Passenger pigeon. Photo by J. St.John, Wikimedia Commons
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The passenger pigeon (Ectopigtes migratorius) is an extinct, North American bird of great legend and mystique. It resembled in size and markings the still living mourning dove (Zenaida macroura), and the two birds were sometimes confused for each other. The plumage of the passenger pigeon, though, was more colorful than the rather drab mourning dove. According to genetic analysis, however, the passenger pigeon was actually more closely related to birds in the genus Patagiones (a genus that includes the band-tailed pigeon (P. fasciata) and the white-crowned pigeon (P. leucocaphala)) than to birds in the genus Zenaida.
The passenger pigeon’s common name is derived from the French “passager” which means “passing by.” This very appropriate designation emphasizes the species’ tendency to be in a nearly constant state of movement across its broad, eastern and midwestern North American range. In addition to migrating seasonally to and from the southern and Gulf Coast states, the passenger pigeon neither stayed in one particular place very long or moved about in any particularly established patterns. A specific town or section of countryside might see a swarm of passenger pigeons one year at a particular time of year and then not see them again for many years and then, often, at a completely different time of year. Their “passagers” moved about quite randomly.
The passenger pigeon’s “home range” was wherever its swarm happened to be. It is thought that there were only a very limited number of these staggeringly large flocks. As John Burroughs’ wrote in his book Birds and Poets, “the whole race (of passenger pigeons) seems to be collected in a few swarms.” Each swarm moved about on its own and was made up of hundreds of millions to billions of birds.
Passenger pigeon swarm. Illustration by F. Bond, Wikimedia Commons
The birds in a swarm flew, often at 60 miles per hour, in dense aggregations relatively close to the ground. The birds were packed so closely together that they blocked the view of the sky and the sun! A swarm could be several kilometers wide and many hundreds of kilometers long. The birds in the swarm sought out hardwood forests that were rich in “mast” (beech nuts, acorns, chestnuts, etc.). American beech trees were, according to John James Audubon, the optimal mast producers for the passenger pigeon. The small size of the beech nut made them easy to swallow and the regularity (big mast crops often every other year) and abundance of their yearly production made them an ideal food source for these birds. Possibly, the abundance of the American beech ‘s mast production was a direct, evolutionary response to counter the immense appetite of the passenger pigeons!
The passenger pigeon’s impact on the trees of North America was significant. It has been suggested that the purpose of the extremely rapid, same season germination of the white oak’s autumnally dropped acorns was to protect the acorns from inevitable spring consumption by the returning flocks of passenger pigeons. Also, the irregular masting cycle of the white oak (years of few acorns alternating with a rare, randomly occurring year of great abundance) is also thought to be a safeguard against total mast consumption by the pigeons. Further, the wide size range in northern red oak and other oak species’ acorns is thought to be an adaptation to guard against total loss of a trees’ acorns to passenger pigeons (the larger acorns could not be swallowed by the birds).
The physical impact of the passenger pigeons on the structure of their transient, forest roosting sites also affected the nature of the forests themselves. The large numbers of birds defecating for several weeks in their roosting forest killed the existing forest floor vegetation and enriched the forest soils with high levels of nutrients. The impact of the sheer weight of the roosting and nesting birds also broke limbs from the trees and added significantly to the potential fire debris on the forest floor. White oaks are more resistant to fire than most other types of oaks. They are also more fire resistant than most of the other eastern North American tree species with which they compete. The observation that the pre-European settlement forests in eastern North America were dominated by white oaks may be at least partially attributable to the influence of the passenger pigeon not only increasing the potential for forest fires but also not being able to mass-ingest white oak acorns!
Martha, the last passenger pigeon. Photo by E. Meyer, Wikimedia Commons
The passenger pigeon has been described as a “keystone species” of our eastern forests. They increased the quality of the forest habitat and helped to create niches for a wide range of forest dwelling species. The extinction of these birds may explain some of the observed degradation and decline in the quality and productivity of these forested ecosystems and in particular may explain the diminishing abundance of white oaks throughout eastern North America.
The great fame of the passenger pigeon is primarily based on its staggeringly huge numbers. It was the most abundant bird not only in North America but possibly in the world! Its population was estimated to be 3 to 5 billion individuals in early 1800’s. This represented 25 to 40% of all of the birds in North America! Its sudden, brutally complete, human-caused extinction (the last passenger pigeon, named “Martha,” died in the Cincinnati Zoo on September 1, 1914) was both shocking and, in hindsight, inevitable.
Accounts of witnesses of the arrivals and the departures of the swarms of passenger pigeons are the stuff of science fiction or horror novels. In an article in Audubon Magazine (2014), B. Yeoman described an account from Columbus. Ohio in 1855:
“…a growing cloud blotted out the sun as it advanced toward the city. Children screamed and ran for home. Women gathered their long skirts and hurried for the shelter of stores. Horses bolted. A few people mumbled frightened words about the approach of the millennium, and several dropped on their knees and prayed.” When the flock had passed over, two hours later, “the town looked ghostly in the now-bright sunlight that illuminated a world plated with pigeon ejecta.”
The massive swarms of the passenger pigeon and their constant migratory behavior were a very effective protection against predators. A billion birds dropping in on a forest would provide food for any number of local predators, but no wild predator could make a dent in the overall size of the population. Also, more distantly located predators could not effectively travel to and exploit the passenger pigeons because the birds, after truly wrecking up their forest habitat, quickly departed for some distant, more pristine site that was rich in mast.
There were many behavioral adaptions that allowed the passenger pigeon to live in such dense aggregations. Some of these adaptations may also have led to the ingrained, imitation behaviors that were observed in the species. For example, for a passenger pigeon to mate or nest they had to see many other passenger pigeons mating or nesting. This dependence on large group sizes and reliance on imitation served the passenger well by stabilizing the intensely social community of the swarm, but it is also laid the seed for rapid cessation of reproduction when the massive swarms were whittled down in size by the intense human hunting of the mid and late 19th Century.
Natural predators could not locate the passenger pigeon swarm easily, and even if some distant predator did locate a swarm, they were unable to get to the birds quickly enough to prey upon it before it departed its selected forest. Humans, though, with rapid communication systems (the telegraph) and rapid transportation systems (the railroad) could locate and travel to a roosting swarm in time to wreak great damage on the pigeons. Using nets, guns, sticks and clubs human hunters took billions and billions of passenger pigeons over a few short decades and sent them in salted barrels to feed city dwellers, farm workers and livestock. It was a breath taking demonstration of human power (and ignorance) that very quickly exterminated all of the wild passenger pigeons in North America.
Recent studies have explored the history and evolution of these giant, passenger pigeon flocks. The absence of passenger pigeon bones in midden piles of ancient humans living in eastern North America coupled with precise analysis of the bird’s DNA (taken from museum specimens) have suggested that until quite recently, the passenger pigeon existed in much smaller aggregations with a much smaller overall population. Further, these populations according to genetic analysis, was subject to great extremes of growth and decline. Possibly the resurgence of the mast producing hardwood forests of North American following the last glacial maximum was the trigger that allowed the passenger pigeon’s population to surge to a unprecedentedly large size where it hovered, on the edge of sustainability until the crushing blow of human hunting destroyed them.
White-footed mouse. Photo by D.G.E. Robertson. Wikimedia Commons
The extinction of the passenger pigeon may also be having another ongoing impact here in the 21st Century. The absence of the pigeon means that there is more mast available to other mast consuming species. One of these mast consuming species is the white-footed mouse whose North American population has greatly increased greatly through the 20th and 21st Centuries. The white-footed mouse is also the major intermediate host for the bacterium that causes Lyme disease (Borrelia burgdorferi). Possibly, the extinction of the passenger pigeon is one of the contributing factors to the recent epidemic explosion of Lyme disease throughout the northeastern United States.
The extinction of the passenger pigeon shocked the people of the early 20th Century. It led to consideration and adoption of a wide range of conservation programs and continues to serve as a viscerally real lesson into the potential damage that uncontrolled human activity can have on natural species and ecosystems.
Thanks again Bill Hamilton!!!
You make this world a better place to live.