I have the Tamron 100-400mm and recently added a D500 to my D750 & D7100 for the sole purpose of bird photography. It delivers consistently very sharp images in good light and it is a pleasure to manage while hand holding. The lens only weighs 39+ ounces. I am blessed to have osprey, owls, hawks, herons, ibis and all manner of other wading birds and smaller birds within 200 ft of my back veranda on a river in Florida and this lens can handle all of that very well in reasonable light. I am 75 and in good health but the weight issue is lurking so I got a 4/3 G9 outfit that takes me out to 560mm, in 35mm equivalents, and it is also amazingly effective except for BIF where it does not work as well as the D500/Tamron 100-400 combination. The G9, lens and TC only weighs 50 ounces. The Olympus system will handle the BIF situation better but apparently not as well as the Nikon solution or at least from what I’ve read. I can and do get good photos at dawn and dusk with the Tamron but I do get some focus hunting at times. If I go crazy and and buy another lens for further reach it will probably be the Nikon 500mm PF because it only weighs 51 ounces but it may be a terminal acquisition because at $3600 my wife will put an end to me. Good luck!

PS- whatever you decide to get, pick up an OpTech harness strap for about 25.00. It gets the weight off your neck and re distributes to back, chest and shoulders. And the strap attaches to your camera lugs and not the tripod connection. It has made such a difference to me carrying a heavy lens. I routinely hike 2-3 miles (with the Sigma 150-600 mm) on fairly level ground without undue physical fatigue and the camera is always ready to shoot. And it provides a little counter resistance that helps your stance.