One of the reasons I enjoy photographing birds is the combination of technical challenges and general accessibility of subject matter. In general, encountering technical challenges gives me better intuitions about how to use my camera to get the shots I can and to think about how to get the shots I can't. Because I may find them out my back door, in an urban landscape, or out in a nature preserve, having birds among my standard subjects provides plenty of opportunity to practice and learn.
Anyway, I have and use a kit lens of similar specification. At long zooms it struggles irrespective of subject. This is, as Michael pointed out, due to the lack of available light at f5.6. One thing I've learned from pursuing technically challenging subjects is that autofocus is there to help me get the shots I want.
If it's not doing that job, I turn autofocus off and focus manually. In general that's my rule for any feature that starts with "auto". The closer I get to shooting at the limits of the camera, the more "auto" features I turn off. Camera designers build automatic modes to make photographing 'common scenes' easier. I'll admit that 'common scenes' is a bit slippery: professional cameras are designed to support a wider array of common scenes than consumer oriented cameras [at the consumer end of the spectrum, the design tends to optimize good snapshots].
Anyway, two items of advice:
- Obviously, turn off autofocus and focus manually when autofocus isn't working.
- Shoot with a one or two stops smaller aperture [higher f-stop number, f8 or f11 over 5.6] to increase the depth of focus. This will increase the range of distances over which the subject will be in acceptable focus and often improve the sharpness of the optics. Compensate with some combination of ISO and shutter speed [shooting in manual mode makes this simpler].
Using a smaller aperture trades sharpness for some 'pleasing bokeh' I suppose...but a kit zoom wide open at long range will still be more dependent on choice of background than depth of field. The sky or foliage distant beyond the subject will work well in either case and foliage just beyond the subject will be about the same at long zoom.