Wildlife, especially bird photography is a difficult and expensive area. Going for an APC-C sized sensor gives you the advantage of that x1.5 crop factor with Nikon and x1.6 for Canon. But as the megapixel count goes up noise generally also goes up. Shooting at dawn, long lens needed, that crop factor can be a helpful and a cost affective way of extending your lens. That consumer 70-300mm f/5.6 lens becomes a 105 (112) – 450 (480)mm with no loss of aperture, but unless light is good you have to crank up the iso until your photos look poor. The other option, a low noise full frame sensor, now you can really crank up the iso and get excellent clean files. But now your 70-300mm f/5.6 is just that, it only reaches 300mm.
Spend good money or make compromises, its a very difficult area. Compromise; well you could try that 300mm f/4 with a x1.4 teleconverter, but again not a cheap solution.
Many top bird photographers are using 400mm f/2.8 with x2 teleconverters. Thats when you start to realise that a D3s is cheap and is only the start of the real expense.