Bird populations have been declining in recent decades, raising concerns among ornithologists and conservationists. Understanding the scope of bird population declines, the species most affected, and the likely causes can help guide efforts to reverse these trends.
What do surveys of bird populations show?
According to data from the North American Breeding Bird Survey, over 300 bird species in North America have lost at least half their populations since 1970. The total breeding bird population in the continental U.S. and Canada has dropped by 2.9 billion birds in the past 48 years.
Some specific examples of population declines include:
- Grassland bird species have declined by 53% since 1970
- Shorebird populations in North America have declined by more than one-third since 1973
- Wading bird populations like herons and egrets declined by 35% between 1978 and 1998
- Raptors like hawks, falcons, and kites experienced population declines of 40% or more from 1966 to 2015
Surveys in other parts of the world have also found substantial population decreases among many common bird species. For example, farmland bird populations in Europe have declined by over 20% in just the past decade.
Which species have been most affected?
Some types of birds have been harder hit than others by population declines. Grassland species, migratory birds, seabirds, and aerial insectivores appear to be among the most rapidly declining groups.
Grassland birds such as meadowlarks, grasshopper sparrows, bobolinks, and dickcissels rely on large open fields and pastures. The extensive loss and fragmentation of grasslands has made these species vulnerable to steep population declines.
Many migratory birds that travel large distances between breeding and wintering grounds are also experiencing serious declines. For example, populations of Barn Swallows and Purple Martins have dropped by up to 45% in the past decades throughout North America.
Seabirds nesting on islands have declined by 70% globally since 1950. Factors including overfishing, pollution, disturbance of nesting colonies, and introduction of invasive predators have decimated many seabird populations.
Aerial insectivore populations also dropped by over 40% between 1966 and 2013. Species like nighthawks, whip-poor-wills, martins, and swallows feed on insects caught during flight. Habitat loss and declines in flying insect populations have contributed to their worrying population decreases.
What are the main causes?
Habitat loss and degradation appear to be major factors driving bird population declines, especially for grassland species and farmland birds. Agricultural intensification, draining of wetlands, development and logging have eliminated vital breeding and foraging habitat for many species.
Climate change may also be contributing to population declines for some species. Shifts in temperature averages, rainfall, vegetation and food resources can affect where birds can breed, migrate and overwinter successfully.
Other threats to bird populations include:
- Collisions with buildings, towers, and wind turbines
- Overhunting
- Poisoning from pesticides and lead
- Invasive predators
- Disease
For migratory birds, habitat loss on breeding grounds, migratory routes, and wintering grounds combines to threaten populations. And for all birds, climate change, pollution, infrastructure, and disease interact to magnify extinction risks.
Are some regions impacted more than others?
Bird population declines are widespread across most parts of North America. However, some regions and habitats have been harder hit than others.
Grassland species have been most impacted in the Midwest and Great Plains, where grasslands have been extensively converted to agricultural cropland. According to the North American Breeding Bird Survey, grassland birds in this region have declined by 53% since 1970.
Seabirds nesting on islands have experienced dramatic declines, including in Hawaii where breeding seabirds have declined by 90% in the past century. Seabird colonies in the Arctic like the Aleutian Islands off Alaska have also been depleted with the introduction of non-native predators.
Aerial insectivores dependent on flying insects have decreased across North America as insect populations have dropped, but the most serious declines have occurred in the Midwest and Northeast.
In general, migratory birds that travel through the eastern half of the U.S. have exhibited steeper declines than western species. This may reflect habitat loss concentrated along eastern migration routes.
Have any common feeder birds declined?
Even some familiar backyard birds commonly seen at bird feeders have declined significantly in recent decades. According to the Breeding Bird Survey data:
- Chipping sparrows have declined by 47% between 1970-2017.
- Wood thrushes declined by 53% between 1970-2017.
- Bobwhites have declined by 83% in the Southeastern U.S. since the 1960s.
- Eastern meadowlarks declined by 97% in the Northeast between 1970-2010.
- Common nighthawks declined by 40% between 1970-2015.
However, some species adapted to backyard feeders and urban parks like cardinals, blue jays, mourning doves, red-bellied woodpeckers, and goldfinches have increased in population over the past decades.
How could climate change impact bird populations in the future?
If current climate change trends continue, models suggest that bird populations could undergo further declines in the coming decades. Some of the key ways birds could be impacted by climate change include:
- Shifting ranges: As climate zones shift, some birds may not be able to quickly adjust their breeding ranges to keep pace with changing temperatures and habitat.
- Mismatch with food sources: Warmer temperatures cause earlier emergence of insects, which could get out of sync with timing of nesting and chick rearing.
- Increased droughts and severe weather: Drought conditions in some areas could reduce food supplies and reproductive success. Increased storms could destroy nests and swallow colonies.
- Loss of coastal nesting areas: Sea level rise could flood important coastal habitats like salt marshes, seabird colonies, and shorebird nesting areas.
- Less migration and hibernation: Warmer winters may result in fewer birds migrating or hibernating, meaning they need more food year-round to survive.
By destroying habitats, disrupting food chains, and creating physiological stress, climate change could compound existing threats to bird populations in the future.
Do citizen science programs help track bird populations?
Volunteer citizen science initiatives can play a valuable role in monitoring bird population numbers and trends over time. Some examples of citizen science projects focused on birds include:
- Christmas Bird Count: One of the longest running wildlife surveys in the world, volunteers count bird sightings during a 2 week period each December and January. Data goes back over 100 years.
- Great Backyard Bird Count: People submit checklists of birds seen over 4 days each February to help track winter bird distributions.
- NestWatch: Volunteers monitor nests to track reproductive timing and success. Helps identify threats to nesting birds.
- eBird: People submit bird sightings year-round via app/website to document real-time distributions and abundance of species.
These programs rely on contributions from thousands of volunteer community scientists. Their data complements more systematic surveys by scientists and helps identify population trends.
Conclusion
Scientific surveys clearly show that many bird species in North America and worldwide have undergone worrying population declines in recent decades. These decreases are most pronounced among grassland birds, seabirds, aerial insectivores, and long-distance migratory species. Habitat loss and degradation, climate change, pollution, and other human impacts are key contributors. Citizen science initiatives help track bird populations over time, complementing scientists’ systematic surveys. Addressing threats from human activities and protecting essential habitats will be key to reversing downward population trends for threatened and common bird species.