Is this a male or female crow? You can’t tell, as with many birds with long-term pair bonds. (Photo by Becky Matsubara) There are other differences besides color. Male and female birds can vary ...

In the famous Darwin's finches from the Galápagos Islands, for instance, there is a 40-fold difference in bite force between the tiny-beaked Green Warbler-finch and the massive-beaked Large ...