The Sixth Mass Extinction and Birds: What the Numbers Mean
The phrase "sixth mass extinction" appears in scientific literature with increasing frequency, and for some readers it carries a quality of hyperbole. The previous five mass extinctions in the geological record — the end-Ordovician, the Late Devonian, the end-Permian, the end-Triassic, and the end-Cretaceous — eliminated between 75 and 96 percent of all species alive at the time. We are manifestly not in the middle of anything that dramatic in any human-observable timeframe. What scientists mean by the sixth mass extinction is that current species extinction rates are measurably and dramatically elevated above background rates — the baseline extinction rate that operated over geological time before the Holocene — and that the trend is accelerating in ways that, if continued for centuries, would produce losses comparable to the geological mass extinction events.
Background extinction rates and what "elevated" means
The background extinction rate, estimated from the fossil record, is approximately 0.1 to 1 species per million species per year (commonly abbreviated as E/MSY). For birds specifically, with approximately 11,000 described species, the background rate suggests roughly 1 bird species should go extinct per century. The documented bird extinction rate since 1500 CE, when European exploration and settlement began systematically altering island ecosystems globally, is approximately 161 confirmed extinctions — roughly two species per year, or 200 times the background rate.
This figure is likely a substantial underestimate. Many island species went extinct before scientific description; the gap between Polynesian settlement and European documentation spans centuries in which losses are incompletely recorded. Paleontological and subfossil evidence from Pacific Island deposits suggests hundreds of additional species extinctions associated with Polynesian settlement that are not in the documented list.
The current decline: numbers that do not require extinction
The most immediately striking evidence for the biodiversity crisis in birds is not the extinction count but the population decline data. A 2019 study published in Science documented a net loss of approximately 3 billion breeding birds in North America since 1970 — a decline of approximately 29 percent of the total bird population across all species, including species still classified as Least Concern on the IUCN Red List. This is not a list of threatened species declining; it is the aggregate abundance of all bird life.
Comparable analyses for Europe show a decline of approximately 600 million individual birds since 1980. The European farmland bird index has fallen by more than 55 percent since 1980. Common farmland species — Skylark (Alauda arvensis), Corn Bunting (Emberiza calandra), Grey Partridge (Perdix perdix) — have declined by 70 to 90 percent across large parts of their historical European ranges.
Island birds and the extinction concentration
Documented bird extinctions are concentrated on islands: approximately 90 percent of all historical bird extinctions have been island species. The pattern reflects island vulnerability — small population size, no evolutionary experience with introduced mammalian predators, limited habitat area, and high endemism producing species with no alternative refugia. Hawaii alone has lost more than 70 bird species since Polynesian settlement: the Hawaiian honeycreeper radiation, one of the most spectacular adaptive radiations in avian evolutionary biology, has been reduced from over 50 species to approximately 17, many of which are now Critically Endangered.
The mechanism for most Pacific island extinctions was the same: introduced rats (Rattus rattus and Rattus exulans carried by Polynesian settlers; Rattus norvegicus introduced by European ships) depredating eggs and chicks of ground-nesting birds with no anti-predator behavior. Introduced pigs, mongoose, cats, and eventually avian malaria (introduced to Hawaii via mosquitoes from tropical regions) compounded the losses.
What the evidence means for conservation
The combined evidence — elevated extinction rates since 1500, accelerating recent declines in common species, and the trend trajectory for tropical forest species under current deforestation scenarios — is what scientists mean when they describe a sixth mass extinction in progress. The conclusion is not that 75 percent of species will be gone by next century; it is that current trajectories, if maintained for the geological timescales over which mass extinctions are measured, would produce losses comparable to the major geological events.
The practical implication is that conservation at the margins — protecting small reserves, preventing the last individuals of Critically Endangered species from dying — is necessary but insufficient. Population-level interventions, landscape-scale habitat restoration, and systemic changes to land use are the scale of response that the data suggests is needed.
What birders can do about biodiversity loss
Birders are uniquely positioned to contribute to extinction prevention because the birding community produces more biodiversity data than any other group of citizen scientists on Earth. eBird alone receives over 100 million bird observations per year — a dataset that drives conservation planning at continental scales. Understanding how birder data translates into conservation action gives individual birders a clearer sense of the significance of their activity beyond personal enjoyment.
eBird data has directly contributed to the delineation of Important Bird Areas (IBAs), the selection of Ramsar wetland boundaries, and the identification of previously unknown migration routes for threatened species. When birders document a species outside its known range or record an unusually high or low count at a monitored site, those records become part of the baseline that researchers use to detect population trends. Complete checklists — every species, not just the unusual ones — are the most scientifically valuable form of observation.
Habitat protection is the most direct conservation lever available. The major bird conservation organizations — BirdLife International, the American Bird Conservancy, the RSPB — fund land purchase and lease programs that protect critical habitat from development. Membership and donation to these organizations provides the financial basis for habitat acquisition. The land trust model, pioneered in North America, has protected tens of millions of hectares specifically for migratory bird habitat.
Supporting shade-grown and bird-friendly coffee certification addresses the most significant threat to Neotropical migratory birds during their wintering season. Over half the North American wood warbler species that breed in the United States and Canada winter in tropical forest habitats that are being converted to sun-grown coffee and cattle pasture. The Smithsonian Migratory Bird Center's Bird-Friendly certification identifies coffee that is grown under conditions maintaining the forest structure that wintering birds require.
Domestic cat management is the most underacknowledged contributor to bird mortality in the developed world: free-ranging domestic cats kill between 1.3 and 4 billion birds annually in the United States alone. Supporting Trap-Neuter-Return programs and keeping domestic cats indoors are direct actions available to any birder regardless of geographic location or budget.
Monitoring extinction risk in real time
The most direct way to track species-level extinction risk in real time is the IUCN Red List's rolling assessment program, which updates species' status annually as new data are received. Subscribing to the IUCN Red List email update service provides automatic notification when species' categories change — either to higher risk categories (a species deteriorating) or to lower risk categories (a conservation success). The annual State of the World's Birds report from BirdLife International summarizes the trajectory of all bird species and is freely downloadable from birdlife.org.