Pesticide mixture effects on physiological stress and morphology of growing wild nestlings

Synthetic pesticides may pose a risk to non-target organisms, notably farmland birds, through their hazardous effects on life-history traits, but their impacts on growth and physiological stress have never been assessed in natura. We investigated these effects on skeletal growth, mass gain, body condition and physiological stress (basal and stress-induced corticosterone secretions) of 35 nestlings of Circus pygargus, a farmland apex predator, blood-sampled twice during their development. We found contamination levels ranging from 0 to 9 pesticides detected out of the 116 searched. We observed an age and sex-dependent pattern of pesticide mixture effects on nestlings' growth and baseline corticosterone. Males (≤17 days old) having shorter tarsus when their pesticide load was high while this was found in older females (≥18 days old). Higher pesticide loads were linked to poorer body condition of young nestlings irrespective of their sex. Similarly, depressed corticosterone secretion was found in more contaminated young nestlings, and in females irrespective of their age. These alterations observed in young nestlings are caught up later during their development. No relationship between contamination levels and stress-induced corticosterone was found. Including other life-history traits would provide more information on other stress endpoints and potential energetic trade-offs between an active defence against pesticides and important functions of the organism.