Bison calves are born a light orange-red, which is so distinctive that everyone who works around them calls the calves red dogs. They keep that colour for two or three months and then darken to brown, so the window in which a herd contains obvious red dogs is short and tells you almost exactly how old the calves are.
They arrive ready to move. A newborn bison is on its feet within an hour or so and can keep up with a running herd within a few days, which is the only workable strategy for an animal that raises its young in the open with no den and no cover.
The cow's behaviour in this frame is the other half of it. Bison cows are aggressively defensive of calves and the herd closes around them when threatened, and a cow with a calf is statistically the most dangerous animal in Yellowstone for visitors who misjudge the distance. Everything about photographing them properly is about a long lens and staying further back than feels necessary.