Mate for Life

“Canada geese (Branta canadensis) mate monogamously and for life” is a generally accepted aphorism. Why are Canada geese faithful to one mate throughout their lives while most waterfowl select new mates yearly or even more frequently? David Quammen in his collection of essays entitled The Flight of the Iguana provides a reason. Although I tried to follow up on some of Quammen’s references, I did not find verification for this specific topic. However, I find the reasoning interesting.

A Canada goose is a large bird, many of whom undertake long annual migrations. Aerodynamically, being already large, a Canada goose cannot tolerate adding much weight in the form of fat reserves before migrating. Instead it must maintain its weight within a narrow range between eating enough food to survive and its ability to get airborne.

Canada geese also have very inefficient digestive systems. Their diet is made up of grasses and other vegetation. Lacking a rumen that contains cellulose-digesting bacteria, they cannot utilize much of the nutrition in their food. Anything a Canada goose eats rapidly passes through its body. A rumen and a full stomach would add more weight to an already large bird. Therefore a Canada goose must be constantly eating.

A Canada goose has little energy to spare because of an inefficient digestive system and lack of fat reserves. In a way it is a life on the edge.

Reproduction also adds to a Canada goose’s precarious situation. Females lose much of their energy reserves in laying a clutch of eggs and incubating them for nearly a month, rarely leaving the nest for a brief meal. Sometimes female Canada geese will starve to death on their nest. While the female tends to the nest, the male stands watch over the female. Even after the eggs hatch he remains as a sentinel while the female devotes her time to the goslings. This constant alert state and fighting off predators is difficult on the male and he too loses weight.

The family remains together through migration and on the winter feeding grounds. Canada geese parents remain responsible for their brood, standing guard and protecting their young, for almost a year until the young leave. At that point the pair is ready to breed again.

With such extensive familial responsibilities and long migrations, time spent courting means less time eating and maintaining nutritional needs and energy reserves. Rather than expend energy with yearly courting and mate selection, Canada geese commit themselves to long-term relationships and better survival odds rather than waste precious energy on finding new partners annually.

It is an interesting theory.

Then again. . . Conover and Dinkins, reporting their research in The Canadian Field Naturalist (2018), showed 15% of female Canada geese and 18% of males “divorced” their mates during their lifetimes. It was a small sample, but perhaps Canada goose fidelity is overrated.

This Canada goose pair was photographed at Baum Lake (Shasta County CA).

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4 Responses to Mate for Life

  1. usermattw says:

    Interesting theory. I’ve read that there are populations of Canada geese that have largely stopped migrating because people feed them, so they settle and stay where the food is. Since migration is part of this theory, I wonder if the lack of migration affects things.


    • gingkochris says:

      True, some populations of Canada geese do not migrate or are weakly migratory. Size, digestive inefficiency and reproductive energy requirements might still make mating for life advantageous. I do not know of any research on the fidelity of non-migratory Canada geese – a research project for some advanced degree candidate?

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  2. Interesting. Thanks for sharing. Who knows if it can be applied to swans too… Swans are also large and monogamous.


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