Foto at top: Holland Kestrel (Torenvalk) cloudy dark weather. Handheld V3 Nikkor 300mm f/4E PF VR TC-14E equivalent 1135mm total camouflage.
High ISO values maximize noise particularly in the Low Lights. Often this is more visible but less significant in dull sometimes less relevant or background areas of the image.
In cases where noise is inevitable for instance in low available light conditions in combination with dark and or backlit surface(s) of the bird(s), noise can be corrected to an acceptable extent according to subjective appreceationt. This ican be accomplished using post production software like Lightroom or specialized tools like Noiseware, DXO qqq and qqq. See fore example the picture on top of this page of a Kestrel mating in dark cloudy weather Holland handheld D300 Nikkor 300mm f/4E PF VR TC-14E equivalent 630mm qqq ISO 1/qqq second, distance 23 meters total camouflage, RAW converted and edited in Lightroom.
Anyway in situations when a bird has dark coloured areas preferably do not underexpose too much or use converters.
Low ISO values will mosttime yield clean, colourful, clear and almost noisefree pictures with fine detail and good colour contrast.
