I have a little story about kestrels at our place.
Last year a pair of kestrels showed up at our farm in April and appeared to set up territory near a nest box. I was able to catch the female and band it. However, when I checked the nest box in May, the female on eggs wasn't banded. So, that female I banded on the left leg (it's just a convention, but I try to band all birds on the right leg). The female and her mate fledged five young from that nest box last year.
Then, a couple of weeks ago, a pair of kestrels started hanging around that same nest box, and when I scoped the female, I could see that she was banded on the left leg, so almost certainly the resident female from last year. However, on Sunday, I had some time to watch the two kestrels hanging around the nest box, and the female is not banded!
I never was able to catch a male near this nest box last year - too wary to go for a mouse in a trap. And, the male that first showed up this spring would also watch the mouse with interest, but make no attempt to go for it. However, on Sunday, the most recent male came immediately to the mouse and was caught quickly.
Without the banding program, I might have assumed that a pair of kestrels nested here last year and again this year - same territory, same nest box. - when, in fact, there have been at least three females and two, maybe three males all occupying the same territory for a time.
I checked with Gary Bortolotti, who has studied kestrels for years at 500 nest boxes at Besnard Lake, and he said he wasn't surprised at all, that there is a lot of turnover in the population and in pairings.
As I've mentioned before, we have had a pair of Great Horned Owls nest here every year for at least the last ten years, but it hasn't been the same two owls. There have been at least three different females and three different males in that time, with floaters moving in very quickly when one of the pair dies.
Some birds may indeed mate for life, as has been suggested by some people. I wouldn't know about that. I do know that, for raptors at least, the birds don't remain single long, if one of the pair dies or leaves for any reason.