Not too fast on the shutter speed? (1 Viewer)

Hi all. Was hoping someone can give me some advice. I realise that a fast shutter speed is usually ideal and many people pump up their ISO or do whatever they can to achieve either 1/1000 or 1/ 2000. However I have found when taking pictures of passerines and fairly small birds i get better feather detail and allround clarity if I keep the speed under 1/1000...usually 1/500 to 1/840 roughly. Now i realise often this may not be fast enough but on occasion when there is alot of light these speeds work great. Is there a case for sometimes not shooting a 1/3200 or 1/2500. I always used to just go for the fastest possible but now I'm not so sure. Any thoughts would be really appreciated. Thank you P.S does high ISO numbers do anything else detrimental to the picture other than just create more noise!

There's no downside to high shutter speed as such. The only way it could have a negative impact on image quality is by its effect on lens aperture. All else being equal, the faster the SS the wider the lens has to be opened up to achieve proper exposure, and a wide aperture always results in less depth of field & often (particularly in the case of zooms) reduced sharpness & contrast as well.

As far as high ISO is concerned, "noise" (in all its ugly forms) is the main problem & is a bad enough one so that you don't really need any other reasons to avoid high ISO settings whenever possible.