Frequently Asked Questions on Pesticides and Birds

Why Pesticides and Birds
What is a Pesticide
When are Birds Exposed
How Can Pesticides Harm Birds

Why Pesticides and Birds

Birds are a visible and important part of the world we share.  They are not only worthy of protection in themselves, but are also environmental sentinels, forewarning us of potential hazards to our environment and our own health.  They are particularly sensitive to many pesticides. For example birds are 100 times more sensitive than mammals to the commonly used insecticide diazinon.  Populations may already be in decline due to other factors, such as habitat loss due to agriculture, urban development and invasive species.  In her landmark book, Silent Spring, Rachel Carson called attention to the decline in some bird species, such as Peregrine Falcons, brought about by DDT and other pesticides, and argued that these declines suggested serious effects for other species of wildlife and for humans.

One study, by David Pimentel of Cornell University, conservatively estimates the number of birds lost each year to pesticides at 67 million on farmland alone, with a potential for ten times that number to be affected in other ways.  We see only a small fraction of the avian impacts of pesticides because birds die in places not normally frequented by people, and when they do, we do not usually look for them.  In addition, poisoned birds hide, carcasses decompose and scavengers do a very efficient job of cleanup (77% of birds, on average, are scavenged by predators within 24 hours of dying - see paper by Nimish B. Vyas).  Sick birds are often misdiagnosed as victims of trauma and the chemical analyses required to determine pesticide poisoning are costly and therefore seldom carried out.

Far from declining in use since Rachel Carson's time, pesticide use, in terms of pounds applied and number of active ingredients registered, has actually increased.  In the United States today, 1.2 billion pounds of pesticides - 20% of the world's total - are applied annually to crops (see EPA's web pages on pesticide usage), lawns, households, golf courses and other areas.  Documented and investigated cases indicate that approximately 50 pesticide active ingredients currently used in the U.S. have caused bird kills.  When we weigh the risks of these chemicals against their benefits, we must ask how we can responsibly use and regulate a tool that inherently poses a risk to wildlife, especially to one of it's most cherished ambassadors: birds.

What is a Pesticide

The term pesticide means 'pest killer'.  Pesticides are chemicals designed to kill target organisms deemed to be pests because they can contribute to human diseases (such as malaria), crop destruction (before or after harvest), livestock disease or are household pests like ants and mice.  The Federal Insecticide, Fungicide and Rodenticide Act (known as FIFRA) defines pesticides as ". . . any substance or mixture of substances intended for preventing, destroying, repelling or mitigating any pest, and any substance or mixture of substances intended for use as a plant regulator, defoliant, desiccant."  

Pesticides work by disrupting one or more vital, biological processes of the target organism eg. photosynthesis in plants or blood clotting in mammals.  They can be divided into groups based on the organisms they target (eg. insecticides, rodenticides, herbicides and fungicides), by their mode of action (eg. cholinesterase inhibitors kill by interfering with nerve impulse transmission) or by their chemical class. The two classes encompassing the majority of insecticides in use today are carbamates and organophosphates.  Others are pyrethroids and biological controls such as viruses and bacteria.  One particular class of insecticides, widely used after WWII - organochlorines - was found to have a variety of severe adverse effects on wildlife soon after their introduction.  In 1972, all uses of the organochlorine DDT were cancelled in the US, closely followed by most other persistent chlorinated pesticides.  There are still some persistent organochlorines in use in the US today, such as endulfosan (Thiodan).  DDT is still used in six countries in the Western Hemisphere (Argentina, Belize, Ecuador, Guyana, Peru, Mexico).  While less persistent in the environment than traditional organochlorines, carbamates and organophosphates are generally more acutely toxic to birds.

Most rodenticides are anti-coagulants.  First generation anti-coagulants such as Warfarin cause hemorrhage.  Second generation types bring about neurological and cardio-pulmonary injury in the target organism before the onset of hemorrhage.

Herbicides act on biochemical pathways specific to plants, for example photosynthesis.  They can act on contact or by root absorption and their effect can be non-systemic (e.g. diquat) or systemic (e.g. glyphosate).  Generally, they are not directly responsible for bird mortalities but declines in bird populations can be attributed to habitat loss and depletion of food resources caused by these chemicals.

New classes of pesticides are constantly being developed and introduced, some less acutely toxic to wildlife but nevertheless raising concerns about accumulation in soils, long-term ecological effects and synergistic action when different classes of pesticide used simultaneously.  While testing procedures for human health risks have been greatly improved over recent years, tests for wildlife toxicity have advanced little thus restricting our ability to adequately assess potential wildlife effects prior to registration or widespread use. To view detailed information on individual pesticides go to our Pesticide Profiles page.

Any given pesticide product contains one or more active ingredient and any number of inert ingredients to enhance the effect of the product.  The active ingredient is the component designed to kill or otherwise harm the target organism and must, by law, be listed on the pesticide label, but inerts can be toxic too, or may enhance the toxicity of the active ingredient, and yet need not be listed on pesticide labels.  In fact, the mixture of inert and active ingredients are often closely guarded trade secrets.  Synergists increase the toxicity of the active ingredient, diluents give the product greater volume for ease of application, and surfactants reduce surface tension, thereby aiding the spread of the chemical.  Pesticides can take a variety of forms: liquid, powder, gel bait granular etc. Pesticides manufactured in granular form may be subsequently dissolved in spray tanks and applied as liquid.  Others are applied in particle form.  Not long ago these particles were embedded in silica or processed corn cob, both of which are highly attractive to birds.  More recently clay granules have been developed which come in many varieties and can break apart in soil much quicker.  While less attractive to birds due to their lower visibility, these clay granules may adhere to food items and so may be eaten by birds inadvertently. The forms a pesticide takes are generally determined by agronomic and human health concerns - wildlife effects are rarely considered.

All pesticide use in the US is regulated via a complex system of registration and review, under the auspices of the Environmental Protection Agency (EPA).  The governing legal act for product registration and use is the Federal Insecticide, Fungicide and Rotenticide Act (FIFRA).  The initial version of this act was passed in 1947, for implementation by the USDA.  It required chemicals to be registered but provided no means of controlling use.  It was not until 1972 that amendments were made to FIFRA to provide mechanisms for risk-benefit analysis and use restrictions, with control handed over to the newly formed EPA. Today, chemical manufacturers bear the burden of proposing new chemicals, testing their toxicity and presenting the case for their use.  EPA scientists and other staff are responsible for evaluating the manufacturers case and making decisions on where and when a chemical may be used and how it should be applied.  The part the U.S. public can play in this procedure is by voicing their concern - an increasing role - which can effect the outcome of an EPA decision. 

There are 20,700 pesticide products on the market today containing 890 registered, active ingredients.  Pesticide users spend about $11.9 billion on 4.5 billion pounds of active ingredient each year and ³/₄ of all households use them.  They are ubiquitous, a fact of modern life and are found in wood preservatives, soaps, disinfectants, Christmas trees and home lawn products.  They are used in zoos, parks and swimming pools and we rely on them to control disease and to guarantee and protect our food supply.  They can be effective tools but are, in the end, poisons and using them entails risk to birds and other wildlife.

When are Birds Exposed

Birds are rarely the target of pesticide application but are often caught in the crossfire of a never ending war between humans and pests.  Peregrine Falcons were nearly wiped out due to DDT poisoning throughout the 50's and 60's.  In 1996 it is estimated that over 20,000 Swainson's hawks, 8% of the species, died in Argentina from 1995-96, due to poisoning by the insecticide monocrotophos (Azodrin, Nuvacron) and the EPA estimated that a single carbamate, carbofuran (Furadan) killed two million birds or more each year in the early 1990's.

Birds are, by their very nature, mobile creatures and cannot usually be kept out of areas where pesticides are applied.  Their ability to cover large distances in a short period of time increases their susceptibility to pesticide exposure.  Birds can come into contact with pesticides by three Routes of exposure:

1. Ingestion

   • on/in the food they eat

   • Primary exposure - birds eat seeds, insects or vegetation sprayed with a pesticide.

   • Secondary exposure -  birds eat a small mammal or other vertebrate which has itself been exposed by eating seed, insects or vegetation sprayed with a pesticide.

   • by drinking

   • spray solution collected on leaves

   • contaminated water

   • via preening
     

2. Inhalation

   • of chemical spray

   • of treated dust

3. Dermal absorption

   • from bathing in tainted water

   • absorption through the feet when standing on treated vegetation/soil

   • through preening, which works the pesticide past the feathers and onto the skin

   • through the brood patch - when spray falls on a nest where birds are incubating

The differences in toxicity of any one chemical varies tremendously between species, sometimes by a factor of 10 or more.  Variations in the application method, time of year, route of exposure and application rate can also have a significant outcome on avian toxicity.  Field studies have shown that 77% of bird carcasses are scavenged by predators within 24 hours and those that remain are often extremely difficult to locate, hampering efforts at assessing the number and scale of pesticide kills.  This suggests that kills that have been identified are only the tip of the iceberg.

Simply because a bird does not feed on vegetation or insects which have been the target of pesticide application (primary exposure), does not mean that it cannot come into contact with that chemical.  Predatory or scavenging species may be exposed when they feed on target or non-target vertebrate species (e.g. rats) that have themselves been exposed.  This is called secondary exposure.  Runoff or wind can carry pesticides to nearby ponds and fields where birds feed, bathe, drink or gather tainted brush or mud for their nests.

Finally, when we think of pesticides, we generally think of agricultural use, but homeowners share some of the responsibility.  In one study in Illinois, homeowners used four times as much pesticide per acre as farmers, on their lawns and gardens.

How can Pesticides Harm Birds

When we think of birds being affected by pesticides, the image conjured up in many minds is of media headlines citing mass mortalities, accompanied by graphic pictures of fields of dead birds.  In reality, while these do occur from time to time, actual effects may be much more subtle and hard to isolate.  Some incidents may eventually cause death in a slower more insidious way, making carcasses harder to find.  Others may cause species numbers to decline indirectly by weakening the animal or reducing food supply, and others still may reduce species numbers without killing individuals, by reducing breeding success.

To simplify matters, we can think of pesticide hazard to birds in two ways; exposure and effect: 

Exposure can be either acute or chronic;

• Acute: These are sudden, short exposures to high levels of pesticides.  Responses of birds to acute exposure can vary significantly.  Acute exposure to organophospates often leads to death of the bird within 30 minutes after symptoms of feather fluffing, labored breathing, muscle tremors and convulsions.
  

• Chronic: these are defined as long-term exposures.  The problem with these exposures is that because the poisoning takes place over a prolonged period birds are more likely to become scattered over a wider geographical area and may never be seen.  If they are recovered it is much harder to correlate the death with specific pesticide applications.

Effects from acute exposures can be immediate or delayed.  Chronic exposure, by definition produces delayed effects. Both types of exposure lead to a range of potential DIRECT EFFECTS, which might be lethal or sub-lethal:

• Lethal: in addition to the highly visible mass mortalities, birds can die in a variety of other ways due to pesticides, from both acute and chronic exposures.  Again, the slower the reaction, the harder it can be to detect.

• Sub-lethal:

  • Weakening: If a bird is hit by a car, we might simply put that down to bad luck for the bird or careless driving by the driver.  But if the bird's vision or reactions are impaired by chronic exposure to pesticide it is neither.   These victims of road accidents are often left unanalyzed by the side of the highway and we are none the wiser of the underlying cause.  Birds that are suffering from pesticide poisoning are more likely to be killed by predators - this may be especially true of females on the nest, as was the case in a study on the one-time treatment of fields with methyl parathion and nearby nesting ducks.  In an unusual reversal, a Great Horned Owl died in 1999 after being wounded by its intended prey, a muskrat.  These wounds are a common occurrence but in this case the owl was suffering from the effects of brodifacoum poisoning, an anti-coagulant rodenticide, causing it to bleed to death.  Tolerance to extreme temperatures and disease may also be reduced after pesticide exposure.

  • Reproductive: these effects include egg shell thinning (as was the case with DDT, which caused Peregrine Falcon eggs to break while being incubated), deformed embryos, reduced fertility and decreased hatchability of fertile eggs.  Most of these effects are never seen unless specifically studied because eggs that fail to hatch may do so for a number of reasons not necessarily attributable to pesticides (parental inexperience, genetic defects, nest disturbance).  Fertility reduction is only noticeable when looked at over successive generations.

  • Behavioral:  these changes are perhaps the most subtle of all potential pesticide effects.  Changes in territoriality, vocalization, courtship and breeding behaviors, reduced attention to young and a reduced response to human presence may all result after pesticide exposure, with latent effects on individuals or populations.

INDIRECT EFFECTS: Birds that use roadside margins or hedgerows for nesting, roosting, cover or food can decline when these areas are 'cleaned up' by civic authorities' 'beautification' programs.  Birds will likewise be effected if their primary food source is also regarded as a pest or is a non-target species that is affected nonetheless by insecticide or herbicide application.  While the pesticide does not necessarily poison the bird, it is nevertheless the indirect effects of the chemicals that are ultimately responsible for reduction in populations.  In addition to the effects on birds to direct exposure to pesticides, birds can be affected by the pesticide without ever coming into contact with it.  These are called Indirect Effects.