SIBLING RIVALRY IN A VARIABLE ENVIRONMENT

Unpredictable variation in food availability is widely thought to be a key factor in the evolution of avian brood reduction. We develop here two models to examine how interannual variability in parental provisioning, M (a function of the level of food in the environment), and the ability to assess M affect the brood reduction decisions of a senior sib in a brood of two chicks. Obligate execution is always favored by natural selection if M never exceeds mCRIT, the minimum level of M that, from the senior sib's perspective, is required to sustain both chicks. Obligate execution can be favored in a variable environment even if M exceeds mCRIT in most years because of the asymmetry in costs and benefits of levels of M below and above mCRIT. Execution will never be favored if M always exceeds mCRIT. If M varies around mCRIT, facultative execution will be favored when M can be assessed accurately, when the cost of assessment is low, and when the cost of an error is small. These costs arise from the possibility, due to sampling effects, that the senior sib arrives at an incorrect decision, killing its sib in good years, or more importantly, allowing it to live in a bad year. A senior sib must allow its junior sib to remain alive in order to derive information about the state of M (e.g., from the pattern of parental provisioning), but has the disadvantage that the senior sib must share food with the possible victim, which yields no payoff if the eventual verdict is execution. The results of a dynamic programming model suggest that early execution is favored, as the senior sib has the most to gain in future provisioning, and uncertainty about M is greatest. Increasing total provisioning through extension of the nestling period favors clemency. © 1992.