Brood Reduction: Narrow Sense, Broad Sense

Abstract

In population biology, is a familiar and quintessentially ornithological concept, among the most visible of David Lack's many compelling insights. Its meaning seems straightforward enough: the two words surely connote numerically decreasing a sibship during the post-hatching period, while the offspring continue to rely on parental care. Unfortunately, the usage of has become more complicated and ambiguous in recent years. In this essay I attempt to make three points: (1) I suggest that the term may be most useful if restricted to its original (and explicitly narrower) meaning; (2) I propose some operational criteria by which such can be recognized in the field; and (3) I call the ornithological community's attention to parallel phenomena found in other taxa. In this way, I hope to ease some growing confusion, while also showing that even a restricted definition of is much more global and intriguing than is generally appreciated. Although I advocate narrower use of the term reduction, I hope to promote a broader view of the phenomenon. Though linked inexorably to David Lack's name, the term was first coined by the then-undergraduate Bob Ricklefs, who was testing Lack's unnamed idea that parents create expendable, marginal offspring as a contingency against variable food conditions (Ricklefs 1965). In a nutshell, the hypothesis hinged on the assumption that fatal levels of sibling competition would trim size to an appropriate level if food turns out to be low. It was assumed that this destructive process would not kick in if food proves plentiful (but see Pijanowski 1992), so the overall effect would be to maximize size in the face of stochastic conditions (i.e., it serves in resource-tracking: Temme and Charnov 1987). The problem of ambiguity arose later, plausibly from two sources. First, as field observations accumulated showing that families sometimes do lose one or two young offspring for a variety of reasons (that include, but are not limited to, fatal sibling competition), the natural tendency to couple observations with an established phenomenon may have led many workers to the same label. This pigeon-holing inadvertantly stretched its meaning. Second, Lack's hypothesis stood virtually alone for decades as the only really attractive functional explanation for the early mortalities (others existed, but were strangely ignored). Conceivably, the temptation to couple one's data with a prominent concept distracted many workers from the implicit assumptions underlying Lack's argument. Whatever the reason behind the emerging ambiguity, the term brood reduction now means different things to different workers. To some it is merely a broad descriptive phenomenon; to others, a particular mechanism. In any case, Lack's hypothesis clearly needed a handle and Ricklefs certainly provided a successful one.