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.