You might want to get a copy of Steve Perry's book "Secrets to Stunning Wildlife Photography". He has some very good information on equipment in Part 2: Tech Talk under "Lenses for Wildlife" in addition to everything you need to learn about technique. I am constantly looking at other birder's work and what equipment they use. The biggest TECHNICAL differences I see between my image's and others' are tack sharpness, contrast and how they can blur the background. Part of it comes from using long, faster, primes like the the Nikon 500mm f4 VR which is about $10,000 or the Canon version at about $9000. Primes are inherently sharper with more contrast than zooms and the larger aperture helps separate the subject from the background providing a nice blur. I use a D500 with a Tamron 150-600mm f5-6.3 G2 and am getting some nice sharp images (but not tack sharp) after tuning the lens to my camera. The others in the class (Tamron 150-600 G1, Sigma 150-600 Contemporary and Sport, Nikon 200-500) are pretty close to each other so pick your poison. I hear others say, and they show, good results with all of these lenses. Good Luck!

The Nikon is sharper, and may have an edge in build quality. But neither are intended for use in harsh conditions. For the same price as the Nikon you can get the Tamron 150-600 G2 which is sharper that the Nikon, moisture sealed, and a 20% longer in focal length, and if you decide you want to use it on a tripod, it's foot is already equipped with an Arca-Swiss compatible dovetail.

I looked a the Nikon, the original Tamron and the two Sigmas - the C and the Sport - and found the Sport sharper and better built, with the Nikon a close second optically. I've since spent some time with a single copy of the G2 and found it to be every bit as sharp as the Sport, and almost 2 lbs lighter.

I bought the Hikkor 200 - 500mm f5.6 over the Sigma and Tamron 150 - 600mm offerings and never regretted it for a moment. I used mine on a Nikon D500 and it matched beautifully. It seemed to snatch birds out of the sky when I was using it for birds in flight - even better than my Nikkor 300mm f4 PF ED VR + TC14EIII. I preferred my 200 - 500mm to my Canon 100 - 400mm f4.5/5.6L IS II, which I used with a Canon 7D mark II. Though I no longer shoot Nikon, I still regard the Nikkor 200 - 500mm as a cracking lens 0withexcellent AF and optical performance. Take a look at UHH member Steve Perry's review on YouTube.