The presence of nestlings and other nest dwelling organisms in cavity nests alters the composition of gasses inside the cavity. Differential concentrations of gasses could be used by some parasites as a cue to localize their hosts. Here, we explored temporal variation in the concentration and isotopic signature of carbon dioxide (CO2) and methane (CH4) inside nest boxes of blue tits (Cyanistes caeruleus) during the nestling period (on days 3, 8, 13, 20, and 21 post-hatching) as well as several variables potentially affecting such variation. Finally, we checked whether differences in gas concentrations affect the abundance of different types of parasites affecting nestlings. Gas concentration and isotopic signature were significantly different between nest boxes and the forest during the nestling period. The CO2 concentration was higher inside nests than in the forest air, whereas CH4 concentration was lower. We expected to observe a positive correlation between the abundance of parasites actively seeking nests (i.e., blackflies, biting midges, and blowflies) and differences in gas concentration for those species that use these differences as a cue for host location. We observed that biting midge abundance was positively related to differences in CO2 between nest and forest air at day 20 of nestling age, indicating that this species can use these differences to locate hosts. We also found a positive relationship between blackfly abundance and differences in CH4 concentration. However, we hypothesize that the concentration of this gas inside nests may be related with bacterial activity; therefore, this relationship may be due to an effect of bacteria on blackflies and not to the effect of CH4 as an attraction cue for blackflies.

中文翻译：

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巢气体作为叮咬飞虫和其他腔内筑巢鸟类的体外寄生虫的潜在吸引力线索

巢穴中雏鸟和其他巢居生物的存在会改变巢穴内气体的成分。一些寄生虫可以使用不同浓度的气体作为定位宿主的线索。在这里，我们探索了在雏鸟期（第 3、8、13、20 和 21 天）蓝山雀（Cyanistes caeruleus）巢箱内二氧化碳 (CO2) 和甲烷 (CH4) 的浓度和同位素特征的时间变化孵化后）以及可能影响这种变化的几个变量。最后，我们检查了气体浓度的差异是否会影响影响雏鸟的不同类型寄生虫的丰度。在雏鸟时期，巢箱和森林之间的气体浓度和同位素特征显着不同。巢内的 CO2 浓度高于森林空气中的 CO2 浓度，而 CH4 浓度较低。我们期望观察到积极寻找巢穴的寄生虫（即黑蝇、蠓和苍蝇）的丰度与那些使用这些差异作为宿主位置线索的物种的气体浓度差异之间存在正相关。我们观察到，咬蠓丰度与雏鸟 20 天时巢穴和森林空气之间的 CO2 差异呈正相关，表明该物种可以利用这些差异来定位宿主。我们还发现黑蝇丰度与 CH4 浓度差异之间存在正相关关系。然而，我们假设巢内这种气体的浓度可能与细菌活动有关。所以，