Once the most abundant bird species in North America, the passenger pigeon (Ectopistes migratorius) was so numerous many observers did not believe they could ever be at risk of extinction. Throughout the 19th century human population growth and migration, as well as agricultural and industrial development, reduced and fragmented the hardwood forests which were the primary nesting habitat for passenger pigeons. Concurrently, hunters and trappers targeted breeding colonies for largely unregulated markets. Over-harvest and habitat loss have each been proposed as the primary factor leading to the extinction of passenger pigeon. In order to quantitatively explore the likelihood of different extinction scenarios involving habitat loss and harvest, I developed a temporally dynamic and spatially explicit modeling framework for the passenger pigeon incorporating multiple lines of historical and biological evidence. I created the pre-settlement model of nesting habitat using climate and vegetation covariates along with historic nesting accounts. Habitat loss was modeled at annual time steps using historic records of population density, settlement, land-use, timber harvest, and transportation. The passenger pigeon population model was parameterized primarily through historic accounts of wild and captive birds.
Results/Conclusions
Although the precise conditions which led to the extinction of this species can never be known conclusively, the results of the model imply that the combination of both habitat loss and harvest led to the precipitous decline. The results suggest that direct mortality from harvest alone was unlikely to have accounted for the observed rate of decline. However, a sensitivity analysis conducted on the population model parameters suggests that the species may have been especially vulnerable to the method of harvesting as it also decreased reproductive success. Meanwhile, the timeline of available habitat illustrates the decreasing availability and increasing fragmentation of suitable nesting sites for large colonies. IUCN Red List criteria applied as if naive to the ultimate fate of the species shows how rapidly the species moved through the risk categories. This study illustrates the importance of conducting risk assessment early and often even for seemingly abundant and common species, particularly when impacted by commercial harvest or rapid changes in land use or land cover.