The question is asked by zoo visitors and biologists alike, but while numerous theories abound, no-one has yet provided a definitive explanation.
Now after conducting an exhaustive study of captive Caribbean flamingos, two scientists believe they finally have the answer.
Flamingos stand on one leg to regulate their body temperature, they say.
Matthew Anderson and Sarah Williams are comparative psychologists based at Saint Joseph's University in Philadelphia, US who are interested in the studying the evolution of behaviour.
"Flamingos captured my attention for a variety of reasons," says Anderson.
"Scientifically speaking, their highly gregarious nature makes them an ideal species for investigating social influences on behaviour."
"Aesthetically speaking, they are large, beautiful, and iconic."
"Perhaps most importantly, I was very surprised to discover how little systematic, hypothesis-driven empirical research had been conducted on flamingos."