Monday, February 01, 2010
THE Bewick swan is an example to us all; it’s one of the world’s most monogamous birds.
News of a divorce in Britain, therefore, comes as a shock. Swans visiting the Wildfowl and Wetlands Trust reserve at Slimbridge are studied intensively. This winter, a bird which had been paired for the last two years, arrived with a new partner. It was assumed that the original mate had died and that the swan had remarried. Then the former spouse turned up. It, too, had a new partner. Julia Newth of the Trust told RTÉ’s Mooney Show that this is only the second divorce recorded among 4,000 pairings over a 60-year period. The former spouses have, so far, been ignoring each other.
First recognised in 1815, the species was named after Thomas Bewick, the famous bird artist, although he had no association with it. Telling the various kinds of swan apart can be tricky. The familiar ‘mute’ swan, present throughout the year, has a ‘Celtic’ S-shaped neck, and a black nose, known as the ‘berry’ over an orange bill. The Bewick, a winter visitor, has a more angular appearance; the neck is straight, there’s no berry and the bill is black with a yellow patch. The larger whooper swan, another migrant, is similar, but there’s more yellow than black on its bill.
The Bewick’s extraordinary lifestyle may explain its penchant for monogamy. It breeds in Siberia just south of the Barents and Kara seas and on the island of Novaya Zemlya, 3,700km from Ireland. This wild region has a brief, but productive, summer when midges and mosquitoes make it unbearable for humans. The swans arrive in late spring. They must nest, raise their young and leave before the weather deteriorates in the autumn. It’s a most demanding task so the pair bond must be rock solid; there’s no time for ‘flings’ or for worrying about possible rivals. Being with the same partner year after year increases the prospects of nesting success; birds come to know each other’s ways and their local area intimately. Pairing begins, at the age of two or three, in the Siberian autumn flocks or at feeding stations along the route to western Europe; at any rate Bewicks are paired when they arrive here. Actual breeding starts at age five.
Monogamy is a high-risk strategy because all of an individual’s eggs are ‘in the same basket’. If its chosen partner has some genetic weakness, all of a bird’s offspring may be doomed. Most species guard against this possibility by having other partners, extra ‘strings to their bow’. The added genetic diversity increases the chances of their genes surviving.
The Bewick’s cousin, the whooper, comes to us from Iceland. Conditions there are less severe than in Siberia and the migration to be undertaken is much shorter. Whoopers, therefore, can afford to be less rigid in their marital mores and divorces are recorded occasionally. Compared with its migrant cousins, the resident mute swan has a cushy number; its breeding season is long, the weather is mild and there’s no need to migrate. More flexible marital relationships are only to be expected. Having ringed and tracked more than 1,500 of them, I was able to estimate the divorce rate of Irish mutes. It’s about 3% per year. A marriage doesn’t exist until it’s consummated, so only swans known to have produced eggs were regarded as married. A bird was deemed to be divorced if, having produced eggs with one partner, it went on to do so with another, while the first spouse was known to be still alive. The actual divorce rate may be a little higher than 3%. Break-ups are probably more common among failed first-time nesters and some of these short-lived nests may have gone undetected. A few cases of partners being swapped during the breeding season were recorded, so it’s not always certain that eggs in a nest belong to the husband. Of course, under the rigorous criteria I applied to mute swans, the Slimbridge Bewicks would not even count as married, let alone divorced. The pair had been together for several years but, as no young ever arrived with them at Slimbridge, we can’t confirm that they had produced eggs. Nor is there anything to suggest either of the new unions have been fertile.
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