Most of my decent wildlife photos have relied on habits of the creatures photographed. There's absolutely no shame in it - the really good guys I've come across are naturalists and observers first and photographers second.
I have a feeling that buzzard from that position is right on the limit of what's possible with your setup. I don't think you'll improve it with a crop, simply because there's insufficient detail.
I have a similar pic of an Osprey (below) - the composition is nice (I caught him taking off), but... the lens isn't long enough, I wasn't quick enough, he isn't in sharp focus, and there's even a bit of camera shake (your fencepost is an excellent idea - I think I had a uselessly-dangling monopod for that shot!).
- Male Osprey Eldorado Park (c)2017 SDM
- (152.95 KiB)
It's one of those "I saw it and I can prove it! photos - certainly not for a big wall print! I can't improve it any further, as more tweaking would make it worse!
In the case of your buzzard, using the raw file would help hugely, as you'd be able to correct for the chromatic aberration visible around the bird (caused by the lens), and significantly increase the sharpness, with no loss of quality.
You can see jpeg or processing artefacts, as you can on my Osprey. Don't over-sharpen in the photo editor - there's a slight halo round the bird. The Osprey is horribly over-sharpened too!
Given there's good light (the bird isn't shaded), a teleconverter would probably have worked quite well. I've no idea if they are available for Panasonic though they probably are.
The light is behind you, which is also very helpful as it will get you the best exposure, at the expense of a little 3D-ness to the image, because of where the shadows fall, and it's a bit harder for him to see you clearly!
It's one of those photos where you'll be fine at the time of day you took the original photo, but at other times, light falling across the front of the lens will dramatically decrease the quality. The image will get duller and you might also get speculars (bright patterns) across it (like car headlights in rainy night movie scenes).
If the lens doesn't have a hood, try some rolled up cardboard around it to keep the sun off (elastic band to keep it on the lens, if you can make it black inside so much the better, but anything to shade the front of the glass will help.