One of the great challenges for biologists is recognizing the limits of a species.  Humans provide a nice example of a highly variable species.  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.  The dark-skinned Chamberlain towers more than two feet over the white-skinned Shoemaker.  That’s variation!  But we know that humans around the world are capable of producing fertile offspring, one definition of a species.

On the other hand, some species are morphologically nearly identical to other species.  We call such similar forms sibling species.  Traill’s Flycatcher is a case in point.  A painting of this “species” appeared in Audubon’s Birds of America.

Field work in the 1950’s and 1960’s showed that some males consistently sang a sneezy “fitz-bew” song and others sang a “fee-be-o” song.  Furthermore, females were only attracted to one of the two song types.  Based on this research, the AOU Check-list Committee split Traill’s Flycatcher into Willow Flycatcher and Alder Flycatcher.  Good luck distinguishing one from the other in the field if they are not singing.

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.  Research showed that the two species do not interbreed where their ranges overlap in the west.

Red Crossbills present an even more complicated and controversial situation.  In 1988, Jeff Groth published an extensive monograph showing that ten types of Red Crossbills occur across North America.  These types are best identified by their flight calls, but subtle morphological differences occur as well.  Groth showed that different types have preferences for different types of conifer cones.

Matthew Young has recently produced a nice update of our knowledge of these crossbill types.  He also has made recordings of the different flight calls.  All can be downloaded at:

Click on “Add to Cart”.  You will be taken to a Checkout page but the price is right: $0.00.  Submit your order and you will get two pdf documents on Red Crossbills and 11 sound files of flight calls.

Much remains to be learned about the distribution of these 10 types of Red Crossbills.  Types 1, 2 and 10 occur regularly in New England and New York and a couple more appear on occasion.  Type 8 is found exclusively in Newfoundland.

What do we do with this information?  Should we recognize ten species of Red Crossbills instead of recognizing a single species?  I suspect that much more work will need to be done, particularly examining the ability of different types to interbreed, before the AOU Check-list Committee would be willing to split Red Crossbill into multiple species.

Nevertheless, it is fun to try to identify the type of Red Crossbill you hear when are birding in crossbill country.  Downloading the sound files will give you the resources you need to train yourself to give type identification a try.

World Life Listing

Many birders enjoy competitive birding as they seek to build the largest year list, state list or list for any other area of interest.  A milestone in world listing was recently passed by Tom Gullick, an expatriate Englishman living in Spain. Gullick is a seasoned world-traveler and bird tour leader.

Last summer at the age of 81, Gullick visited the remote Indonesian island of Yamdena where he saw Wallace’s Fruit Dove.  That species represented his 9,000th bird!  Gullick is the first birder to ever reach that milestone.

About 1,500 species remain in the world for Gullick to see but he indicates he does not have the 10,000 species milestone on his radar.  I’m sure those remaining birds are either extraordinarily rare or highly restricted in distribution.

[First published on October 28, 2012]