To give you an idea of setting up some perches in the garden have a look at some of the pics below. What I have done is got some copper tube and tied perches to them and placed feeders on them. With the copper tube they are easier to move around if needed. I also have some wood perches but need to get some larger moss branches / logs for a more pleasing perch. I also have some squirrel proof feeders hanging form heavy duty string/cord this feeder is to attract the birds to the garden and a further away from the house the birds don't mind me now and come to the closer perch which is about 3.5meteres from the back door. I have my tripod set-up at the back door and take the photos through the glass so there is probably a little loos of IQ.
I have recently changed to the D810 from the D750 and both camera will allow for some excellent cropping and as mentioned you could probably crop a bit tighter on your photos. The Red Kite is a very large crop. The longest lens I have had is the Nikon 300mm f4 prime and paired with a 1.4TC. You will get better results with your 600mm.
I have disabilities and this way of capturing nature in the garden helps keep me sane
, and it's been a great learning curve for me in the last 18months or so, of taking an interest in wildlife photography.
Thought the post may help and give you some ideas on the D750 and set-up and Jeff and the more experience users of the lens will help you with that!
Loads of tits at the feeder! by
Swansea Jack, on Flickr
Loads of Tits! by
Swansea Jack, on Flickr
Long Tail Tit in some winter sun by
Swansea Jack, on Flickr
Coal Tit in winter sun. by
Swansea Jack, on Flickr
Red Kite over House this morning. by
Swansea Jack, on Flickr