The color it appears on infrared footage depends on the other pixels in frame. It uses dynamic ranges to map infrared values to a visible light spectrum. If the rest of the frame was ice, or you were looking up into space, a bird would probably be rendered as very warm.

If the rest of the frame is a warm ocean surface and warm wind turbines, then a flying bird may be rendered as cold relative to those pixels.

Balloons can also show up as a different temperature than the background of the frame depending on what the balloon is made of, altitude differences (ambient temp at high altitude is colder than at the surface), etc.

For example, If I take a blurry photo of something I see outside on a full moon that's probably a raccoon and proclaim its a photo of the elder god Nug, spawn of Azathoth, the Lord of All Things, and someone points out that its probably a raccoon but the photo is so bad there's really no way to ever tell the right attitude isn't to say:

"It can be compelling and attractive to fill the silence or the unknown with an invention of certainty - sort of like a prophet or shepheard - but the edge of known demands more curiosity and wonder for an honest approach."

IR imagery can be flipped between black=hot or white=hot. These systems are about creating contrast to aid visualization, not recording scientific data.

>> What sort of balloons show up as contrast instead of neutral?

A hot air balloon? Any balloon that has recently changed altitude? Any reflective balloon reflecting sunlight (Mylar is common). Or, in thin air, a non-reflective balloon absorbing sunlight and warming faster than it can dissipate that heat.