How did the two birds in the cage die?

Question

Near the ending of Close Encounters of the Third Kind, Roy and Jillian are frantically trying to reach Devils Tower in Wyoming.

The U.S. Army has issued road blocks and they have made false claims about toxic nerve gas in the air to prevent the public from being in the vicinity.

Roy purchases a gas mask and a cage containing two live birds. They both put on gas masks. As they drive closer to their destination, they encounter several dead animals on the side of the road. Jillian checks the two birds and they are still alive. They both realize that it's a hoax and remove their masks.

They are stopped by uniformed workers with gas masks who take them into custody. When they are stopped, the birds are still alive. When we see Roy and Jillian being taken away, we see one of the uniformed workers carrying the cage and both birds appear to be dead moments after Roy and Jillian are removed from the car.

How did the two birds die? The movie makes it clear that the Army created this hoax and that the air was fine. Did the uniformed officers do something to the birds to convince Roy and Jillian that the air was toxic?

Answer 1

It's been a long time since I have seen the movie, but you made me curious with your great question. It was part of the government cover-up to use non-lethal gas to knock people and animals out. I found these clues:

From filmsite.org, it appears that government helicopters used sleeping gas -- on everybody.

Ground troops search behind them on foot with "nothing to report at mid-station. It looks up ahead that there's thousands of places to hide. We're gonna need at least three times the men if you want this covered in one hour." The searchers are ordered "off the northern face" so that helicopters can begin dusting the area with sleep aerosol. As night begins to fall, they press on and scramble for hiding places when another helicopter approaches. Larry falls behind and pauses to rest in the open, exhausted by the climb. The sleep-inducing helicopter, one with nightmarish sand, passes over him and releases a cloud of dust. Like so many other fallen birds, he quickly drifts off to sleep after inhaling the dust. [Emphasis mine]

From IMDb, we get a similar, but briefer, comment about the people and the birds being gassed.

Did Larry die after being gassed by the helicopter?

No, it was only a sleeping gas, the same agent used to knock out the livestock and caged birds.

Answer 2

I believe an agent sprays the birds with a chemical.

This happens when agents in hazmat suits take Roy and Jillian out of the car. At the same time, another suited agent reaches into the back seat for the cage. As he reaches, IIRC you can hear a "tssst" hiss that sounds like compressed air spraying out of a canister. The next shot, the agent is carrying the cage with the two sleeping birds.