Malathion applied at standard rates reduces fledgling condition and adult male survival in a wild lesser kestrel population

The lesser kestrel Falco naumanni is a globally threatened colonial bird that has suffered a generalized population decline in Europe. Recent studies have suggested that the land‐use changes linked to agriculture intensification are the main factors accounting for the population regression. However, there has been little analysis of the role of pesticide applications. In this study, we examined the consequences of a malathion treatment in a wild population of lesser kestrels, specifically its effects on breeding performance, adult survival and population size. We found that the larger the area treated with malathion around the colony, the lower the size and the body condition of the fledglings, although no effects were found on the number of fledged chicks per pair or their sex ratio. Survival of adult males, but not females, was lower in the malathion‐treated areas. These results show that organophosphorus pesticides applied at standard rates might disrupt the lesser kestrel population dynamics by reducing their breeding performance and increasing adult mortality in a sex‐biased way. The proportional area treated around the colony did not affect colony size in the following year, indicating that an increased adult male mortality was not enough to lead to a detectable population size reduction and/or that the arrival of immigrants could have masked it. Both the direct malathion toxic effects and the expected reduction in prey availability due to fumigation are likely to underlie the observed effects. Hence, keeping non‐treated buffer areas around kestrel colony centers should be a strictly applied conservation measure to avoid the observed negative side effects of malathion applications.