

Guillemots choose breeding sites on long narrow ledges, usually on sheer cliffs, and will squeeze as many bird as possible onto each ledge, with nesting pairs often in bodily contact with one another. They find strength in numbers and hope that this strategy will prevent predatory birds with larger wing spans from being able to land.

Breeding season usually begins in early May, when the Guillemot will lay a single egg onto the rock ledge. The egg is large and pointed towards one end, the theory behind this is that if the egg falls over it will just roll in a circle rather than off the edge. Each egg has an individual colouration and pattern so the birds are able to recognise their own eggs.

The parent birds will take turns to incubate the egg for around 30 days before it hatches. Then after around 20 days the chick will fledge the nest, gliding (falling) from the ledge, they will try to slow themselves by fluttering their wings. Although they are capable of diving as soon as they hit the water, they are not able to fly properly until 2 weeks after fledging.

The male then stay with the chick for up to 2 months, as they both swim out to sea away from the island and any predators.

Many birds will stay close to the islands over winter, while others will go as far as North West Africa and the Western Mediterranean, where they will moult and gain their winter plumage.