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Canada geese famously mate for life, but it turns out that some of them aren’t ready for the commitment.

A 25-year study of Canada geese with identification collars in Connecticut showed that 15 to 18 per cent of Canada geese dump a living mate somewhere along the line. And usually, a divorcee will pair up with a new mate that is younger than the ex.

Around this point, someone will say that birds don’t get divorces, and newspapers should stop dumbing down science. So it’s worth noting the title of the paper: “Divorce in Canada Geese (Branta canadensis): frequency, causes, and consequences.”

In fact, studies of divorce among bird species that usually mate for life go back at least into the early 1980s. They involve many other species — snow geese, trumpeter swans, great skuas, oystercatchers and more.

The latest study, in a journal called the Canadian Field-Naturalist, tosses around several possible reasons why what’s good for the goose is not always good for the gander. Most of them have something to do with the ability to raise babies.

• The “incompatible mates hypothesis.” This says that two birds simply can’t co-operate in the way necessary to have a family. The authors discard this one, saying that many geese break up after several successful breeding seasons.

• A search for better territory. Good territory is essential to nesting and breeding, and this theory says one bird leaves its mate in order to move to better territory.

The two biologists reject this one as well, saying that many divorcees stay on the same territory after the split.

• Birds are accidentally separated during migration. It sounds logical, but the Connecticut goose population stays year-round in the same spot.

• Their new theory is that “at least some divorces occur when one mate is willing and able to nest, but its partner is not. Under such circumstances, the willing partner can increase its direct fitness by finding another mate and reproducing rather than foregoing the opportunity to nest that year.”

Splitting up and finding a new partner did not seem to affect the birds’ ability to reproduce, they found. One bird from each pair, however, usually took at least a one-year break from breeding after a split.

The authors — Michael Conover of Utah State University and Jonathan Dinkins of Oregon State — don’t address the question of whether it is the male or female geese that decide to initiate the split.

tspears@postmedia.com

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