Wildlife near agricultural lands is exposed to pesticides, particularly organophosphorus and carbamates, where birds appear to be more sensitive to their toxic effects than other vertebrates. One of the main effects of pesticides is the disruption of the nervous system through the inhibition of cholinesterase enzymes. The aim of this study was to determine the plasma cholinesterase activity in native birds of pesticide-exposed agricultural lands within the Grassland Priority Conservation Area El Tokio, located in northeastern Mexico. The study was conducted during three summer seasons (2008–2010), when the reproduction of birds and pesticide spraying occurred. Forty-four birds of 13 different species were captured, sampled, and released. High variability values among individuals and species were found, ranging from 0.200 ± 0.055 to 4.960 ± 0.150 µmol/min/L. White-winged doves' values were significantly smaller than basal reference, showing 29–49% of plasma cholinesterase inhibition and possible pesticide exposure. Mean plasma cholinesterase activity values for 10 of the species had not been reported previously. These data can serve for future interpretations of plasma cholinesterase activity values in wild birds within agricultural lands and for decision making in priority conservation areas.