Hypervariable genetic markers, including a novel locus-specific marker detected by a mouse major histocompatibility complex probe, reveal that multiple paternity is common in families of polygynous red-winged blackbirds (Agelaius phoeniceus). Almost half of all nests contained at least one chick resulting from an extra-pair fertilization, usually by a neighboring male. Genetically based measures of reproductive success show that individual males realize more than 20% of their overall success from extra-pair fertilizations, on average, and that this form of mating behavior confounds traditional measures of male success. The importance of alternative reproductive tactics in a polygynous bird is quantified, and the results challenge previous explanations for the evolution of avian polygny.
To study parentage in wild blackbirds, we collected blood samples from adults and 6-day-old nestlings from one low-density (0.1 territorial males per hectare) and two high-density (0.4 males per hectare) marshes near the Queen's University Biology Station, Chaffey's Lock, Ontario, in 1986.