The 600mm f/4 is probably the flagship of any manufacturer's lens range, and it's arguably the best lens there is for photographing small birds. This Nikon one is just about perfect as far as the optics are concerned, but the downside is that is a very big, very heavy lump of glass.

This is the older non-VR version of the lens, but we think the lack of VR isn't a major issue. The lens is so heavy, you'd almost certainly be using it with a monopod or tripod anyway. (And even if you're strong enough to use it hand-held, the weight tends to dampen camera shake. It's much less 'twitchy' than a lighter lens!)

If 600mm isn't enough for you, this lens works very well with Nikon Teleconverters and it will still deliver superb image quality. The 1.4x Teleconverter makes it an 840mm f/5.6 lens whch will retain autofocus on all compatible cameras. The 1.7x Teleconverter makes it a 1000mm f/7-ish lens, and the 2x Teleconverter makes it a 1200mm f/8 lens, but these latter two combinations might be manual focus only depending on the camera and the conditions.

Suggested extra: You might find the Manfrotto MN393 long lens bracket useful when mounting this lens on a monopod or tripod.