It was quite cold during my visit, about 5 degrees below zero Celsius in fact.
As you can see in this image African waterside aviary (Jan 20th, 2019) | ZooChat
the aviary does have a heated and sheltered bird house the birds can go in when they want or have to, and usually when it is cold but sunny the birds get to choose whether they want to be indoors or outside. This "open doors" during cold but sunny weather is a common kind of policy at zoos around here I believe.
It also possible that some more sensitive species could be cooped up inside, but I don't know about that for sure. In any case I did not see nearly all species during my most recent visit (for example no avocets, lesser flamingos or cranes were out).
