Before you set out please do not scare wildlife and please do not photograph nests unless you feel you are capable of doing so without causing undue stress, as this will cause birds to abandon their young, also some birds are protected and you must have a licence to photograph them near or on the nest.
If your not sure don't do it and please go to the R.S.P.B site and check out their bird photography and the law files, also please check out all the relevant wildlife and countryside acts on the government site as it is not only birds that are protected but all sorts of other things like flowers, insects and mammals, just use common sense and and don't get caught out as pleading ignorance won't help if you end up in court.
When going out photographing wildlife a few things can make all the difference from seeing something and not. A few examples like wearing dark clothing or better still cammo, I know some people think it's a bit ott but it helps break up you outline so making it harder for wildlife to spot you. Make sure you are as quiet as you can be the less noise the better, practice to see how quiet you can be. Good clothing that dosn’t creak and make noises as you walk is a must if you want to get close, as you will probably be crawling along the ground for some shots When stalking make sure you stay up wind of you intended target and use the undergrowth and other natural obstacles to hide behind. If birds are on water try setting up down stream as lots of birds will just go with the flow and if you are well hidden they will just float down to where you are. You can also try baiting an area with titbits and the birds will find these, if you do it for a few days before hand, When they are used to it just set up and wait for them to turn up. Check out where your intended target breeds and what sort of food they like and research them to find out as much as you can about their habitat to find the best places to go looking for them. If using a hide you must try to let the wildlife get used to it before you start using it as they will be weary of it to start with, If you have a friend to go with you you can both enter the hide then your friend can leave the birds will think you have gone as they car't count, one way to help is to use a natural hide made out off the local fauna if possible but don’t go chopping things down to do it as it not nice and also illegal. By using the local fauna I mean hiding in a bush or using a willow screen or reeds and the like.