Abstract: In this paper, we review the relationships that link avian parental behavior, stress (acute or chronic) and energetic constraints to the secretion of prolactin, the ‘parental hormone’. Prolactin secretion is stimulated by exposure of the parent to tactile and visual stimuli from the nest, the eggs or the chicks, while prolactin facilitates/stimulates the expression of parental behaviors, such as incubating, brooding or feeding. Because of this role of prolactin in the expression of parental behaviors, we suggest that absolute circulating prolactin levels may reflect to the extent to which individuals provide parental care (i.e., parental effort). Stressors and energetic constraints (acute or chronic) depress prolactin levels (‘the prolactin stress response’) and this may be adaptive because it may disrupt the current parental effort of an individual and promote its survival. Alternatively, an attenuation of the prolactin stress response can be considered as a hormonal tactic permitting the maintenance of parental care to the detriment of parental survival during stressful situations. Therefore, we suggest that the magnitude of the prolactin stress response may reflect parental investment. Finally, we detail the interaction that links corticosterone, prolactin and stress in bird parents. We suggest that corticosterone and prolactin may mediate different components of the stress response, and, therefore, we emphasize the importance of considering both hormones when investigating the hormonal basis of parental investment.