I read an article recently which talked about the Hummingbirds
Hummingbirds have race car hearts that eat oxygen at an eye-popping rate. Their hearts are built of thinner, leaner fibers than ours. Their arteries are stiffer and more taut. Their hearts are stripped to the skin for the war against gravity and inertia, the mad search for food, the insane idea of flight.
They are tiny little birds and their hearts beat 10 times a second. So even if you put your huge ear to its chest, it would be hard to discern the heartbeat.