Lifetime expression of egg rejection behaviour in Eurasian magpies is associated with variation in a polymorphic genetic marker.

Obligate avian brood parasitism is a reproductive strategy in which parasitic birds rely entirely on host species for incubation and chick rearing, reducing host reproductive success. This pressure has led many host species to evolve defenses, particularly the ability to recognize and reject parasitic eggs. However, not all individuals within a host population exhibit this behavior-some accept parasitic eggs, while others reject them. One proposed explanation for this behavioral variation is genetic polymorphism. Previous research on Eurasian magpies (Pica pica) linked egg rejection behavior to a specific microsatellite locus (Ase64), though these studies used genotypes from nestlings, not adult females, who are the ones expressing the behavior. Since egg rejection can change with age, accurately categorizing females as acceptors or rejecters requires repeated observations across their lifespan. This study genotyped adult females whose behavior had been reliably classified over their lifes to test for genetic differences. Results showed significant differences in the genotypic and allelic frequencies at the microsatellite marker MSLp4, which was more common in acceptors. These findings support the hypothesis that a genetic polymorphism contributes to individual variation in egg rejection behavior, a defensive trait that may shift over an individual's lifetime in magpies.