Bird populations around the world are in decline. According to data from the North American Breeding Bird Survey, bird populations in the United States and Canada have declined by 29% since 1970, equating to a loss of around 2.9 billion birds. This decline is occurring across diverse groups of birds, including common backyard species as well as endangered birds. What factors are contributing to this concerning drop in global bird populations?
Habitat Loss and Fragmentation
One of the biggest threats facing birds worldwide is habitat loss and fragmentation. As humans convert natural landscapes for our own uses like agriculture, urban development, and resource extraction, we fragment contiguous habitats into smaller, disconnected patches. Birds rely on large, interconnected habitats to support healthy populations; habitat fragmentation leaves them with less space to nest, forage, and raise their young.
Agriculture is a major driver of bird habitat loss. Since the 1800s, over 1 billion acres of natural habitat in the United States has been converted to farmland. As crop production expands to meet global food demand, prime bird habitats like grasslands, wetlands, and forests are being cleared. Urbanization also consumes crucial habitat; U.S. urban land area grew by 100 million acres between 1982-2010. Extractive industries have destroyed important habitats through mountaintop mining, tar sands extraction, and more.
Fragmentation from major road networks further divides and degrades bird habitats. Roads bifurcate landscapes into smaller sections and facilitate predator and invasive species movement. Noise, light, and chemical pollution from roads also interfere with bird behavioral patterns. Research shows bird diversity and density declines near roads.
Agricultural Intensification
Modern industrial agriculture relies heavily on chemical inputs and mechanization. The transition away from low-intensity small farms toward intensive monoculture crop production has negatively impacted biodiversity, including birds.
Widespread pesticide use reduces available insect prey for insectivorous bird species. Herbicides kill off vegetation, eliminating food sources and nesting habitat. Fertilizers decrease plant diversity in field edges, hedgerows, and adjoining wetlands used by birds. Farm machinery destroys nests and causes direct mortality. Earlier and more frequent cutting of hay meadows disrupts breeding cycles. High-yield crop varieties ripen uniformly, causing boom-bust cycles rather than staggered food availability. These agricultural intensification pressures combine to make cropland inhospitable for many bird species.
For example, Bobolinks rely on hayfields for nesting and foraging, but modern industrial hay harvesting methods have led to steep declines in this species. Cornell Lab of Ornithology researchers found bobolink populations declined by 88% between 1980-2011 in Ontario and by 98% between 1967-2010 in Quebec due to agricultural intensification.
Climate Change
Birds are highly sensitive to changing climatic conditions. Scientists have already documented shifts in bird migration and distribution patterns due to global warming. Climate change is expected to impact birds through:
– Shifting habitat ranges. As climate zones shift, so do the locations of suitable habitat for bird species. Not all birds will be able to shift their breeding ranges fast enough to keep up.
– Disrupted migration patterns. Many birds use seasonal cues like changing daylight hours to begin migration. If climate change leads to warmer weather occurring out of sync with daylight cues, migrations could start at the wrong times, reducing survival.
– Extreme weather. Increased frequency and severity of heat waves, droughts, wildfires, flooding, and storms disrupt nesting, destroy habitats, and directly kill birds.
– Mismatched food availability. Birds time their reproduction and migration to match peak food abundance. If climate change causes shifts in timing of food availability, it could impact bird survival and reproductive success.
– Sea level rise. Coastal habitats like beaches, tidal flats, and estuaries are being degraded and destroyed by sea level rise, impacting numerous shorebird species.
– Increased disease. warmer weather may increase prevalence of diseases that affect birds, especially pathogens spread by mosquitoes.
Invasive Species
Invasive animal and plant species are a major threat to birds worldwide. These non-native species disrupt ecosystems, compete with native birds for resources, and degrade habitats. According to a study published in Conservation Biology, invasive species contribute to 39% of bird extinctions worldwide.
Some problematic invasive species affecting birds include:
– Domestic cats: Kill billions of birds in the U.S. alone each year. Especially impact ground nesting species.
– Rats, mice, snakes: Threaten eggs, chicks and adult birds, especially on islands.
– Emerald ash borer: Kills ash trees, destroying habitat for over 30 bird species dependent on ash ecosystems.
– Parakeets: Compete with native cavity nesting birds for nest sites.
– Brown tree snakes: Caused extinction of all forest bird species on Guam.
– Northern snakehead fish: Voracious freshwater predator that impacts waterbirds.
– Cheatgrass: Alters prairie habitat structure and increases wildfire risk.
Invasive plants like kudzu, garlic mustard, and purple loosestrife degrade habitats by outcompeting native vegetation. Prevention and management of invasive species is crucial to reducing pressure on bird populations.
Overexploitation
Humans have long relied on birds as a resource for food, feathers, and more. But excessive harvesting has driven numerous species toward extinction.
Historically, many coastal bird species were hunted to near extinction for their plumes, which were used in ladies’ hats in the late 1800s. Passenger Pigeons, once numbering in the billions, were pushed to extinction by overhunting and habitat loss. Today, poaching for meat, eggs, and trophies continues to threaten rare species.
Bycatch, the accidental capture of seabirds by fishing vessels, kills hundreds of thousands of birds each year. Longline fisheries are a major culprit, as seabirds try to take bait off longline hooks and get pulled underwater and drowned. Gillnets are another significant cause of bycatch mortality.
Pollution
Environmental pollution poses varied threats to bird health and survival. Dumped plastics and discarded fishing gear harm seabirds and shorebirds. Oil spills affect birds both directly by coating feathers and indirectly by contaminating prey.
Chemical pollution like DDT and neonicotinoid pesticides accumulate in food chains, harming reproduction. Heavy metal pollution from mining and industry decreases survival. Light and noise pollution disrupt natural behaviors. Birds face respiratory health impacts from air pollution as well as habitat damage from acid rain.
Even renewable energy sources have unforeseen consequences. Wind turbines kill hundreds of thousands of birds and bats annually through collisions. Birds mistake solar panel arrays for bodies of water and fatally crash into panels. While offering important climate change mitigation benefits, renewable energy development must be properly sited to minimize harm to birds.
How Do These Factors Interact?
The threats facing birds rarely operate in isolation. Habitat loss, climate change, pollution, and overexploitation combine to magnify pressure on bird populations already stressed by human activities.
For example, habitat loss compounds the impacts of climate change by giving birds nowhere to go as ranges shift. Light pollution worsens habitat degradation by disrupting navigation and natural rhythms. Overharvesting makes rare species more vulnerable to inbreeding and extinction.
Interactions between threats make mitigating bird declines even more challenging. Conservation must consider the multifaceted, synergistic ways human impacts intersect to drive bird declines.
Protecting Birds into the Future
With many bird species in peril, implementing science-based conservation strategies is crucial. Some key approaches for protecting global bird populations include:
– Preserving and restoring habitats through protected areas, sustainable forestry, ecological restoration of degraded land, and more natural agriculture.
– Making manmade structures like buildings and bridges more bird-friendly through retrofits and design changes.
– Reducing pollution, transitioning to renewable energy responsibly, and enacting sustainable fishing reforms.
– Controlling invasive species spread and managing existing invasives.
– Addressing climate change by reducing emissions and working to conserve carbon sinks like forests.
– Continuing long-term bird population monitoring and research to assess trends over time.
– Educating people on how to help birds in their communities through citizen science programs, policies for bird-safe buildings, keeping cats indoors, and avoiding plastic pollution.
From professional conservationists to concerned citizens, we all have a role to play in reversing the troubling declines in avian biodiversity worldwide. By understanding the threats driving changes to bird populations, we can better target solutions to keep our skies vibrant with the sight and sound of birds for generations to come.
References
[1] Rosenberg, Kenneth V., et al. “Decline of the North American avifauna.” Science 366.6461 (2019): 120-124.
[2] North American Bird Conservation Initiative, U.S. Committee. The state of the birds 2014 report. U.S. Department of Interior, 2014.
[3] American Bird Conservancy. “Causes of Bird Mortality.” Accessed October 19, 2023. https://abcbirds.org/threat/mortality-causes/
[4] Loss, Scott R., et al. “The impact of free-ranging domestic cats on wildlife of the United States.” Nature communications 4.1 (2013): 1-7.
[5] Wilcove, David S., and Joon Lee. “Using economic and regulatory incentives to restore endangered species: Lessons learned from three new programs.” Conservation Biology 18.3 (2004): 639-645.