Q: Do any local breeding birds mate for life? Why? [Leo, San Francisco]
A: Some local birds do form long-lasting pair bonds of several different kinds. Ravens and scrub jay pairs hang out together all year, not just at breeding time. Wrentits and resident Canada geese also have long relationships that can last more than ten years.
Most songbirds have the same mate every spring, but this has more to do with site fidelity than with long-term partnership. Males usually arrive early and use songs and displays to define, control, and defend the same territory each year. If a female arrives and finds the same male, courtship ensues, copulation follows, and both tend to the young. But if last year’s male lost his territory, then the new guy will do just fine. So real estate is more important than individuals. But there is a direct correlation between fitness and the ability to secure good territory.
Most of the 12 seabird species that breed in our region mate for life. But it’s fidelity to territory, not to individuals.
Scientists are discovering through DNA analysis that pair bonding does not mean complete faithfulness. There is a lot of cheating going on. The proper term for this is —extra pair copulation,— and both males and females do it. This may increase the genetic diversity and therefore the viability of the offspring. But we are far from understanding the complexities of avian sexual relations.
The evolutionary bottom line is how many thriving offspring you leave behind. There are successful species of birds that mate for life or for a season or for one copulation. Each strategy works for whatever species uses it.