Like mentioned filling the frame with small birds at a distance of 18' or so can be tough without going for long (read expensive) glass. If using a crop body camera or even a good point and shoot can help. Though if you can get the the bird to fill at least 60-70% of the frame and use good technique than cropping down and lead to excellent results.
But nothing beats a big, long and fast lens to fill up the frame except maybe some luck and can get very close < 10' or so if the lens will focus down to that distance.
This Bushtit was right about 15' from the camera, a 50D using a 500mm f/4 L on a gimbal and tripod. It does have a portion of the right of the frame cropped out.
This hummingbird was at about 9' and using the 100-400mm L at 400mm on a 30D, with just a minor crop
This Pine Siskin was at about 10' using the 100-400mm @ 400mm again with the 40D.
The last two shot in my backyard and at about half the distance you are mentioning, so you can see that to fill the frame one has to be close, very close. And like I mentioned while these are most of the frame if I had been another 4 or 5' away I still could have captured close to the same detail that a crop would have pulled up.
Good luck!