Max,
As Chevy Chase put it in
Caddyshack , "Be the Ball". Or in this case, Be the Bird
You are a hunter, hunting for images. So you need to think like your quarry. What do birds think about? Food, safety and sex. Sort of like human Males....LOL
So exploit this knowledge. Stakeout locations where the food source is. For waterfowl, get close to water. For raptors, checkout fields with ground large ground squirrel or field mouse populations. For other species, as mentioned, stay close to feeders or other possible food sources.
Their desire to stay safe will keep them away from noises and bright colors and cause them to take flight if they see any fast or aggressive movements in their direction. So be a quiet as you can, wear muted colors and move slowly. I find with many species, if you don't make eye contact, you can get quite a bit closer (works well with raptors). As if they think you don't see them.
During nesting season, it is usually pretty easy to pick out a spot to get some nice captures. The birds willl be leaving and/or returning to the nest. Many times using the same routes. They also need to visit the "feeding grounds" and return to the nest. Again, usually using the same routes. So you can locate yourself along these routes, with respect to the direction of light. As with feeding birds, it is unwise to get TOO CLOSE to nesting birds. This may cause the parents to abandon the nest. So please keep this in mind when capturing nesting birds.
The more you stalk birds and practice your stalking techniques, the closer and closer you will be able to get.
Steve