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.

亲代抚育子代时，可采取不同策略以应对子代需求的提升。本研究通过调整育雏规模，对野生大山雀（Parus major）的育雏需求进行了实验操控，并比较了连续两年（2010年与2011年，两年生态环境差异显著）亲代所采取的育雏策略。2011年的气温更高，降水日数更少，且毛虫（大山雀的首选猎物）在亲代的食物组成中占比显著更高。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）、降低猎物选择偏好，来应对实验性提升的育雏需求，使得不同育雏规模操控组的雏鸟获得的长期平均食物投喂量保持一致。2010年则未发现育雏规模操控对平均访巢间隔或猎物选择偏好存在影响。因此，育雏规模扩大组的雏鸟，其长期平均食物投喂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操控处理发生的变化，符合方差敏感觅食理论（variance-sensitive foraging theory）：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”中观测到的模式存在显著差异。因此，在生态环境更为严苛的年份中，我们的研究结果为方差敏感抚育策略提供了一定支持。综上，本研究结果表明，育雏需求的差异可导致亲代根据不同生态环境采取截然不同的觅食抚育策略。