Nikon 600mm f/4D ED-IF AF-S Nikkor

A big lump of glass and metal!! For small songbird photography this lens is a must. Some may say a Sigma 800mm is the answer but that's f5.6 and the subject to background seperation just isn't as convincing. Besides, stick a 1.4x teleconverter on and you outreach an 800mm Sigma yet still have rapid AF, stick a 1.7x on and you still have AF (though your technique and support will have to be perfect to get sharp results)

Optical clarity, colour rendition stand out in the results. Backgrounds are thrown out to perfection, seperating the subject from any distractions.
AF-S is rapid and quiet.

The 4 focus lock buttons on the lens body don't seem quite as important with modern cameras these days.

The huge HK23 hood has the benefit of being two sections, allowing just one section to be used when conditions allow... flare being less of an issue in cloudy northern europe.

Personally, I would have preferred a more hand-friendly tripod foot with finger grooves (although more comfortable lower profile 'feet' are available)

Minimum focus distance isn't quite up to modern standards but is alleviated with a teleconverter or extension tubes for smaller subjects.

Look for this on the used market for a real bargain, I picked up an optically mint ex paparazzi model (with an exciting history) for a steal.

For goodness sake, realise just how big and heavy any 600mm f4 is before purchasing, for most a 500mm f4 will be a more sensible option. Lens support will need to be the best and this means legsets like the Gitzo 1548 and a gimbal type head.

More details and images at http://www.digiscoped.com/600mm.html