Some flightless varieties of island birds are closely related to flying varieties.

Some flightless varieties of island birds are closely related to flying varieties, implying flight is a significant biological cost.

The island birds, owing to reduced exposure to infections and low genetic diversity, appear to be more susceptible to infections due to a weaker immune response.

Like many other island birds, they lost the power of flight because it was no advantage where they lived.

By one measure, he writes, "an island bird faces about 50 times as great a likelihood of extinction as a mainland bird."

Many island birds become flightless, including the Aldabran rail and the extinct dodo of Mauritius.

The susceptibility inherent of many island birds is undoubtedly due to evolution in the absence of mainland predators, competitors, diseases and parasites.

Clearly, island birds face a much higher probability of extinction than mainland birds do.

The mainstay of their food supplies was the profusion of island birds, especially gannet and fulmar.

Conservation of island birds.