If there's a sudden huge change in the weather (like a 20-degree change) it would be good to bring them in at night, but you can keep them outside when it's more moderate so they can adjust to the seasonal changes. Wild cockatiels have to deal with freezing temperatures sometimes (I'm talking below 32 F) so they're evolved to deal with it as long as the change isn't too extreme and abrupt. Once they've adjusted to the cooler temps they'll be able to stay out even at night.
It's possible to put a heat lamp or heat panel in the aviary. There are heat lamps made for reptiles that emit heat but not light, and these are OK for birds. Some of the UV lights made for reptiles aren't good for birds, but that's a completely different type of bulb that serves a different purpose. There are heat panels made for birds but they're expensive:
http://www.avitec.com/Cage-Heaters-AviT ... s-s/50.htmWe just had a big change in the weather here too. It was 90 a couple of days ago. The forecast for tomorrow calls for a high of 69.