Neurotoxic pesticides and behavioural effects upon birds.

Organochlorine, organophosphorus, carbamate, pyrethroid and neonicotinoid insecticides and organomercury fungicides are all neurotoxic and therefore have the potential to cause behavioural disturbances in birds. A number of studies have described behavioural effects caused to captive birds by neurotoxic pesticides, but it is very difficult to measure such effects in the field, which is a serous limitation given their potential to cause adverse effects at the population level. The mode of action, and the neurotoxic and behavioural effects of these compounds are briefly reviewed before considering evidence for their effects in the laboratory and field. Behavioural effects may cause adverse changes at the population level either directly or indirectly. Direct effects upon avian populations may be due to disturbances of reproduction, feeding, or avoidance of predation. Indirect effects on predators may be the consequence of direct action upon the prey population leading to either (1) reduction of numbers of the prey population, or (2) selective predation by the predator upon the most contaminated individuals within the prey population. Attention is given to the historic evidence for neurotoxic and behavioural effects of persistent organochlorine insecticides, raising the question of retrospective analysis of existing data for this once important and intensively studied class of compounds. Less persistent pesticides currently in use may also have neurotoxic effects upon birds in the field. Sometimes, as with some OPs, their effects may outlast the persistence of their residues, and the ecotoxicity and persistence of some may be affected by interactions with other environmental chemicals. The development of new mechanistic biomarker assays could improve understanding of behavioural effects and possible associated effects at the population level caused by such compounds in the field.