Bird Photography Ethics is a very mine-filled topic at many birding and nature photography forums. Understandably birders want to protect and conserve threatened and endangered species from further harm. Bird Photography requires clear frame-filling shots of which magazine editors will take notice. Does this mean that bird photographers have to work at cross-purposes with birders?
No. Most photo editing software programs have crop functions and working with a DX or crop sensor will help, especially with the high megapixel sensors that seem to populate crop sensor cameras nowadays. You can also use resizing programs to resize your image up to a usable image. Of course it is better to resize the original image and then crop it to what you want, though it will take some time to resample the image. Resizing first then cropping is the preferred route to go.
I work with a D300s (1.5x crop sensor) camera and utilize cropping for most of the framing in my images mainly because I will not get close enough to agitate the birds. For me, staying "on the dyke trail" at Boundary Bay means staying on the dyke trail.
This is a great blue heron shot that I took at Iona Beach Regional Park. Partway along the causeway, there is Mud Bay. That bay is a great little place for wading birds and herons. I took the shot from the road with my 600mm. This is the original image from my camera/600mm lens "uncropped".
And this was the image that I got cropped and resized back up to a usable image.
A 600mm and teleconverter will not solve your image problems with regards to composition, however the resizing and cropping will go a long way to giving you a lot more flexibility with your photography.