Data and code from: Fitness consequences of parasitism in a changing world: A case study with bird blow flies and sagebrush songbirds

Host-parasite interactions increasingly are influenced by human-induced rapid environmental change (HIREC), and the fitness effects of parasitism may be compounded or exacerbated by host traits and/or exposure to additional extrinsic stressors associated with HIREC. Potential interactions between parasitism and different stressors associated with environmental change, however, remain poorly understood for most systems. We examined how parasitism by bird blow flies (Trypocalliphora braueri), ambient weather conditions, and habitat disturbance jointly affected offspring traits and juvenile mortality for two declining species of sagebrush songbirds (Brewer’s Sparrow, Spizella breweri; and Sage Thrasher, Oreoscoptes montanus) in Wyoming, USA. We evaluated two alternative hypotheses: that parasitism could act (i) in an independent and additive manner with temperature and habitat alteration (Multiple Stressors Hypothesis), or (ii) synergistically to exacerbate the effects of temperature and habitat alteration (Parasitism-HIREC Interaction Hypothesis) on offspring traits and juvenile mortality. We assessed morphometric traits of nestlings and survival of fledglings in relation to parasite loads, temperature, and habitat disturbance associated with natural gas development using liner mixed models and Cox proportional-hazard models. Higher parasite loads and colder temperatures were associated with different effects for nestlings of each host species, reducing tarsus and wing chord length for Brewer’s Sparrow and increasing mass for Sage Thrasher. Despite differences in the effect of parasitism on nestling traits, post-fledging mortality risk for both species increased with higher parasite loads. The effects of parasitism and temperature mainly were additive, with limited evidence that weather exacerbated the effects of parasitism. Habitat disturbance had a weak positive effect on nestling tarsus length and post-fledging survival probability for Brewer’s Sparrow. Although parasitism rarely results in direct mortality of hosts, parasites can nonetheless exert considerable fitness consequences, especially when combined with extrinsic stressors associated with human-induced environmental changes.

宿主-寄生虫互作日益受到人为快速环境变化（Human-induced Rapid Environmental Change, HIREC）的影响，宿主性状与/或暴露于HIREC相关的其他外源胁迫因子，可能会加重或放大寄生虫寄生的适合度效应。然而，针对多数生态系统，寄生作用与环境变化相关的各类胁迫因子之间的潜在互作机制仍未得到充分解析。我们以美国怀俄明州两种处于衰退状态的灌丛鸣禽——布氏鹀（*Spizella breweri*）与薮嘲鸫（*Oreoscoptes montanus*）为研究对象，探究了鸟丽蝇（*Trypocalliphora braueri*）寄生、环境气象条件以及栖息地干扰三者共同作用对雏鸟性状与幼鸟死亡率的影响。我们评估了两种对立假说：其一，寄生作用可与温度、栖息地改变以独立加性的方式发挥效应（多重胁迫假说）；其二，寄生作用可与温度、栖息地改变产生协同效应，进而加剧二者对雏鸟性状及幼鸟死亡率的负面影响（寄生-HIREC互作假说）。我们采用线性混合模型与Cox比例风险模型，结合寄生虫负荷、温度以及与天然气开发相关的栖息地干扰因子，分析了雏鸟的形态学性状与离巢幼鸟的存活情况。更高的寄生虫负荷与更低的温度对两种宿主的雏鸟产生了差异化影响：布氏鹀的跗跖长度与翅弦长度显著降低，而薮嘲鸫的体重则有所增加。尽管寄生作用对雏鸟性状的影响存在物种间差异，但两种鸣禽的离巢后死亡风险均随寄生虫负荷升高而上升。寄生作用与温度的效应主要以加性方式呈现，几乎无证据表明气象条件会加剧寄生作用的负面影响。栖息地干扰对布氏鹀的雏鸟跗跖长度以及离巢后存活概率存在微弱的正向影响。尽管寄生作用极少直接导致宿主死亡，但寄生虫仍可对宿主的适合度产生显著影响，尤其是当与人类活动引发的环境变化相关的外源胁迫因子共同作用时。