Long-term insights into who benefits from brood reduction.

The resource-tracking/facultative brood reduction hypothesis suggests that, under food stress, many altricial birds sacrifice the youngest brood members to enhance the growth and survival of their siblings. Studies examining staggered hatching and food shortage have generally supported this idea, although staggered hatching may serve additional purposes. However, the direct beneficiaries of this selective mortality remain unclear, as most research has overlooked parents and post-fledging outcomes. A life history perspective has rarely been applied to brood reduction. Using a 34-yr individual-based dataset on blue-footed booby () reproduction, we investigated whether siblicidal reduction of food-stressed two-chick broods benefits mothers, fathers, or surviving offspring. Results revealed that mothers of reduced broods were ~16% more likely to survive to the next breeding season than mothers of intact broods, indicating a significant maternal benefit. And cessation of sibling competition allowed surviving chicks to achieve fledging body condition, breeding probability, longevity, and lifetime reproductive success comparable to chicks from intact broods. Thus, sacrificing the subordinate chick ensures adequate-not privileged-growth of the surviving sibling and enhances maternal survival, while apparently providing no survival benefit to fathers. However, experimental confirmation is required to validate these findings and further explore the underlying mechanisms.