From All Other Pigeons
Jason Weckstein and Joel Greenberg
Not a Carrier Pigeon, Not a Messenger Pigeon, Not a Rock Pigeon
The passenger pigeon and the rock dove (Columba livia, aka rock pigeon, carrier pigeon, etc) are often confused in the public’s mind but they are not closely related. The dock dove is a Eurasian species that has been semi-domesticated for centuries and has been introduced into North America. They like to nest on ledges, which is one reason they have proliferated in cities around the world in a feral state.
Passenger Pigeons Were Unlike Any Other Bird in the World
in at Least Three Important Ways
The Passenger Pigeon was a bird solely of North America, with the vast majority inhabiting a region from the Gulf States to Hudson’s Bay, and from the Atlantic Ocean to the upper Missouri River. Three things made them unique in all the world:
1) they were the most abundant bird of the continent, if not the world: no one knows for sure how many there were but the most careful figure offered ranges from a low of three billion to a high of five billion individuals;
2) they aggregated in numbers that darkened the sky for as much as three days: individual flights might have exceeded two billion birds; and
3) in literally decades, human actions reduced this incredible bounty to zero, when on September 1, 1914, the last of the species died. Given that it is extinct, very little was known about its relationships to other birds until recently.
Where Passenger Pigeons Belong in the Tree of Life
Scientists divide the vast array of life forms into categories based on the similarities and relationships between organisms. From broadest to narrowest, these categories are Kingdom, Phylum, Class, Order, Family, Genus, species. All the birds of the world are in the Phylum Chordata, Kindom Animalia, Class Aves and then are divided into 26 orders. (based on the taxonomy of James Clements). These orders include the Falconiformes (hawks), Anseriformes (ducks and geese mostly), Strigiformes (owls), and Passeriformes (the largest order of all which includes the perching birds like warblers, swallows, flycatchers, finches, thrushes, and sparrows).
All pigeons are members of the order Columbiformes which have a number of characteristics that together set them apart from other birds. These include a bilobed crop that produces a sort of “milk” that is fed to the chicks (“crop” is a pocket like space near the throat); monogamous mating behavior; the ability to drink by sucking or pumping; and thick feathers set close to the skin. On a general level, pigeons possess stocky bodies with small heads, bills, and feet. (Passenger pigeons were among the sleekest of pigeons). There are 42 genera and 308 recognized species of Columbiformes.
Pigeons of the World: From 8 Pounds to 22 Grams
There is no difference between pigeons and doves: the terms are interchangeable. Pigeons are found throughout the world. Some pigeon species eat mostly fruit, whereas others forage on seeds. The fruit-eating Columbiformes tend to be much more vividly colored than the seed eating ones. The largest species is the Victoria crowned pigeon (Goura victoria) of New Guinea which approaches the size of a turkey and can weigh in excess of 8 pounds. The smallest species are members of the ground dove genus (Columbina): they can be as small as house sparrows and weigh not more than 22 grams.
Until recently, the relationship of Passenger Pigeon with respect to other pigeon species has been simply speculation based on gross plumage characteristics. However, recent genetic data published in 2010 by Johnson and colleagues (Molecular Phylogenetics and Evolution 57:455) show that, despite the gross similarity in appearance to mourning doves and its relatives (the genus Zenaida), the Passenger Pigeon is not closely related to this group of pigeons at all. In fact, its closest relatives are a group of large-bodied pigeons from the New World in the genus Patagioenas, which includes the western Band-tailed Pigeon among others. Even so, scientists believe that Passenger Pigeon is still different enough from other extant pigeons to remain in its unique genus, Ectopistes. Based on an analysis of the evolutionary tree constructed from genetic data, Johnson and colleagues (2010) hypothesized that eons ago an Asian cuckoo dove crossed into North America and provided the ancestor to both Ectopistes and Patagioenas.
Jason Weckstein is an ornithologist at the Field Museum who specializes in bird parasites. Joel Greenberg, affiliated with the Chicago Academy of Science’s Peggy Notebaert Nature Museum and the Field Museum, is an author who is active in Project Passenger Pigeon.
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