LOL! I've got to have that lens! Where can I get one?
My experiences have been mixed. I've got a 70-300 on my Nikon D300, and sometimes I can get a full-frame shot, sometimes 1/4, and sometimes what seems like just a speck in the frame. It depends on the species of bird, what the bird is doing at the time, and what kind of environment you're in. Birds will typically let you get closer if there's something between them and you (natural cover). Even if the cover isn't complete, they'll often be more comfortable if there's something in the way. Another trick is to bird by car wherever you can. For some reason, wildlife feels a lot less threatened by four wheels, as opposed to two legs.
Another thing I've seen, if they've already spotted me, I've had more luck should try to approach them slowly, but standing upright, rather than trying to hide and creep closer. Predators like to be stealthy and creep closer. If you stand upright and just move slowly, you look less like a threat. I recently used both of these to get really, really close to a Little Blue Heron, a bird that's normally not very tolerant. He squawked at me a couple of times, but he never flew off, and I got close enough to take a full frame head-only shot.
One more thing to look for is the bird's behavior. If the bird is feeding or courting, it'll often be more tolerant of strangers. Just stop moving when it picks its head up and starts paying attention to you. Wait for it to get used to you and go back to what it was doing, and then you can inch a bit closer. Just make sure you take some shots early, in case you get a bit too close (you'll at least have something that way).
Each species tends to have its own personality too. You can study the birds in your area and get a feel for how close they'll let you approach.
So, I guess my answer is no, don't give up just yet
There are lots of ways you can get closer. Probably the most important thing you can do is learn as much as you can about your quarry. If you know their normal routine, you can anticipate where to go to get the best shot. If you know how they'll react, you can plan your next move more effectively.