The most scientifically backed theory as to why we yawn is brain temperature regulation. If the brain’s temperature has increased too much, inhaling air can cool it down. (hearingsol.com)
We aren’t the only ones who yawn. Birds do it as well.
Today, at Lambton woods, we saw a sleeping Great Blue Heron awaken and produce a terrific yawn.
That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang.
– William Shakespeare (1564-1616) Sonnet 73