The hummingbird is a very tiny bird that has a very big heart. In fact, of all bird species, the hummingbird has the largest known relative heart size. The heart represents 2.4 percent of their body weight. Also, the heart of a hummingbird must be very strong in order for it to complete a non-stop flight across the Gulf of Mexico in 18-22 hours. It is no wonder that these tiny and amazing creatures seem to so easily capture the hearts of birdwatchers everywhere!