Project Pn0902: Effects of Seed Treatments on Birds

PROJECT PN0902: EFFECTS OF SEED TREATMENTS ON BIRDS

SUMMARY

Seed treatments can be an efficient, environmentally friendly, low-input way of using pesticides. However, they may present a high risk to seed-eating birds and in the past some of them have caused poisoning incidents on a sufficient scale to warrant regulatory action. The objectives of this project were to measure the actual risks to birds from seed treatments, identify factors affecting exposure, evaluate current methods of risk assessment and, if necessary, develop improved methods.

It was necessary to focus most of the work on one pesticide and two species of birds. Pigeons (woodpigeons and feral pigeons) and the insecticide fonofos were chosen. It is important to stress that these were used solely as experimental models for other species and seed treatments. A variety of methods were used including both field and laboratory studies and modelling. Some additional work was supported by grants from the EU and the Spanish Government. Most of the detailed results are specific to pigeons and fonofos, but the conclusions and recommendations are generally applicable except where otherwise stated.

The actual risks to birds from seed treatments

The actual impact of seed treatments on woodpigeons was assessed using three sources of data: poisoning incidents reported to the Wildlife Incident Investigation Scheme (WIIS), surveys of affected birds in roosts at Woodwalton Fen during 14 winters, and monitoring of radio-tagged woodpigeons in the winters of 1994/5 and 1995/6. The results showed that the frequency of poisoning of woodpigeons by fonofos and other OP seed treatments was low and probably close to zero in the 1990s, but reached appreciable levels in some years in the 1980s.

Factors affecting exposure of birds to seed treatments

The factors which influence exposure can be divided into three groups: those which determine the distribution of treated fields, those which determine how many birds visit those fields, and those which determine the degree of exposure during each visit to a treated field.
Cropping patterns and pesticide usage determine the spatial and temporal distribution of treated fields. Factors which determine how many birds visit those fields include the geographic distribution of bird populations, their dietary preferences, the density of seed available on the soil surface, the availability of alternative food sources, the degree of disturbance by shooting and other human activities, and the tendency of some species to feed in flocks.

Factors which determine individual exposure during each visit to a treated field include the concentration of pesticide on the treated seed and the rate at which it decays, the density of exposed seeds, the ability of birds to avoid treated seeds, the rate of feeding, the tendency of birds to feed faster when in flocks, hunger, the ability of some birds to regurgitate treated seed, and the potential for birds to learn from previous experience with treated seed. All of these factors were investigated to some extent.

Field studies showed that woodpigeons do avoid fonofos-treated seed in the wild. This was the first demonstration of birds avoiding pesticide-treated seed in the wild. It is a very important result because it confirms the potential for avoidance to reduce exposure to pesticides in the wild, and implies that it should be considered in risk assessment.

A second new and important finding was the crucial and pivotal role of feeding rate. Together with the concentration of pesticide on the seed, the rate of seed consumption is crucial in determining the speed at which the active substance is ingested. It is pivotal because it is the mechanism through which many of the other factors above influence exposure. Birds feed faster when they are hungry, when food is available for limited periods during the day, and when feeding competitively in flocks. The rate of feeding may be reduced if the density of exposed seed is low, or if birds are disturbed or wary during feeding. A series of experiments under different conditions showed a close positive relationship between the rate of feeding and the frequency of mortality.

Another novel finding was that regurgitation can be crucial in enabling pigeons to avoid poisoning after consuming a lethal quantity of treated seed. Information in the literature suggests that species and individuals vary widely in their ability to regurgitate, making it difficult to take account of in risk assessment.

The results showed that in some conditions there is a fine balance between mortality and survival, depending on whether the dose absorbed by a bird reaches the lethal level before the avoidance and regurgitation responses can intervene. For pigeons and fonofos, mortality is likely only when feeding is very rapid, and the concentration of the pesticide is close to the application rate. Field observations indicate that only 1% of woodpigeons on newly-sown cereal fields feed at the rate required to cause mortality.

Recommendations for improving risk assessment

Prior to the start of this project there was considerable scepticism in some parts of the scientific and regulatory community (especially in North America) about the role of avoidance in reducing risk in the wild. If avoidance is ignored, then the very low toxicity-exposure ratios (TERs) for pigeons feeding on fonofos-treated wheat would lead regulators to expect very frequent mortality. If on the other hand avoidance is considered, simple tests suggest that the response is very strong and no mortality would be expected. In practice, neither conclusion is correct. This project has demonstrated that avoidance does occur in the wild, and revealed the crucial role of feeding rate in determining the conditions under which avoidance breaks down. With this information the risk assessment can be refined in a way that was not possible before, correctly predicting that mortalities would occur but only rarely and under specific types of conditions. While the details of this are specific to fonofos it leads to a total of 29 general recommendations for improving risk assessment, which are listed on the following pages. Most importantly, the results of this project provide the basis for moving from qualitative assessments of risk for seed treatments to more quantitative estimates of the magnitude and frequency of effects.