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.
Do you have a wildlife question you’d like answered? Email your question to wildquestions@immediate.co.uk or post it to Q&A, BBC Wildlife Magazine, Immediate Media Company, Eagle House, Bristol, BS1 4ST.