A large APERTURE value will result in faster more accurate autofocus and maximal bokeh.

However in bird photography APERTURE value usually will be more or less fixed because you need all the light you can get and lightweight lenses usually have lower light aperture values equal or larger than 4.0, 5.6 or 6.3. Therefore you will set lenses to maximum light value, to start with, unless you prefer to gain (some) extra detail by stopping down the lens. Anyway most of the time you will be working with fixed values of APERTURE.

Suggestions for stopping down lenses to gain some detail:

• Nikkor 300mm f/4E PF VR: l stop, clearly visible effect.
• Nikkor 500mm f/5.6E PF VR: Nikon rightfully claims no loss of detail using TC-14E III (lens will go to f/8 so one stop down). (TC-14E II has the same effect in our experience)
• Nikkor 200mm-500mm f/5.6E: 1.0 stop visible effect but loss of advanced AF functions dynamic 3D and auto (AF dynamic 25 points is still supported through f/8)
• Sigma 150-600mm /f 5.0 – 6.3: 1.0 stop (also when using 1.4 converter visible effect)
• Sigma 150-600mm /f 5.0 – 6.3: 1.0 stop (also when using 1.4 converter visible effect)