I'm not sure if birds can get them (though I don't recall hearing that they do), but I know that cats can be one of the worst flea carriers among animals. Don't you have ferals around where you live? That could be why it's such a constant battle for you too. When I lived in southern MO, we struggled with fleas on the collies a lot too. We had a neighbor with at least 30 or 40 cats that lived outside around his house and strayed all over, and I'm sure that was a big producer of our problem too (Very little animal containment law enforcement there).

The best luck we had was with Advantage. We got it the year it came out and it was the first time we could really get rid of the fleas on the dogs. But we moved to MN that year, and I have never seen a flea since. In MO, we had to constantly flea bomb the house, spray the yard, flea dip/bathe the dogs, as well as flea repellents and so on on the dogs. They had cedar shaving stuffed beds because cedar tends to repel fleas. But unfortunately, no matter what we did it was always a struggle, and a very frustrating and expensive one. What made it doubly worse was that Willie had a flea allergy and every summer had big oozing sores on him from flea bites .