The colors and patterns of feathers provide some of the most important clues we have for identifying birds and are generally quite reliable. Only a few things alter the appearance of feathers. They become worn and faded over time. They can be temporarily shifted out of place, disrupting the typical color pattern and sometimes revealing an unexpected grayish or whitish color of the exposed feather bases.
Feathers can also become stained or discolored by external substances, and the most common of these is simple water. Feathers are water repellent, and water usually just rolls off (like water off a duck’s back), leaving the feather unchanged. When feathers are subjected to a real dousing, some of the water sticks to the feather, and they become wet. This happens most often when water is somehow pressed into the feathers: sparrows pushing through wet grass, doves exposed to an extended heavy downpour of rain, or warblers taking a bath.