Monitoring exposure of northern cardinals, Cardinalis cardinalis, to cholinesterase-inhibiting pesticides: enzyme activity, reactivations, and indicators of environmental stress

Northern cardinals (Cardinalis cardinalis) frequently use agricultural field edges in northeast Arkansas, USA, and may be at risk of exposure to cholinesterase (ChE)‐inhibiting pesticides. We monitored northern cardinal exposure to ChE‐inhibiting pesticides by comparing plasma total ChE (TChE) activity to reference‐derived benchmarks and TChE reactivations. Total ChE and acetylcholinesterase (AChE) were measured for 128 plasma samples from 104 northern cardinals from nine study sites. Of birds sampled from sites treated with ChE‐inhibiting pesticides, 4.3% of the samples had TChE activities below the diagnostic threshold (2 standard deviations [SD] below the reference mean) and 8.7% of the samples had TChE reactivations. No difference was found in TChE (p = 0.553) and AChE (p = 0.288) activity between treated and reference sites; however, activity varied among treated sites (p = 0.003). These data do not suggest uniform exposure to individuals, but rather exposure was variable and likely influenced by mitigating factors at individual and site scales. Furthermore, monitoring of TChE reactivation appeared to be a more sensitive indicator of exposure than the diagnostic threshold. Fluctuating asymmetry (FA) was greater at agricultural sites than reference sites (p = 0.016), supporting the hypothesis that FA may be useful for assessing a combination of habitat‐ and contaminant‐related environmental stress.