Seabirds are birds that have adapted so they are better able to live at sea or near it. Many seabirds live "at sea", that is, far away from land. They may only come on land to breed. Other seabirds live in colonies on islands. Very often, seabirds use the sea for food, most often fish or shellfish. The first seabirds evolved in the Cretaceous period, and modern seabird families first appeared in the Palaeogene. The similarities between them are examples of convergent evolution. The same environmental problems and feeding niches have resulted in similar adaptations.