Carotenoids are pigments that give plants, algae, and some bacteria their yellow, orange, and red colors. When birds eat these plants or animals, the carotenoids are absorbed into their bodies and deposited in their feathers. The amount of carotenoids in a bird's diet determines the intensity of its red color.

Some birds, such as cardinals and tanagers, are naturally red. These birds have a high concentration of carotenoids in their diet. Other birds, such as robins and blue jays, are not naturally red. However, they can turn red if they eat a diet high in carotenoids.

In the fall, many birds migrate south to warmer climates. During migration, they often eat a diet high in carotenoids. This is because the fruits and berries that they eat are ripe and contain a high concentration of carotenoids. As a result, many birds turn red in the fall.

When birds molt in the spring, they lose their old feathers and grow new ones. The new feathers are often brighter and more colorful than the old feathers. This is because the birds have a higher concentration of carotenoids in their diet during the spring.

The color of a bird's feathers can also be affected by other factors, such as genetics, age, and health.