Most birds are very tolerant of hot weather, but any bird left in the heat can be subject to overheating or heat stroke. Make sure to take the proper precautions so your pet bird can enjoy the summer weather without worrying about the dangers of too much sun.
Never leave a pet bird in direct sun or heat where they cannot cool down or seek shade. A bird that is too hot will pant, fluff up its feathers to try to cool down, and will become listless like they are sleeping while standing. They may also pace back and forth or make more noise than usual due to distress. Vomiting is also common when a bird has been left in the heat for too long.
If you provide a shady area or a nesting box for your bird to go into when it gets too warm, in addition to a bowl of water for bathing in, your bird should be just fine in the sun. Keep in mind that smaller tropical birds, like parakeets and cockatiels, don't like drafts, so only allow them in the sun with a breeze if the wind is warmer and they have protection from the winds.
Birds love sunbathing and can enjoy a summer day just as much as we humans can, just make sure you give them a safe scenario to enjoy the weather in so you don't end up with a bird that has gotten too much sun. The sun can be fatal to even larger birds if they are left unattended in the summer sun and heat.
If your bird is showing signs that they are too hot, remove them from the sun and place them in a cool area. Gently mist them with warm (not cold) water to help bring down their body temperature, and keep loud stimulants away which may frighten your bird. Get them veterinary care as soon as possible, even as they start to show signs of recovery. A bird left untreated for over exposure to the sun can become very sick or even die. Taking precautions to keep your pet bird out of direct sun for long periods of time is the best way to protect them and keep them safe.