The size of the heart is determined by the size of the animal, because large animals need to pump a higher volume of blood through the body than smaller animals. However, the relationship is not linear. That means an animal that is 10 times larger does not have a heart that is also 10 time larger. In fact, as an animal gets larger, the heart to body weight ratio actually decreases. That may be partly due to the fact that as an animal gets larger, the amount of body surface in proportion to its body weight also decreases, and therefore there is less heat loss to the environment. Therefore there is less need to circulate warm blood to all parts of the body to replace the heat lost, and the heart does not have to be as large in large animals.
Animals that do not generate body heat to stay warm tend to have smaller heart to body weight ratios. Fish, amphibians and reptiles fall into this category. Because of energy requirements of flight, birds generally have larger heart to body ratios than comparably sized mammals. In general, small birds tend to have larger heart to body weight ratios. Active birds also tend to have larger hearts. Among birds, hummingbirds generally have the largest heart to body ratios because they are both small (which means they must replace heat lost to the environment more than larger birds or mammals) and because they are very active. Not surprisingly, falcons also have noticeably larger hearts than hawks and vultures, because falcons rely more on speed to catch prey than do hawks. Also not surprisingly, the sandpipers also have very high heart to body weight ratios, because they are small and they spend a good deal of time near the shoreline, a cool environment. There also seems to be no correlation between intelligence and heart to body weight ratio, because crows and ravens are among the most intelligent birds but their heart to body weight ratios do not differ much from birds of the same size.