Data from: Provisioning tactics of great tits (Parus major) in response to long-term brood size manipulations differ across years

Parents provisioning their offspring can adopt different tactics to meet increases in offspring demand. In this study, we experimentally manipulated brood demand in free living great tits (Parus major) via brood size manipulations and compared the tactics adopted by parents in 2 successive years (2010 and 2011) with very different ecological conditions. In 2011, temperatures were warmer, there were fewer days with precipitation, and caterpillars (the preferred prey of great tits) made up a significantly larger proportion of the diet. In this “good” year, parents responded to experimental increases in brood demand by decreasing mean inter-visit intervals (IVIs) and reducing prey selectivity, which produced equal average long-term delivery of food to nestlings across the brood size treatments. In 2010, there was no evidence for effects of brood size manipulations on mean IVIs or prey selectivity. Consequently, nestlings from enlarged broods experienced significantly lower long-term average delivery rates compared with nestlings from reduced broods. In this “bad” year, parents also exhibited changes in the variance in inter-visit intervals (IVIs) as a function of treatment that were consistent with variance-sensitive foraging theory: variance in IVIs tended to be lowest for reduced broods and highest for enlarged broods. Importantly, this pattern differed significantly from that observed in the “good” year. We therefore found some support for variance-sensitive provisioning in the year with more challenging ecological conditions. Taken together, our results show that variation in brood demand can result in markedly different parental foraging tactics depending on ecological conditions.为子代提供抚育供给的亲代可采取不同策略，以应对子代需求的提升。本研究通过育雏规模操控实验，对野生大山雀（Parus major）的育雏需求进行人为调控，并对比了其亲代在生态条件差异显著的两个连续年份（2010年与2011年）所采用的抚育策略。2011年气温偏高，降水日数偏少，毛虫（大山雀的偏好猎物）在其食谱中的占比显著更高。在这一“优质年份”中，亲代会通过缩短平均访巢间隔（inter-visit intervals, IVIs）、降低猎物选择性，来响应实验诱导的育雏需求提升，最终使不同育雏规模处理组的雏鸟获得的长期平均食物供给量保持一致。2010年则未发现育雏规模操控对平均访巢间隔或猎物选择性存在影响。因此，育雏规模扩大组的雏鸟，其长期平均食物供给率显著低于育雏规模缩减组的雏鸟。在这一“恶劣年份”中，亲代的访巢间隔方差随实验处理的变化模式，与方差敏感觅食理论（variance-sensitive foraging theory）相符：育雏规模缩减组的访巢间隔方差最低，而育雏规模扩大组的访巢间隔方差最高。值得注意的是，该模式与“优质年份”中观测到的结果存在显著差异。因此，在生态条件更具挑战性的年份中，本研究为亲代采取方差敏感抚育策略提供了一定支持证据。综上，本研究结果表明，育雏需求的差异可因生态条件的不同，导致亲代觅食策略出现显著分化。