This document summarizes three studies on extra-pair paternity (EPP) in birds. A study of little auks found high rates of extra-pair copulation (EPC) but very low rates of EPP. Zebra finches in captivity showed high EPP unlike those in the wild, suggesting environmental factors influence EPP. Mountain bluebirds with more ornate plumage had higher EPP, potentially increasing sexual selection. Overall, the document finds EPP varies between species and can increase variance in male reproductive success.