Why do white terns lay their eggs on branches?

BBC Wildlife contributor Mike Toms explains why these small birds lay their eggs in such a precarious location. 

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White tern on Midway Atoll in Hawaii

White tern on Midway Atoll in Hawaii © Danita Delimont / Getty

 

The white tern (sometimes mistaken called fairy tern) is a widespread species that breeds on tropical islands. While these birds also nest on rocky slopes, cliffs and buildings, they are unique among terns in using branches, a habit famously observed on Bird Island.

Here, females often lay a single egg directly onto the bough of a Casuarina tree, typically in a fork or depression. Though the eggs risk being blown or knocked from these precarious sites, they are safe from ground predators, which take many of those laid elsewhere on the island.

White terns lay just one egg that requires 30–41 days incubation, so there is a good chance that the nesting attempt will fail; fortunately, they may breed over a 15-year period. 

 

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