If you think that penguins and puffins perch atop a close set of branches on the avian family tree, think again. Genetically, penguins and puffins aren't siblings or even cousins. Penguins belong to the family Spheniscidae, while puffins are members of the family Alcidae. There are 17 different species of penguin and four species of puffin, without any overlap. In fact, they aren't even geographical neighbors. Puffins reside in the Northern Hemisphere, with about 60 percent of the global puffin population residing in Iceland [source: Project Puffin]. You'll find penguins on the other side of the world living throughout the Southern Hemisphere, from Antarctica to South America.
But the clearest distinction between these animals is flight. Puffins, which are much lighter and more compact, can fly. Thanks to penguins' chubby bodies and rigid wings, they're permanently grounded. To understand why that is, let's compare their sizes. You could easily tote a puffin around for a day since they weigh slightly more than a pound (0.45 kilograms) [source: Street and Emily]. On the other hand, carrying an emperor penguin that tips the scale at around 66 pounds (30 kilograms) would be like hauling hefty luggage.