Genetic studies have revealed that a surprising number of chicks are fathered by males outside the primary pair bond.
Researchers have found evidence that female reproductive tracts can affect sperm survival, storage, transport, and fertilization success.
Some sperm may be stored longer, moved more efficiently, or given a better chance of reaching the egg.
This creates intense sperm competition.
Males compete not only through courtship displays, songs, territory defense, and physical condition, but also after mating through the performance of their sperm inside the female reproductive system.