Bald eagle in flight

The one thing I that definitely did not work for me last weekend was panning. When I tried to get birds in flight, the pictures turned out blurry. One reason for sure was the shutter speed, the light in the afternoon was not as good as earlier that day, when I was photographing in bird blind. I could have increased ISO some, but then my shutter speed would not reach anywhere near 1/1000 anyway, and the grainy pictures could not be submitted to stock, even if it worked. But the main reason is that I had no experience in panning, especially not handholding such a heavy lens. Panning exercise this weekend?