Birds spread their wings on the ground primarily to sun themselves, a widespread behavior that helps them control parasites, regulate body temperature, and maintain feather health. If you’ve spotted a bird lying flat with its wings fanned out and its beak open, it probably looks distressed, but this is normal maintenance behavior observed in dozens of species around the world.
Parasite Control Is the Leading Explanation
The most well-supported reason birds sun themselves is to fight off feather parasites. Birds carry ectoparasites like lice, mites, fleas, and ticks, and sunning appears to be one tool in their defense. When a bird lies prone on the ground with its wings and tail feathers spread and its head feathers raised, it exposes as much plumage as possible to direct sunlight. Research has shown that the surface temperatures feathers reach during bouts of sunning are high enough to kill feather lice. Scientists believe short blasts of heat, UV radiation, or a combination of both from the sun’s rays do the damage.
This isn’t the bird’s only line of defense. Preening, scratching, bathing, and dust bathing all play roles in keeping parasites in check. But sunning requires almost no energy. The bird simply has to hold still and let the sun do the work, which may explain why the behavior is so common across unrelated species.
Warming Up on Cold Mornings
Thermoregulation is another clear driver. Turkey vultures, for example, fly up to a perch on cold mornings and spread their wings to catch the first rays of sunlight, burning off the overnight chill. Mousebirds in sub-Saharan Africa sun themselves communally after rain or heavy dew, lining up together like swimmers drying off at a pool. For smaller birds especially, the energy savings from passive solar warming can be significant, reducing the calories they need to burn to maintain body temperature.
Drying Wet Feathers
Some species spread their wings specifically to dry out. Cormorants and anhingas are the most familiar examples. Unlike most waterbirds, their outer feathers are “wettable” rather than water-repellent. This is actually an advantage underwater, because reduced buoyancy helps them chase fish more effectively. The trade-off is that they need to air-dry afterward. You’ll often see them perched with wings fully extended after a dive. Cormorants do maintain an insulating layer of air next to the skin even when swimming, so only the outer portion of the plumage gets soaked, but that’s still enough to require a good drying session before they can fly efficiently again.
The Vitamin D Question
An older theory suggested that birds produce a vitamin D precursor in their preen gland (the oil gland near the base of the tail), spread it across their feathers during preening, and then ingest the UV-converted vitamin D the next time they preen. It’s an elegant idea, and one study did find a vitamin D precursor in the preen glands of domestic chickens. However, that study used older analytical methods, and the finding has never been confirmed with modern techniques. Most researchers now consider this hypothesis unproven and largely abandoned, though it hasn’t been definitively ruled out.
A Related Behavior: Anting
Sometimes birds spread their wings on the ground to rub ants into their feathers, a behavior called anting. The birds pick up ants (often species that produce formic acid, which can make up more than half of their defensive secretion) and wipe them through their plumage. The long-held explanation was that formic acid acts as a natural insecticide, essentially shampooing the feathers to kill parasites. But experiments have found that avian ectoparasites aren’t actually harmed by ant secretions under real-world conditions.
A more recent explanation is that anting is actually food preparation. By rubbing the ants through their feathers, birds trigger the ants to empty their acid sacs, effectively “disarming” them. Once the formic acid is depleted, the ants become a safer, less unpleasant meal. The formic acid itself appears to be what triggers the behavior in the first place.
Which Birds Do This
Wing-spreading behavior shows up across a remarkable range of species. Songbirds of all kinds typically lie on their fronts with wings and tail spread flat. Pigeons and doves take a different approach, often reclining on one side with the opposite wing raised to expose the underwing to the sun. Blackbirds, starlings, pheasants, laughingthrushes, and even large birds like secretarybirds have all been documented sunbathing belly-down with wings fanned to the sides. Canada geese sometimes sit with their wings hanging loosely away from the body to expose the back. Domestic chickens do it too.
If you see a bird in your yard lying flat with wings spread, looking half-dead in a sunny patch, it’s almost certainly fine. Give it a few minutes and it will likely shake itself off and go about its day.
The Predation Trade-Off
Lying flat on the ground with wings spread is not without risk. A sunning bird is conspicuous, less mobile, and slower to take flight. Research on wintering sparrow flocks found that birds routinely preferred foraging in shade over sunlight, even when temperatures were well below their comfort zone. Bright sunlight makes birds easier for predators to spot and can cause visual glare that reduces their ability to scan for threats. The preference for shade was strongest at warmer temperatures, when the thermal benefit of sunlight was smallest, confirming that birds are constantly weighing warmth against safety.
This helps explain why sunning sessions tend to be relatively brief and why birds often choose sheltered spots with good sightlines. The benefits of parasite control and warmth are real, but so is the cost of being exposed. Birds that sun on the ground are taking a calculated risk, and they don’t linger longer than they need to.