Abstract: An emergency life-history stage is expressed in breeding vertebrates when the immediate survival is threatened by poor energetic conditions (i.e. allostatic overload). This emergency life-history stage shift energy investment away from reproduction and redirects it toward immediate survival. In birds, this emergency life-history stage is promoted by a release of the stress hormone corticosterone. However, how corticosterone functionally reduces the expression of parental cares remains to be clarified. One hypothesis is that the release of corticosterone affects prolactin levels, a pituitary hormone widely involved in regulating parental behaviours. We tested this hypothesis by experimentally increasing corticosterone levels of chick-rearing black-legged kittiwakes (Rissa tridactyla) over a 3-days period and by monitoring prolactin levels over an 8-days period. We also investigated whether this hormonal manipulation affected nest attendance and breeding success. Corticosterone treatment resulted in a significant increase in plasma corticosterone levels during the first 2 days, which returned to pre- treatment values at day 3 . This short term corticosterone increase was accompanied by a 30% decrease in prolactin levels. Prolactin levels where reduced in a progressive and persistent manner and did not return to their initial levels when corticosterone levels returned to baseline. Moreover, although corticosterone levels had returned to pre-treatment values, low prolactin levels were associated with a reduced nest attendance. Finally, this hormonal treatment significantly reduced breeding success