Gone Birds

Last Friday, September 1st, was the anniversary of the death of Martha, the last Passenger Pigeon–a species that was, for centuries, the most abundant bird in North America. Martha was probably born in captivity in Charles Otis Whitman‘s aviary in about 1885, and died in the Cincinnati Zoo in 1914. I say probably because there is some debate about her origin (Schorger 1955). However, I think there are good reasons to consider Lord Rothschild’s (1907) account about her being raised by Whitman to be correct.

On the centenary of Martha’s passing, three excellent books (Avery 2014, Fuller 2014, Greenberg 2014) summarized just about everything you might want to know about Martha and the Passenger Pigeon. All of these books are worth reading for their different perspectives and the various ideas about why this most-abundant of species went extinct.

There can be little doubt that humans caused the extinction of this bird, but the process responsible for their million-fold decline, from billions in the 1870s to thousands by the 1880s, is still a bit of a mystery. Some have suggested that they were just wiped out by relentless hunting as were the Great Auk, the Labrador Duck, and the Dodo, for example. But those species were relatively rare (Labrador Duck and Dodo) or flightless (Dodo and Great Auk), unlike the Passenger Pigeon.

Others have wondered whether the pigeon’s demise may be an example of the Allee Effect whereby individual fitness declines with population size leading eventually to the extinction of local populations. While the Allee Effect might explain the extinction of some small colonies, it is hard to imagine how it would cause a species that still numbered in the thousands to go extinct across its entire range.

A new study by Ben Novak (2016) at UC Santa Cruz presents some intriguing new information that might help us to understand this massive extinction event. Genomic analysis reveals that the Passenger Pigeon probably numbered in the billions for at least 30,000 years, during a time when the forest ecosystems in its eastern North American breeding range were changing dramatically. Because the birds relied on masting tree seeds (oak acorns, chestnuts, pine seeds, maple seeds) as their main food supply, Novak suggests that the birds might have had a dramatic effect on forest ecosystems. By consuming a large portion of the plants’ reproductive outputs but also by depositing several inches of guano each year under their roosting and nesting sites, the pigeons may have been agents of habitat disturbance and destruction. For example, their massive roosts and nesting colonies broke trees and branches, opening up the canopy, and the several inches of guano they left on the ground, may have increased the incidence of forest fires. This sort of habitat disturbance may have been one reason they were so nomadic, returning to previous roosting and nesting sites only 5-20 years later, after the forests had recovered.

Passenger Pigeon populations were decimated by hunters, but Novak’s findings suggest to me that their final decline to extinction may have been due to both habitat destruction by humans settling eastern North America, and by the birds themselves. With much of their roosting and nesting habitat cleared for agriculture, the birds may have found their nomadic lifestyle simply unsustainable.

Clearly, the Passenger Pigeon still has stories to tell us about the causes of extinctions. Though this species is gone, we are fortunate to have specimens like Martha that can be mined for genomic data and the application of yet-to-be-invented tools. Some researchers have even suggested that we use these genomic tools to recreate and re-wild extinct species, but I am neither enthused nor optimistic about that prospect. Moreover, if Novak is correct, the forest ecosystems of eastern North America are now unlikely to be able to support a species that they last interacted with almost 150 years ago.

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References

Avery M (2014) A Message from Martha: The Extinction of the Passenger Pigeon and Its Relevance Today. Bloomsbury, London.

Fuller E (2014) The Passenger Pigeon. Princeton University Press, Princeton, NJ.

Greenberg J (2014) A Feathered River Across the Sky: The Passenger Pigeon’s Flight to Extinction. Bloomsbury, London.

Novak BJ (2016) Deciphering The Ecological Impact Of The Passenger Pigeon: A Synthesis Of Paleogenetics, Paleoecology, Morphology, And Physiology. Mc thesis, University of California at Santa Cruz. Accessed 3 Sept 2017 from http://escholarship.org/uc/item/3260s35t

Rothschild W (1907) Extinct Birds. Hutchinson & Co., London.

Schorger AW (1955) The Passenger Pigeon: Its Natural History and Extinction. University of Wisconsin Press, Madison, WI.