Pay attention to your shoes. Basically, normal hiking shoes will do nicely. Depending on the location, sneakers may also be OK, or you may even need climbing boots. Rubber boots are very useful for birdwatching in tidal flats or on reclaimed land. For snowy places, winter fishing-type boots are recommended.
Birds are very sensitive to colors.
Birds are not color blind and do indeed distinguish colors. Therefore, you should be especially careful not to wear obviously artificial-looking colors, which may frighten them. Earth tones such as khaki, olive and brick red are ideal. Cloth or leather can sometimes be quite noisy and may frighten birds.