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.
Chicks from the first brood sometimes feed those of the second. By helping their baby brothers and sisters, with whom they have genes in common, they are furthering their own genetic prospects. This is enlightened self-interest, not altruism.
The ostrich-like South American rheas also get help. The male creates a nest, around which several itinerant females lay. Having gathered the two dozen or so eggs into the nest, he may recruit a younger male helper, who takes over incubation. Then he constructs a second nest. The benefits for the helper are unclear.
He gets to ‘know the ropes’ during the apprenticeship, valuable experience when becoming an alpha male in his own right. Also perhaps, unobserved by the boss, he mates with visiting females and some of the eggs contain his genes.
Whooper swans don’t indulge in such shenanigans. In 2003, pairs began breeding around fishponds in southern Poland, where ornithologist Krzysztof Dudzik and colleagues ring cygnets each year. In 2006, an additional male was observed on an occupied territory.
This ‘lodger’ was not a lover. Both the alpha male and the incubating female accepted the outsider, but chased away intruding mute swans and white-plumaged domestic geese. The intruder’s ring identified him as a two-year-old son of the alpha male.
Whoopers expel all previous cygnets before starting to nest in spring. That an extra male was allowed to remain, and approach to within a few metres of the nest, was very strange indeed.
The ménage-a-trois continued in 2007. Three fully-grown cygnets from 2007 were allowed to stay on the territory until May 17, 2008, a very late date. There was a two-day-old chick in the nest by then.
In April 2009, the alpha male was found dead. The lodger, still present, now took over defence of the territory, remaining close to the nest where the widow was incubating. They “became an item” the following year and bred, though unsuccessfully. They did so annually until 2012.
Initially, the researchers dismissed this as a freak one-off occurrence. Then, in 2013, another threesome was found. The female and the extra male, this time, were siblings of the 2007 pair. All three swans defended the territory aggressively. The strange liaison continued until late May 2014. In 2016, the intruder’s place was taken by yet another male, an offspring of the female.
Similar behaviour has been recorded in mute swans. A pair nesting at Knock Lake in Co Dublin tolerated a male lodger on the territory for two years. Mutes are fiercely territorial, but the trespassers-will-be-prosecuted rule is not as rigid as it seems; they will even nest communally.
I counted up to two dozen nests, separated from each other by a metre or two, on a Donegal island during the 1980s. There are similar colonies in Denmark. That outsiders are tolerated on territories in Poland suggests that whoopers have a similar latent propensity.