Parakeets love the sunlight for a reason -- they need it to stay healthy. Too little exposure to sunlight has been linked to several health and emotional problems in birds. But keep in mind -- too much sunlight isn't good for parakeets, either.
Parakeets and Vitamin D
Like people, parakeets get their vitamin D from exposure to sunlight. And, like people, parakeets need vitamin D in order to process calcium, which is vital to nerve, brain and muscle function and for laying healthy eggs, and phosphorous, which is critical to bone formation, acid-base balance, metabolizing fat, protein and carbohydrates, and egg formation. Lack of vitamin D can also lead to depression in parakeets kept in low light, even to the point of self-plucking.
The question is not whether parakeets benefit from direct sunlight, it's about how much direct sunlight you expose them to. Generally, five minutes per day is enough, though factors such as season, hour of day, temperature and the location of the bird’s cage must be considered. Some areas are too hot in the summer to place a parakeet in direct sunlight at high noon, for example. Outdoor light is best, as windows block ultraviolet rays. Do not leave your bird unsupervised when he's in direct sunlight.
Sunlight might be necessary for parakeets, but they can overheat rapidly. Most birds suffer heatstroke in hot cars, often when being taken to the vet's. If your parakeet pants and holds his wings out from his body, heat stroke is setting in. He may also become agitated or sport a vapid facial expression. Immediately, but gently, soak him in room-temperature water -- cold water shock could be dangerous -- and remove him from the heat and light. Untreated, heat stroke could lead to convulsions, fever and death.
For Nordic parakeet parents living where sunlight is low, artificial lamps can provide a full spectrum of light to help keep your parakeet healthy. Full-spectrum bulbs can also help a housebound parakeet who does not get exposure to ultraviolet light by sitting next to glass windows. Keep the lamp in the room, but not too near your parakeet. And make sure he can't get to the lamp's cord. Birds love to chew on electrical wires.