Whooper swans now opting for threesome

Whoopers visit Ireland in winter, moving about in noisy flocks. However, on returning to Iceland in spring, their behaviour changes; each breeding pair establishes an exclusive territory, off limits to other swans. Intruders aren’t tolerated.
In about 9% of bird species, pairs get help from others when raising their young. House martins, for example, fledge two broods from mud-cup nests under the eves of Irish buildings.