Daphnias in birdbath

I've been very busy lately and have not cleaned my birdbath nearly as often
as I should. I glanced at it this morning and noticed that there are
daphnia swimming in it! It's kind of ironic that I'm *trying* to grow
daphnia in some buckets, but haven't had much luck with them because I don't
really have time for them either....

Anyway, I'm guessing they arrived in the birthbath on the feet of birds as
eggs, but that's just a guess. Any other speculations?