A difficulty here was shutter speed, sometimes as low as 1/15s, not so good w/ a 700mm lens. I balanced the lens on a sturdy tripod, turned off IS, locked the mirror, and used a cable release with live view. That achieved a sharp image as long as the birds were reasonably still. I discuss this in a photo tip on my website at http://garyluhm.net/big-lens-low-light/.
In Photoshop I changed the background of the image because the original was full of hot spots. I dodged and burned as well, trying to emphasize the light shaft from above.