One of the great challenges for biologists is recognizing the limits of a species.\u00a0 Humans provide a nice example of a highly variable species.\u00a0 As a demonstration of this variability, I like to show my students a portrait of the basketball great, Wilt Chamberlain and the equally famous jockey, Willie Shoemaker.\u00a0 The dark-skinned Chamberlain towers more than two feet over the white-skinned Shoemaker.\u00a0 That\u2019s variation!\u00a0 But we know that humans around the world are capable of producing fertile offspring, one definition of a species.<\/p>\n
On the other hand, some species are morphologically nearly identical to other species.\u00a0 We call such similar forms sibling species.\u00a0 Traill\u2019s Flycatcher is a case in point.\u00a0 A painting of this \u201cspecies\u201d appeared in Audubon\u2019s Birds of America.<\/p>\n
Field work in the 1950\u2019s and 1960\u2019s showed that some males consistently sang a sneezy \u201cfitz-bew\u201d song and others sang a \u201cfee-be-o\u201d song.\u00a0 Furthermore, females were only attracted to one of the two song types.\u00a0 Based on this research, the AOU Check-list Committee split Traill\u2019s Flycatcher into Willow Flycatcher and Alder Flycatcher.\u00a0 Good luck distinguishing one from the other in the field if they are not singing.<\/p>\n
A more recent example is the 2010 split of the Winter Wren into two species, the western Pacific Wren and our eastern form, still called Winter Wren.\u00a0 Research showed that the two species do not interbreed where their ranges overlap in the west.<\/p>\n
Red Crossbills present an even more complicated and controversial situation.\u00a0 In 1988, Jeff Groth published an extensive monograph showing that ten types of Red Crossbills occur across North America.\u00a0 These types are best identified by their flight calls, but subtle morphological differences occur as well.\u00a0 Groth showed that different types have preferences for different types of conifer cones.<\/p>\n