We documented two cases of polygyny in Eurasian Kestrels (Falco tinnunculus) and studied the behavior, morphology, age, relatedness, and heterozygosity of bigamous and monogamous individuals. Bigamous males tended to have shorter wing chords compared with monogamous males. The provisioning rate of bigamous males was almost twice that of monogamous males, and at the secondary nests was lower than that at the primary nests. Secondary females were smaller (in wing chord) and younger (<2 yr old) than primary and average monogamous females. The neutral heterozygosity of bigamous adults did not differ from that of monogamous adults. We observed minimal aggression between bigamous females, perhaps due to their relatedness. It is unclear why secondary females engaged in polygyny with less investment from the male; however, accepting the bigamous male allowed immature females the opportunity to mate.