Why Birds Sun
There are several theories about bird sunning behavior, and in fact birds often sun for different reasons. In cold weather or early in the day, birds sun themselves for warmth by taking advantage of solar radiation. This allows them to maintain their body temperature without expending as much energy from food, and it can increase their chances of survival in cold climates or when food is scarce.
Many birds are observed sunning even on the hottest days, however, and it is believed that sunning can fulfill purposes other than just temperature regulation. Sunning can help birds convert compounds in their preening oil – secreted from a gland at the base of the tail – into vitamin D, which is essential for good health. If the birds have been in a birdbath, sunning can help their feathers dry more quickly. It is even believed that some birds sun themselves for pure enjoyment and relaxation, much the same way humans will sunbathe.
The most important reason for sunning, however, is to maintain feather health. Sunning can dislodge feather parasites because the excess heat will encourage insects to move to other places in a bird's plumage. This will give the bird easier access to get rid of those parasites when preening, and birds are frequently seen preening immediately after sunning. It is essential to get rid of these parasites – the tiny insects that infect feathers can cause problems for a bird's flight, insulation and appearance, all of which can impact its survival.