Koenig Walter D, Walters Eric L, Haydock Joseph
Behav Ecol Sociobiol. 2009 Sep;63(11):1659-1665. doi: 10.1007/s00265-009-0773-y. Epub 2009 May 20.
In cooperatively breeding acorn woodpeckers (Melanerpes formicivorus), helper males have a large positive effect on fledging success in good acorn crop years but only a small positive effect in poor acorn crop years, while helper females exhibit the opposite pattern. Based on these findings, we tested the "concealed helper effects" hypothesis, which proposes that laying females reduce investment in eggs (with respect to their size, number, or quality) in a way that confounds helper effects and results in an absence of a relationship between helpers and breeding success. Results generally failed to support the hypothesis. Mean egg size was positively related to temperatures during the 10 days prior to egg-laying and negatively related to the food supply as indexed by the prior fall's acorn crop, but there were no significant differences vis-à-vis helpers except for interactions with the acorn crop that only partly corresponded to those predicted. With respect to clutch size, females laid larger clutches when assisted by female helpers, opposite the pattern predicted. Although our results suggest that egg size is adjusted to particular ecological circumstances, we conclude that neither egg nor clutch size is adjusted in a way that confounds the apparent effects of helpers, as proposed by the concealed helper effects hypothesis.
在合作繁殖的橡树啄木鸟（Melanerpes formicivorus）中，帮手雄鸟在橡子丰收年份对雏鸟成活率有很大的积极影响，但在橡子歉收年份只有很小的积极影响，而帮手雌鸟则表现出相反的模式。基于这些发现，我们检验了“隐藏的帮手效应”假说，该假说认为产卵雌鸟会以一种混淆帮手效应的方式减少对卵的投入（在卵的大小、数量或质量方面），从而导致帮手与繁殖成功率之间不存在关联。结果总体上未能支持这一假说。平均卵大小与产卵前10天的温度呈正相关，与前一年秋季橡子产量所反映的食物供应呈负相关，但与帮手的情况相比，除了与橡子产量的相互作用仅部分符合预测外，没有显著差异。关于窝卵数，在雌鸟帮手的协助下，雌鸟会产更大的窝卵数，这与预测的模式相反。虽然我们的结果表明卵大小会根据特定的生态环境进行调整，但我们得出结论，卵大小和窝卵数都不会以隐藏的帮手效应假说所提出的那种混淆帮手明显效应的方式进行调整。
