The increase in male blue-headed vireo parental care is thought to be attributed to the levels of testosterone and prolactin found in the plasma of these birds during the mating season. Testosterone, a steroid hormone that is secreted by the testes, has been found to increase aggressive behaviour in male birds during the breeding season and that these males display less parental care if their testosterone levels are maintained. Conversely, prolactin is a peptide hormone that is regulated by the hypothalamus that has been found at higher levels in birds that do the majority of incubation. It was found that in the early breeding season, male blue-headed vireos have a low testosterone level and will maintain this low level at all stages of the season. This low level does not affect the Vireo's ability to compete with other males for territory however, as a high testosterone level has been found to not be necessary for territory establishment. Prolactin levels of the males were found to be high very early on in the breeding season and are maintained for the duration of the season. This high level of prolactin along with a low testosterone level, explains why male blue-headed vireos are so involved in the construction of the nest, daytime incubation, and feeding of the nestlings.