Variable Helper Effects, Ecological Conditions, and the Evolution of Cooperative Breeding in the Acorn Woodpecker

Abstract

The ecological conditions leading to delayed dispersal and helping behavior are generally thought to follow one of two contrasting scenarios: that conditions are stable and predictable, resulting in young being ecologically forced to remain as helpers (extrinsic constraints and the habitat saturation hypothesis), or that conditions are highly variable and unpredictable, leading to the need for helpers to raise young, at least when conditions are poor (intrinsic constraints and the hard life hypothesis). We investigated how variability in ecological conditions influences the degree to which helpers augment breeder fitness in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), a species in which the acorn crop, territory quality, and prior breeding experience all vary in ways that have important effects on fitness. We found that the relationship between ecological conditions and the probability that birds would remain as helpers was variable but that helpers generally yielded greater fitness benefits when ecological conditions were favorable, rather than unfavorable, for breeding. These results affirm the importance of extrinsic constraints to delayed dispersal and cooperative breeding in this species, despite its dependence on a highly variable and unpredictable acorn crop. Our findings also confirm that helpers can have very different fitness effects, depending on conditions, but that those effects are not necessarily greater when breeding conditions are unfavorable.

Temporal variability and cooperative breeding: testing the bet-hedging hypothesis in the acorn woodpecker

“…across years. n ¼ 33 years except for the 'presence of helpers' category, for which n ¼ 23 (fecundity variance) and n ¼ 32 (estimated fitness food supply (the acorn crop), which drives highly varying annual reproductive success of the birds [17]. Second, per capita reproductive success declines with increasing group size, as is generally true for social species and as reported earlier for this population [32].…”

“…This approach makes the reasonable assumption that group size is an index of sociality, but ignores spatial variability ( primarily differences among territories) and annual variation (differences among years), both of which can have significant effects on all aspects of group living [17,21].…”

“…We estimated the overall size of the acorn crop, the main ecological variable influencing reproductive success, each autumn on 250 individually marked trees distributed among the five oak species commonly found within the study area [17] In order to determine whether fecundity variance decreases or estimated fitness increases in relatively poor years, we compared the mean acorn crop in years in which these variables increased with sociality versus years when they decreased with sociality using Wilcoxon tests.…”

“…According to this idea, often called the 'benefits-of-philopatry' hypothesis [14,15], cooperative breeding is hypothesized to enhance the direct fitness of individuals by increasing the success of group foraging, territorial defence, predator deterrence or some other life-history character. A particularly likely scenario is that cooperative breeding enhances fitness when there is high variability in territory quality [15] or when successful reproduction by pairs is difficult or impossible [16], referred to as the 'hard life' hypothesis [17].…”

Sex Allocation Patterns across Cooperatively Breeding Birds Do Not Support Predictions of the Repayment Hypothesis

“…Species living in such conditions may lack adaptive cues for sex allocation. In acorn woodpeckers (Melanerpes formicivorus), helpers have a generally positive effect on breeders' reproductive success, but this effect becomes negative when conditions are poor (Koenig et al 2011). A change in environmental conditions may therefore cancel any adaptive benefit of facultatively producing philopatric males.…”