Defining a species is, like any other scientific endeavor, subject to revision as we learn more. The provisional nature of taxonomy is surely evident in birds. The American Ornithological Society (AOS) has a committee, the North American Checklist Committee (NACC), that maintains the official checklist of North American birds.
Ornithologists who believe that a taxonomic change (combining two species into one or splitting one species into two or more) is in order, submit proposals to the NACC. The proposal may include observations on interbreeding or lack of interbreeding, morphological differences and, increasingly, comparisons of genes. The committee considers the proposals throughout the year and publishes a committee report in July, updating the North American checklist.
Bird listers eagerly await these yearly reports. It is possible for birders to get a life bird on their life list if a split is accepted. For instance, the Pacific wren was split a few years ago from our eastern winter wren. On the other hand, lumped species diminish one’s life list.
This year’s supplement has many changes that affect Caribbean or Central American birds that I will not cover here. However, there are three decisions that affect Maine birds.
The first decision addresses a proposal to combine common redpoll with hoary redpoll. Hoary redpolls are paler with a very short, conical bill. They have less streaking on the breast and lack any dark streaking on the undertail coverts. The NACC decided to lump the two species into one species, common redpoll. Many of you, like me, have just lost a species on your life list.
The evidence that convinced the NACC to lump the two species is based on a genetic phenomenon that I find fascinating. Stick with me for a little cell biology. Birds like us, have a number of paired chromosomes in their cells. The thousands of genes of a bird are lined up along the chromosomes.
To form reproductive cells (sperm or eggs), cell division must occur to produce cells with a single copy of each chromosome. The single chromosomes in a sperm cell combine with the single chromosomes of an egg to make an embryo with two copies of each chromosome, half from mom and half from dad.
Before a cell divides to form eggs or sperm, the tips of paired chromosomes may overlap and trade a portion of the tip of each chromosome. Imagine each of your arms is a chromosome. Cross your arms at the wrist. Now imagine that your left and right hands switch arms. That process, called crossing over, happens about half of the time and promotes greater diversity among the chromosomes of the reproductive cells.
Occasionally, the process goes awry. One of the tips of the chromosomes that is exchanged flips around before it attaches to the opposite chromosome. This time, one of your hands that is exchanged is attached by the fingers to the forearm with the wrist at the tip.
For birds that inherit one chromosome with the inversion and one without, crossing over is not possible in that region of the chromosome because the corresponding genes do not line up. So, the genes in the inversion are linked together to make a supergene. And guess what those inverted genes produce? Pale plumage, short bills, limited streaking – the marks of a hoary redpoll.
Since all the other genes of redpolls are freely exchanged between hoary and common redpoll, the NACC chose to lump the two species, recognizing there are two distinct forms based on the presence or absence of the inversion. Pretty cool!
The barn owl, found occasionally in southern Maine, has been regarded as a cosmopolitan species, found on all continents except Antarctica and on many oceanic islands. A proposal to split barn owl into three species was headed up by Maine’s own Louis Bevier and colleagues. The proposal was accepted and now we have American barn owl in North America. The other barn owl species are eastern barn owl and western barn owl.
The committee made one change based on grammar. They removed the hyphen from night-heron so that we now have black-crowned night heron and yellow-crowned night heron.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu
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