Why do you believe this? Do you believe cats and other animals have no consciousness, so every behavior they exhibit is just instinct? Or do you believe they have some conscious behaviors, but killing birds is not one of them, this thing in particular is just an instinct?
For the first position, I think it is quite clear to anyone who studies and spends time with animals that they have something that is at least of the same kind as our consciousness. I just don't see how you can ascribe the wide gamut of complex, situatuonally and mood appropriate but still varied behaviors of animals to being purely instinct driven.
For the second position, I would like to see some study or some rationale behind it - especially since cats don't kill every bird they encounter, so if it's an instinct, it must still have some trigger, and hunger is not a viable explanation for most of the killings referenced here.
I wasn't arguing for whether cats have consciousness, and I agree that they do, to some extent. (We just lost a 23 1/2 year old cat who had lots of personality.) But killing birds or mice is one of their instincts. One of the triggers is nearness. Young cats will watch a bird through the window with their tail twitching, and the closer the bird is the more excited the cat gets. If they could get through the glass I'm pretty sure they'd go after it. Older cats (my old cat in particular) watched, but either they understand what glass is, or they're too tired to do much about it.