Your pet cockatiel's environment should stay between 65 and 80 degrees Fahrenheit, and her cage must be free from drafts. A captive cockatiel can stand a wide range of temperatures, as long as the changes are gradual. A sudden drop in temperatures, though, is dangerous for her.
Cockatiels are native to arid and semiarid parts of Australia, where they can experience temperatures as low as 30 degrees and as high as 100 degrees Fahrenheit. That's a broad range in temperatures, but your pet cockatiel has had the luxury of living indoors all her life -- she isn't acclimated to handle sudden drops in temperature. That's not to say that she couldn't become used to a change in the weather if it was gradual; she just shouldn't be subjected to an abrupt drop of 15 degrees or more.
Danger From the Cold
Experiencing an extreme drop below her comfort zone can leave a captive cockatiel vulnerable to illness, shock and death. If yours catches a chill, she could become sick. Shivering and fluffed feathers indicate she is cold. Increase the heat around her slowly, as an abrupt increase in temperature is just as dangerous as a sudden drop. Incandescent light bulbs are useful for providing gradual ambient heat, as are heating pads set to low and placed under her cage. Cockatiels are susceptible to frostbite, too, a condition that is indicated by cold, dry, pale skin. Slowly warm the affected area of skin with warm but not hot water, and contact your avian vet for treatment.
Keep Babies Warm
While an adult cockatiel can adjust to gradual drops in temperature, babies are more vulnerable to dying from becoming too cold. Adequate heat is so important to a baby cockatiel that she will be unable to function -- including eating and digesting properly -- unless she is warm and comfortable. The room temperature for baby cockatiels under three weeks old should be around 92 degrees. Older babies can tolerate a temperature in the mid-80s.
Heat Can Cause Problems, Too
Even though your cockatiel is a warm-weather bird, she cannot withstand extreme heat any better than she can tolerate extreme cold. Never leave a cockatiel in a car, even when the weather is temperate outside, for instance. If you take the cage out on the patio during the spring or summer, don't place her in a spot with direct sunlight. Doing so can put her in danger of overheating, dehydrating and possibly dying. Always put her in an area of at least partial shade so she has a place to retreat if she becomes too hot, and make sure she has ample water so she can stay hydrated.