ECOVIEWS: Why are no cranes found in South America?
I received two intriguing questions about cranes this month. Both remind us that biological mysteries abound, even for commonplace animals.
Q. The following is a quote from a poster at a botanical garden: “Cranes are large birds with long beaks found on every continent except Antarctica and South America.” I understand cranes not being found in Antarctica. It’s always odd man out when geographical distribution is the topic. But not found in South America? Do you know why this would be so?
A. Questions about distribution and abundance of animals and plants are among the most basic ones in ecology, and the explanations are often as diverse as the scientists giving them. Why is a species one place and not another nearby? Why are some species abundant one year and absent the next, only to reappear the next year? For some questions, the answers are accepted by ecologists. For others, the answers may be in dispute or simply not exist.
Of the 15 distinct species of cranes, two migratory cranes (sandhill and whooping) seem to be prime candidates for colonizing South America. An isolated population of sandhill cranes lives and breeds in Florida although most of these are migratory. Most springtime breeding activity of American cranes occurs in northern states and Canada.
The reason neither crane species has gotten to South America is unknown. Fossils from North America reveal that cranes have been here for at least a couple of million years, so there’s been plenty of time. But after traveling 2,000 miles from Canada, why go farther south? Many other long-distance migrating bird species do not make it to South America either. An isolated population of sandhill cranes is endemic to Cuba, which is as far south as cranes go in the Western Hemisphere. According to an article last year in the Mexican Journal of Ornithology, only an estimated 550 of the nonmigratory Cuban sandhill cranes still exist. One possibility is that cranes did populate parts of South America but went extinct and left no record of their existence there.
According to the International Union for Conservation of Nature, whooping cranes remain on the endangered list. On the upbeat side, the IUCN places sandhill cranes in the category of least concern. Although sandhill crane populations dipped drastically when European settlers came to the New World, their climb back since the mid-1900s has been steady. The U.S. Geological Survey states that sandhill cranes, which range throughout the United States, Canada and across Alaska to Siberia, are now “the most abundant of the world’s cranes.” Presumably they will stay here and not go to South America.
Q. My wife and I were sitting on our back porch in mid-June and heard what sounded like a sandhill crane. The sound was repeated several times, but the source of the noise did not move. Generally, we hear the cranes when they are in flight. There were lots of frogs calling that night, and we wondered whether frogs can mimic sounds.
A. The frog you heard was a southern leopard frog, which sometimes makes a croaky rattling sound similar to a sandhill crane. Many birds and frogs have a wide repertoire of sounds they use for different situations. The interesting twist with your observation is that sandhill cranes are more likely to mimic leopard frogs than vice versa, especially when walking in or around a wetland. I once watched a great blue heron wade along the shoreline of a lake where bronze frogs (closely related to leopard frogs) were calling from the bank. As the heron approached, it began croaking like a bronze frog, which they will eat in a heartbeat when they can find them. Sandhill cranes will eat leopard frogs and are probably delighted when one responds to what they think is a calling neighbor. A study to see if cranes and herons intentionally mimic frog calls to locate a quick snack might be revealing.