Impacts of pollution and toxins on Vermont birds on their breeding grounds range from direct mortality to sub-lethal effects to reduced habitat quality.
Consequences are typically difficult to detect, and rarely documented at an individual or population level.
- Acidic ions (e.g., mercury and lead, oxidants, organic compounds).
- Atmospherically deposited often thousands of miles from their origins.
- Accumulate in aquatic and terrestrial environments.
- Acidic precipitation, or “acid rain”, has contributed to extensive mortality of red spruce in the Green Mountains during the 1970s and 1980s, and may have impacted the aquatic food chain and compromised habitat quality for piscivorous birds (e.g., Common Loon). Leaches calcium from forest soils, resulting in calcium deficiencies (e.g., Wood Thrush).
- Inorganic mercury from industrial sources can affect both aquatic and terrestrial birds. Bioaccumulates in aquatic food chains and can cause dramatic behavioral or reproductive impairment at high concentrations (seen in Common Loons). Some Vermont loon eggs have high mercury levels. Mercury also bioaccumulates in the terrestrial food web.
- Lead toxicity from ingestion of spent lead hunting shot and fishing sinkers causes direct mortality of aquatic birds like. Between 1984 and 2009, nearly 40 percent of adult Common Loons recovered dead in Vermont died from lead poisoning after ingesting sinkers.
- Birds may ingest treated seeds and fruits, absorb chemicals through contact with treated plants or water, or bioaccumulate chemicals in their tissues.
- Many of the insecticides used in North America’s agricultural crops “give rise to regular bird mortality in a measurable proportion of treated fields” (Mineau 2002).
- More insidious are the potential effects of insecticides on insect prey availability, which can compromise avian productivity.
- Despite the risks they pose, pesticides are relatively low in Vermont’s agricultural fields compared to fields elsewhere in the United States.
- Hay dominates Vermont’s agricultural portfolio, followed by corn (~92,000a), and to a lesser extent, Christmas tree farms and apple orchards (~8,000a total; USDA/NASS 2009).
- Although corn is among the crops with the highest potential for bird mortality in the US from organophosphate insecticides, overall estimated risk in corn decreased from 1990 to 2002. Declines in risk are generally due to the replacement of harmful pesticides with less toxic alternatives, rather than a reduction in applications.
- Although risk in small fruit crops – and notably in blueberries – have increased, blueberry production in Vermont accounts for less than 400acres. Still, blueberries offer an abundant food source to birds, causing localized avian pest problems and potential exposure to pesticides (e.g., Cedar Waxwing, Gry Catbird).
- Forest and shrubland (e.g., powerline right-of-ways) often includes the use of pesticides. Direct, negative effects of organophosphate insecticides used have been documented. There is no evidence that herbicides, when used properly in forests, directly affect birds or bioaccumulate or bioconcentrate. The indirect effects caused by changes in vegetation depend on species, season, time, and site.
- Long-term goal to reduce reliance on pesticides: maintain balanced ecosystems that are resistant to outbreaks and prevent the spread of exotics. As efficient insect predators, birds are part of that solution.
- Alternatives to pesticides include the non-toxic bacterial agent Bacillus thuringiensis (Bt). This too, however, can lower bird productivity by reducing insect prey availability.
- Integrated Pest Management (IPM) minimizes chemical dependency through prevention and disruption of pest outbreaks, and targeted, limited applications of least-risk pesticides. The concept can be applied to farms and home gardens alike.
- Organic farming includes practices that encourage the recovery of beneficial insects that are pest predators.
- Fortunately, many of the known threats posed by contaminants have been, or are being, addressed by regulatory actions.
- “Clear Skies in Vermont” initiative limits atmospheric emissions of mercury and other airborne pollutants.
- Clean Water Act (1977) strictly regulates pollutant discharges and water quality standards for US surface waters.
- Recent statewide bans on use of lead shot and sinkers have greatly restricted their use.
- In North America removal of insecticides with demonstrated high toxicity has been occurring, although at a slow pace compared to Europe (Mineau 2002).
- Most recently, the insecticide carbofuran has been on the brink of cancellation in the United States because it is so highly toxic.
- Technological advances are needed to adequately document the impacts of applications of newly-approved pesticides on birds.