The former abundance of the now extinct passenger pigeon amazed all of the pioneers who witnessed their migration and roosts. Some researchers estimate passenger pigeons composed 25% of the total bird population in eastern North America during colonial times. Their migrations consisted of billions of birds that could eclipse the sun for as long as 14 hours. Pigeon dung fell like “snowflakes” underneath this eclipse. They nested in enormous colonies, covering many square miles. Their survival strategy was predator satiation. With synchronized hatching there were billions of squabs on the forest floor during the week they left the nest and were learning how to fly. There were just too many for predators to eat all at once. They nested and reproduced in the great deciduous and coniferous forests of the Midwest, then migrated to the middle south during the winter where they still roosted in large colonies. Passenger pigeon roosts destroyed vast areas of the forest. The weight of all those birds broke limbs off trees and even busted thick tree trunks in half. The armies of pigeons vacuumed all the acorns, beechnuts, and chestnuts off the forest floor, leaving no mast for other animals. But their dung was the most detrimental element of their roosts–overfertilizing the soil and killing all the trees in the vicinity. Yet, ecologists believe passenger pigeons played a critical role in creating habitat diversity across the landscape.
Illustration showing how passenger pigeons created more diverse habitats.
Flocks of migrating passenger pigeons could eclipse the sun for as long as 14 hours. Areas of the forest where they roosted were devastated.
Stuffed specimen of a male passenger pigeon.
The response of the environment following the aftermath of a cataclysmic pigeon occupation would have been interesting to study. Unfortunately, passenger pigeons were overhunted to extinction before scientific studies of their ecological impact could be conducted. However, we can safely assume the environment recovered in stages. First, plants; such as ginseng, pokeberry, and Virginia creeper; that thrive in soils rich in nitrogen were the initial species to grow in the open conditions strewn with fallen dead trees and limbs. Second, as rain reduced the concentration of nitrogen in the soil over the years; ragweed, grasses, sedges, and composites returned. A shrubby stage with pioneer trees including cedar and pine gradually replaced the grassy stage. Acorns carried by jays and squirrels sprouted into oaks that grew with the pioneer trees before eventually outcompeting them. If undisturbed for centuries, shade tolerant species such as maple and beech took over from the oaks. The dead wood from the original pigeon roosts was flammable during dry weather, and wild fires were likely more common, explaining why fire-tolerant species of oaks (burr, white, and black) predominated in the Midwest.
In many areas of the Midwest some species of oaks are in decline, especially white oak, while red oak is increasing. White oak germinates during the fall when passenger pigeons were absent in this region. But the pigeons were able to consume spring-germinating red oak acorns after the snow melted. Moreover, red oaks are less fire tolerant than white oaks. Oaks are also shade-intolerant and are being replaced by shade tolerant maples. Ecologists think white oaks are missing the passenger pigeon invasions that created the natural disturbance they need and reduced the competition they now face from red oaks.
Scientists with the Revive and Restore Project hope to genetically engineer the passenger pigeon by cutting and pasting their genes into the embryos of their closest living relative, the band-tailed pigeon. I doubt they will successfully be able to re-establish passenger pigeons in the wild. Passenger pigeons fail to breed unless they live in enormous colonies. To survive predation, they must exist in large numbers…the sheer size of their population was the survival mechanism they required. Researchers would need to release at least 10,000 birds to establish a successful breeding population in the wild. Passenger pigeons evolved their survival strategy millions of years ago. Though 1 genetic study suggests their overall numbers fluctuated with changes in climate phases, I am convinced they always occurred in large colonies. The task of re-establishing these numbers is probably an impossible one.