When bird mating goes wrong

OLDER readers will remember the notorious Ne Temere decree, which discouraged Catholics from marrying outside the fold. Such unions are acceptable nowadays but a ‘matrimonia mixta’ in Virginia, Co Cavan, is raising eyebrows.

A mute swan and a goose have taken to living as husband and wife on the local river. Swans have a reputation for virtue and decorum, so how odd is this union?

There are seven swan species worldwide. When kept together in ornamental collections, such as those of the Wildfowl and Wetlands Trust in Britain, swans mate occasionally with members of the ‘wrong’ species. They may do so even when suitable partners of their own kind are available. Liaisons between mutes and five of the other swan species have been documented. Only the black-necked swan of South America seems to have an unblemished record. The offspring of such unions are generally sterile but some may be fertile.

Greylag, snow and Canada geese have teamed up with swans. Such abominations, however, are only to be expected among birds raised in unnatural surroundings. Cygnets may have hatched under a brooding bird of a different species. Raised with ducklings or goslings, they ‘rub shoulders’ with creatures they would rarely, or never, encounter in the wild. The crucial learning and imprinting processes of young chicks can be disrupted, resulting in confused identities and aberrant mate selection later in life. A bird subjected to such an upbringing can scarcely be expected to know the ropes, let along the whips and the chains, of normal marital relations.

Cross-breeding is very rare among swans in the wild but it does occur. During the 1960s, a mute and whooper pair bred for several years on the Corrib and I remember seeing two mute-whooper hybrids in a flock at Dundalk docks some years ago.

Greylag white-fronted and Canada geese will readily embark on illicit relationships. Nor are such tendencies confined to captive birds. Hybrid youngsters turn up from time to time in wild wintering flocks.

These shenanigans give the impression that swans and geese are more prone than other birds to indulge in such excesses. No so. These are high-profile species and, like human celebrities, can’t so easily cover up a scandal. Swans are large and conspicuous. They don’t hide their nests.

Mating for life and remaining constantly together, an unusual pairing will inevitably come to light.

Strange liaisons are far less noticeable among birds whose breeding and family lives are conducted out of sight. The nests of garden birds are seldom seen and, since hybrid young tend not to reach the fledging stage, abnormal pairings aren’t easily noticed. Swallows and house martins nest in, or on, buildings, making their domestic arrangement much more visible to us than those of other songbirds. Not surprisingly, house martin-swallow crosses are fairly often reported.

Around 10% of all bird species are known to hybridise but this may only be the tip of the iceberg. Cross-breeding is usually doomed to failure but, very occasionally, by combining ‘good’ characteristics from both parents, it can confer benefits; genes of survival value may be imported into a species line by the occasional immoral fling.

The tendency to breed across the species divide presents a serious threat in some cases.

The black duck population of the US and Canada is being depleted by interbreeding with the more numerous and successful mallard. American ruddy ducks escaped from wildfowl collections in Britain and began nesting in the wild. Now they are interbreeding with native white-headed ducks in Spain, threatening the survival of that species.

We need a Ne Temere decree for ducks!