Since we have had a bit of a game of musical chairs going on here this year, it seemed appropriate to revisit this question ...
Princess is with Ivy this year at the Radisson because Trey didn't/hasn't returned. In 2003, Trey took a sabbatical from his last mate Madame before returning in 2004 with Princess. Now that Princess is on eggs, I'm not sure what would happen if Trey arrived at this point. Trey's dad, T-Rex took over his father Pop's eggs, when Pop disappeared mid-incubation.
Jules is with her new beau in West Winnipeg, despite Ivy still being around town. Keep in mind that the West Winnipeg and the Radisson birds can see each other from their nestsites - yes, their eyesight really is that good!
And in Brandon, we think that it might be the 2008 resident pair Zeus and Holly that have returned (we have tried to read their bands but they aren't cooperating!). If that's the case, then both Zeus and Holly have taken sabbaticals. They both took 2003 "off", then Zeus did again in 2007 and Holly did in 2009. In 2003 we had not nest in Brandon, but in 2007 Holly paired up with Zeus' brother (Keener-Screech) and in 2009, Zeus was paired with a young Radisson female, Hurricane. When both Zeus and Holly are around however, they are the resident pair.
So yes they mate for life, but life is means as long as your mate returns.