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-year individual-based dataset on blue-footed booby
(Sula nebouxii) 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. Moreover, 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.