Achieving focus exact focus at a distance handheld can be very difficult to say the least, but even more onerous when the bird is moving either by flight or because what he's resting on is moving. I got so frustrated when I started capturing wildlife I blamed everything including the camera, the lens and then me!
To save frustration I now usually try to get closer eliminating the large crops which, as has been said, degrades the quality greatly. Achieving the desired focus point can sometimes be frustrating and near impossible. The attached image is one of 600 taken in 1 hour and this was one of only 4 I nailed with the focus on the birds head. The conditions were perfect and although Stan has quoted the difficulty of a blue sky, worse is a cloudy white sky which will usually end up with the bird in silhouette. It just takes practice, lots of it and now I restrict most to my images to kind light and closer distance. Keep the speed high and the aperture at best f7-f9 or just either side. This applies equally to birds in flight or static.
This image shows the focus point and the camera details. Take no notice that's it a 70-200mm f2.8, I took loads of keepers with my 300 f4 and sure I would have achieved the same with my 70-300mm. Everyone here knows how I struggled with my D7000 on focus and even I sent my first copy back, although the dealer did admit it was the camera not me. The second took me 6 months to master, but when I did got some good results.