Nest predation risk, but not demography, drives dynamics of conspecific brood parasitism

Conspecific brood parasitism (CBP) is an alternative reproductive tactic in several animal taxa. Although various behavioral aspects related to CBP have been studied in several species, understanding spatial and temporal dynamics of CBP and its drivers is still limited. We studied roles of nest predation risk and demography as possible drivers of dynamics of CBP in common goldeneyes (Bucephala clangula), a cavity-nesting duck. We provided decoy nests for parasitic laying in an experimental setting of 15 lakes for 7 consecutive years irrespective of local nest predation, being thus able to control for effects of predation-determined host nest availability. Individual parasites were recognized using protein fingerprints from egg albumen of parasitic eggs laid in the decoy nests. We found considerable spatial and temporal variation in the frequency of CBP within the experimental setting. Variation in CBP was driven by nest predation risk: the rate of CBP tracked the number of nonpredated nesting attempts at the lakes during the previous year. Neither variation in lake-specific number of potential homing first-time breeding females (i.e., demography) nor variation in lake-specific number of nesting females present explained the variation in lake-specific frequency of CBP. Our findings provide evidence that parasitically laying females pursue a genuine and flexible safety-seeking tactic in nest selection and that nest predation risk drives spatial and temporal dynamics of CBP.