I live in a fairly cold area, even in the summer we get fairly cold days so my tiel doesn't get too much sun. The last vet we took her to basically said it was 100% necessary to get a UV bulb and to take her outside on any days warm enough.
As soon as we turned the uv bulb on, she ran over to it and took a nap right in front of it (i made sure it wasn't close enough for her to touch it). She loves her UV bulb, but only for about 4-5 hours. After that she runs to the other side of her cage and chirps at me to turn it off.
Uv lights are a pretty good investment, though our vet did mention don't get ones for reptiles. I just get my avian bulb at a local pet store, and one of the people working there told us the ones for reptiles are basically the same thing and are cheaper - apparently this is not true and they put out UV differently, so look for ones specifically for parrots. And they're also good for people, too, UV light is very good for our mental health. So by helping your bird out, you're helping yourself out too