The passenger pigeon, also known as the wild pigeon, was a beautiful gregarious dove that migrated in flocks so enormous that they seemed to block out the sun when they flew over. Their rapid decline in numbers between 1870 and 1890 is often used as the poster child of extinction and has been blamed on everything from overhunting, destruction of habitat, and their extreme socialness or a combination of all these factors. Martha, the last passenger pigeon, died at the Cincinnati Zoo on September 1, 1914 and signaled one of the most recognizable examples of anthropogenic extinction. (Science break: the Holocene extinction or Anthropocene extinction is the ongoing extinction event of species mainly as a result of human activity.)
This species was found in deciduous forests across North America east of the Rocky Mountains to the Atlantic coast, from the area along the US/Canadian border all the way down to the Mississippi in the south. They wintered from Arkansas, Tennessee, and North Carolina south to Texas, the Gulf Coast, and northern Florida and were at one time arguably the most abundant species numbering an estimated 3-5 billion birds at the time of Europeans discovering the Americas. Yes, that’s billion with a ‘B’. With this being said what could have happened that to cause this extreme population loss in a matter of a generation?
In 1857, a bill was brought before the Ohio legislature seeking protection of the passenger pigeon but the senate filed a report that repudiated the evidence of declining numbers stating they were “wonderfully prolific” and did not need protection. They scoffed at the idea of a whole species being wiped out. Similar attempts at conservation were made in other areas but proved futile and by the 1890’s they were extinct in the wild.
Pigeon meat became a cheap staple in a North American’s diet and these birds were consequently killed off in droves to feed the masses. The sociability of their nature made them an easy target for hunters and it is said that one shotgun blast could kill 5 or 6 because of their close proximity to each other. This does not answer the question as to why there weren’t small isolated flocks that could have survived the slaughter. A study of the DNA of passenger pigeon specimens from Museums across North America suggests that their large numbers may have also been a contributing factor to their demise. Are you scratching your head over this one? Paradoxically, these pigeons were well adapted to living in huge flocks and did not (obviously) adapt well to living in small flocks when their population began to drop so quickly. They just didn’t have the time to evolve to living smaller numbered groups.
Another surprise of this DNA study found a relatively small genetic diversity amongst the museum specimens. It showed that it was lower than anticipated; each individual was genetically very similar to the next. Without genetic diversity, it is almost impossible for a species to adapt quickly to any change within their environment. This came as a surprise to the researchers because normally a large population will also have a high chance of genetic mutations and in turn genetic diversity.
The scientists looked at the 32 genes that appeared to be heavily beneficial to the passenger pigeon to live in huge flocks like stress control, the consumption of high-energy foods used for long migrations, and better immunity needed for living in tight quarters with many, many others. But along with these beneficial traits other genes close to them on a chromosome were also inherited through natural selection even if they were not as helpful. It seems the pigeon had an abundant amount of less important genes linked to these important genes so when their numbers started dwindling, the remaining population did not have enough diversity to adapt to smaller groups.
Although this study is too late by more than a century to help the ill-fated passenger pigeon, it may be used to assist breeding programs of severely endangered animals. It may also show that although an animal may have a large population, it may not have the genetic diversity needed to survive a big population drop. This is evident in cheetahs where although their population numbers are not as bad overall as other species their lack of diversity has caused a physical homogeneity that results in poor sperm quality, focal palatine erosion (a dental abnormality where the first molar penetrates the palatine mucosa),susceptibility to the same infectious agents, and kinked tails. Hopefully, the research being done will help bring other species back from the brink and not follow the passenger pigeon’s fate.
–Janice Willson
References:
https://en.wikipedia.org/wiki/Passenger_pigeon
https://www.eurekalert.org/pub_releases/2018-01/nuos-wdt010518.php
Photo Source: Wikimedia Commons
