Data from: Reproductive effort and future parental competitive ability: a nest box removal experiment

The life history trade-off between current- and future reproduction is a theoretically well-established concept. However, empirical evidence for the occurrence of a fitness cost of reproduction is mixed. Evidence indicates that parents only pay a cost of reproduction when local competition is high. In line with this, recent experimental work on a small passerine bird, the Great tit (Parus major) showed that reproductive effort negatively affected the competitive ability of parents, estimated through competition for high quality breeding sites in spring. In the current study we further investigate the negative causal relationship between reproductive effort and future parental competitive ability, with the aim to quantify the consequences for parental fitness, when breeding sites are scarce. To this end, we 1) manipulated the family size of Great tit parents and 2) induced severe competition for nest boxes among the parents just before the following breeding season by means of a large scale nest box removal experiment. Parents increased their feeding effort in response to our family size manipulation and we successfully induced competition among the parents the following spring. Against our expectation, we found no effect of last season’s family size on the ability of parents to secure a scarce nest box for breeding. In previous years, if detected, the survival cost of reproduction was always paid after midwinter. In this year parents did pay a survival cost of reproduction before midwinter and thus before the onset of the experiment in early spring. Winter food availability during our study year was exceptionally low and thus competition in early winter may have been extraordinarily high. We hypothesize that differences in parental competitive ability due to their previous reproductive effort might have played a role, but before the onset of our experiment and resulted in the payment of the survival cost of reproduction.

当前繁殖与未来繁殖之间的生活史权衡（life history trade-off）是一个理论上已被充分证实的概念。然而，关于繁殖适合度成本（fitness cost of reproduction）存在性的实证证据却并不一致。有证据表明，亲本仅在局部竞争激烈时才会付出繁殖成本。与此一致，近期针对小型雀形目鸟类大山雀（Parus major）开展的实验研究表明，繁殖投入会对亲本的竞争能力产生负面影响，该竞争能力通过春季对优质繁殖位点的竞争情况进行评估。本研究进一步探究繁殖投入与亲本未来竞争能力之间的负向因果关系，旨在量化繁殖位点稀缺时对亲本适合度造成的影响。为此，我们开展了两项实验处理：1）调控大山雀亲本的雏鸟抚育规模；2）在下一个繁殖季来临前，通过大规模移除巢箱的实验，引发亲本间对巢箱的激烈竞争。针对雏鸟抚育规模的调控使亲本提高了取食投入，且我们成功在下一个春季引发了亲本间的竞争。但与我们的预期相悖的是，上一繁殖季的雏鸟抚育规模并未对亲本获取稀缺繁殖巢箱的能力产生显著影响。在过往研究中，若能检测到繁殖的存活成本，该成本通常会在仲冬之后显现。但在本研究年份中，亲本的繁殖存活成本在仲冬前便已产生，即早于本实验春季开展的时间节点。本研究年份的冬季食物可获得性异乎寻常地低，因此初冬时节的竞争程度可能远超常规水平。我们据此提出假说：亲本因前期繁殖投入产生的竞争能力差异可能发挥了作用，但该作用发生在本实验启动之前，并最终导致了繁殖存活成本的产生。