Migratory landbirds are exposed to a variety of threats on their wintering, breeding, and migratory stopover habitats. Pollution is one of the least known threats due to the difficulty in measuring its effects on birds. In this project CIAD will use a methodology specifically designed to measure the impacts of organophosphate and carbamate pesticides on migratory landbirds. These impacts will be measured by the percent of cholinesterase inhibited in blood samples. Blood samples of migratory landbirds will be collected in Bahía Santa María, Sinaloa and in the Colorado River delta, Sonora. CIAD will select common species in order to compare values between these two sites, since both areas are influenced by large agricultural districts. A significant result of this project will be to determine the levels of cholinesterase inhibition of migratory landbirds before they cross the border to the United States, particularly learning where on their migration they tend to experience the greatest exposure (Sinaloa or Sonora).