You may or may not know this, but birds do not have sweat glands, so in principle they require less water than mammals due to their ability to retain more water. However, this isn’t to say birds don’t lose any water. Of course they do, and it is mainly through respiration and droppings.
Depending upon the specific dietary requirements of a bird, for instance one that eats mainly seeds, there may be a different level of rehydration needed. Despite this, most birds get the majority of their water from the food they eat, for instance grubs, caterpillars and earthworms.
Water, being as abundant as it is, can be taken from a variety of sources such as ponds, streams and even droplets that form on the end of leaves. In fact, this is particularly common among forest-dwelling birds.