Herbicides and Birds

What are herbicides?

Herbicides are chemicals formulated to kill plants. Millions of pounds of them are applied annually to gardens, lawns and crops across North America to control unwanted plants and weeds. They are sold under a variety of trade names containing numerous different active ingredients. Herbicides can be applied to plants at various stages of growth or directly to the soil, and affect plants at a cellular or tissue level. Some inhibit plant growth by targeting certain essential amino acids or growth regulators, some prevent the plants from photosynthesizing, while others kill plants by destroying cell membranes on contact. Complete explanations of many of the modes of action in common herbicides can be found in Appendix.

Selective herbicides are those that are more toxic to some plants than others at specified concentrations, simplifying application over large areas. Selective herbicides are commonly applied along with fertilizer directly to home lawns and golf courses, to kill weeds such as crabgrass and sedges, without harming the existing turf. Non-selective herbicides will kill any plant they contact, and are usually applied only to target plants, individually, or to an area prior to planting with a specific crop to kill competing weeds. Round-up, a trade name for the common herbicide formulation containing the active ingredient glyphosate, is an example of a non-selective herbicide commonly used by homeowners to kill individual weeds and other undesirable plants on lawns and gardens. It is applied by hand directly to individual plants. The brand names of common herbicides and their active ingredients can be found in here.

Where are Herbicides used?

In agricultural applications, herbicides are applied to fields to restrict the growth of weeds that compete with crops for space and nutrients. Herbicides are used very commonly in forestry to kill understory plants, usually annuals and bushy species that appear in natural succession after fires and tree falls. These native species compete with tree seedlings when they are replanted for commercial timber. Herbicides are applied by various government institutions on roadsides, railways, and parks to control unwanted plant growth and to restrict exotic plant species that have become invasive to natural habitats. Finally, many homeowners regularly apply herbicides to their lawns and gardens.

How do herbicides affect birds?

Merely because herbicides are designed to kill plants does not prevent them from also being hazardous to animals - certain herbicides have been shown to be directly toxic to birds (e.g. 24-d). Perhaps the more common effect herbicides have on birds is the alteration of the habitat in which they live, feed, and reproduce. Plants form the foundation of every terrestrial food chain and are essential for the survival of every animal above them. Animals and plants have coevolved to be mutually reliant on each other in natural ecosystems. Just as plants may depend on animals for pollination or seed dispersal, birds depend on plants as the basis of their food supply. For some e.g., seedeaters such as finches and sparrows, removal of a plant species from their habitats directly reduces their food supply. For others the relationship is indirect yet equally important. Herbicides kill the plants relied upon by insects, which can in turn reduce the available food supply for insectivorous birds. When herbicides are transported from the original application site to rivers and streams through run-off and evaporation, aquatic life is altered, affecting birds such as waders, shorebirds, waterfowl, kingfishers, and fish-eating raptors. Plants provide shelter and nesting areas for birds, as well as contribute to the microclimate of a habitat by affecting the temperature, humidity, and even rainfall over a geographic region. Consider the following realistic scenarios:

Herbicides used in Agriculture: The non-selective herbicide Roundup is commonly sprayed over entire fields to kill all competing weeds before planting a crop. This practice is becoming more prevalent with the availability of genetically- engineered (GE), "Roundup Ready" seeds. These GE crops are resistant to the effects of Roundup, and farmers are free to use it in very large quantities without fear of harming their crop. When Roundup is applied aerially, quantities of the herbicide drift onto adjacent hedgerows and shelterbelts - important habitat for many bird species, especially in the central and northern prairie states.

Herbicides in Forestry: After forests are clear-cut to harvest timber, the land is treated with non-selective herbicides in order to reduce competition between newly planted tree seedlings and natural undergrowth. Once commercially planted trees are aided by herbicides in gaining dominance over competing plant species, the former natural diversity of plants is wiped out to form monocultures of commercial timber. Without the diversity of habitat, the diversity of birds cannot be sustained. Some forest bird species may be unable to effectively feed or reproduce in commercial timber lands where herbicides have been extensively used.

Herbicides in your backyard: Homeowners will often apply selective herbicides to their lawns in order to reduce the growth of clover, crabgrass, plantains, dandelions, or other plants that commonly invade lawns. By reducing the diversity of plant life in your yard, the diversity of insects species are reduced. This can result in an imbalance of important predator species of insects to the number of pest species prompting some to apply toxic pesticides to control pests. Some herbicides are toxic to invertebrates and can reduce their numbers, directly, reducing the available prey base to birds.

Herbicides' effects on soil: In many instances, herbicides are toxic to beneficial soil organisms, such as nitrogen-fixing bacteria and mycorrhizae. Mycorrhizae are fungi that have evolved with the root systems of plants and facilitate the uptake of essential nutrients. Bacteria "fix" nitrogen essential to plants for amino acid synthesis and growth. Herbicides have been shown to kill beneficial soil micro-organisms, resulting in the overgrowth of other harmful bacteria and fungi. Pre-fixed nitrogen can be supplied to crops through fertilizers, but natural habitats rarely benefit (and aquatic habitats frequently suffer) from applied fertilizer. Some agricultural lands are not productive for a period of time after extensive herbicide use, or are only productive with extensive artificial fertilization.

Herbicides' effects on natural waterways: Whenever herbicides are applied, they are inevitably transported to areas distant from the original site of application. This can be by means of rain-induced run-off, wind or rain erosion of soil particles containing traces of herbicide, and evaporation into the atmosphere and subsequent transportation in fog and clouds (recent analyses of rainfall in Canada and Europe showed that herbicides were found in significant levels in rainwater). Whatever the method of transport, much of the herbicides we apply end up in our rivers and streams. Many herbicides and their additives are directly toxic to aquatic invertebrates and fish. Herbicides kill algae and aquatic plants in waterways which can lead to nitrification and eutrophication and destroy the food base for other aquatic organisms at the base level.