Call me dumb

[Colt]

Another good method for identifying a draft I've used is to light a candle and place it near the cage. (Obviously not something heavily scented and you only need it there for a minute or so.) If the flame is bouncing around or leaning in one direction there is a draft. If the flame stand upright with very little movement then you should be ok.

[debbie276]

A cold draft (breeze) is a little different then a warm breeze. There are many people that turn on fans during heat spells to help the birds cool off. Warm air will not be a problem it's cold air that will give them a chill. I doubt very much you will ever find a place without air movement and that's a good thing otherwise the air gets stale. Fresh air is good for you and your birds.
Blocking off direct vents like Sally and Nerien explained is a good idea but don't try to stop all air movement in the house. JMO

[Sally]

A cold draft (breeze) is a little different then a warm breeze. There are many people that turn on fans during heat spells to help the birds cool off. Warm air will not be a problem it's cold air that will give them a chill. I doubt very much you will ever find a place without air movement and that's a good thing otherwise the air gets stale. Fresh air is good for you and your birds.
Blocking off direct vents like Sally and Nerien explained is a good idea but don't try to stop all air movement in the house. JMO

Good points, Debbie. It is the cold draft that can be harmful and give the birds a chill. And the only reason I use the plexiglass panels to block the cold air conditioning for those cages is because those are stacked cages, with the top cages being only about 12" from the vent. For cages in my dining room, which are under a vent but not nearly so close, I don't bother with the plexiglass panels. For those cages, I simply put some hanging baskets of plants on top of the cages--looks nice and helps to block/divert the air at the same time.

[Harley2013]

Good idea Sally, maybe I can do that. I can place silk plants on top of the cage draping down the side to kinda divert the AC this summer on that side of the cage, although it is a bout 3 feet above and to the side of the vent, I know I feel warm air flowing that direction when the heat is on.