Graham Catley at the excellent Pewit blog is discussing his finding a Bonaparte's Gull and a Ring-Billed Gull on his local patch. He says finding rarities is "80% luck and 20% persistence". So I guess I'm just not very lucky.
But on further reading he says "have checked the gulls three times today", and then having noticed an odd bird writes "in the next three and a half hours it was asleep for 90% of the time".
Three and a half hours!!! Well, firstly I would have had just the one look through and found nothing, and then even if I'm with him at the start of his third look, its a good three hours after I've gone that he finally nails the bird.
So, as I suspected, its 50% skill, 50% persistence, and 0% luck.