Why do some birds mate for life?

We put this to Rebecca Kilner, Department of Zoology, University of Cambridge...I don't think there's a consensus in the scientific literature on this, but there are a couple of good ideas and the general theme is that if it takes two to rear babies effectively then you stick together with the one that you already know. So, that might be the case for example if nest sites are in short supply and it takes two of you to defend the site, then it's better to stick with the team-mate that you've got than to change for the next season. Similarly, if you're feeding your chicks on complex food that's hard to forage for, it's better to stick with an experienced bird than to try your luck with a potentially naive individual.