Partial incubation during egg laying reduces eggshell microbial loads in a temperate‐breeding passerine

Incubation prior to clutch completion may be adaptive if it maintains egg viability by inhibiting eggshell microbial growth, thus reducing the likelihood that the embryo becomes infected. To test this hypothesis, we examined the effect of partial incubation during egg laying on eggshell microbial loads in eastern bluebirds Sialia sialis breeding at a temperate‐zone site. We sampled eggshell microbes prior to and following four days of exposure to either partial incubation during the laying period or ambient environmental conditions without incubation (experimental eggs). Microbial colony counts declined significantly for eggs left in the nest during the laying period but did not vary significantly for eggs exposed to ambient conditions. Initial microbial loads were more similar to those previously reported from tropical than temperate environments, and microbes from potentially pathogenic groups were detected on 88% of first‐laid eggs on the day of laying. Egg viability was maintained when eggs were held indoors for four days without incubation but declined sharply thereafter. Our results suggest that partial incubation during egg laying may enhance egg viability in eastern bluebirds by reducing eggshell microbial loads; these effects appear stronger than those usually reported from the temperate zone.

若能通过抑制卵壳微生物增殖以维持卵的活力，进而降低胚胎感染风险，那么产卵完成前的孵化行为或许具有适应性意义。为验证该假说，我们针对温带繁殖地的东蓝鸲（Sialia sialis），研究了产卵期部分孵化行为对其卵壳微生物负载量的影响。我们分别在产卵期进行为期4天的部分孵化，或置于无孵化的常温环境（即实验组卵）的前后，对卵壳微生物进行了采样。产卵期留在巢内的卵的微生物菌落数显著下降，而置于常温环境的卵的菌落数无明显变化。初始微生物负载量与此前热带环境的报道结果更为接近，而非温带环境；且在产卵当日产下的首枚卵中，有88%检测到了潜在致病类群的微生物。若将卵置于室内无孵化条件下保存4天，其活力可维持，但此后活力会急剧下降。本研究结果表明，东蓝鸲产卵期的部分孵化行为可通过降低卵壳微生物负载量，提升卵的活力；且该效应似乎强于温带环境下已有报道的相关结果。