Data from: Cohabitation with farm animals rather than breeding effort increases the infection with feather-associated bacteria in the barn swallow Hirundo rustica

Feather-associated bacteria are widespread inhabitants of avian plumage. However, the determinants of the between-individual variation in plumage bacterial loads are less well understood. Infection intensities can be determined by ecological factors, such as breeding habitat, and can be actively regulated by hosts via preening. Preening, yet, is a resource intensive activity, and thus might be traded-off against reproductive investment in breeding birds. Here, we studied barn swallows Hirundo rustica to assess the bacterial cost of reproduction in relation to nesting site micro-habitats. Barn swallows prefer to breed in the company of large-sized farm animals, although the presence of mammalian livestock in barns assures a warm and humid micro-climate that favours bacterial proliferation. Thus, we experimentally manipulated brood sizes of birds breeding in barns with, or without, farm animals and measured total cultivable bacteria (TCB) and feather-degrading bacteria (FDB) from the plumage. We found that the abundance of feather-associated bacteria (i.e. both TCB and FDB) in females, but not males, breeding in barns with livestock were significantly higher than in conspecifics breeding in empty barns. Plumage bacterial loads, however, were not affected by brood size manipulations in either sex. In addition, we report a negative relationship between both TCB and FDB and hatching date in females, and several sex and seasonal differences in plumage bacterial abundances. Our study is the first to show that breeding micro-habitat (i.e. livestock co-tenancy) has consequences for the abundance of feather-associated bacteria.

与羽毛相关的细菌（feather-associated bacteria）广泛定植于鸟类羽毛体表。然而，目前学界对鸟类羽毛细菌载量个体间差异的决定机制仍缺乏深入认知。细菌定植强度可受繁殖生境等生态因素调控，宿主亦可通过理羽行为主动调节该定植水平。但理羽属于高资源消耗行为，因此繁殖期鸟类可能会在理羽与繁殖投资之间产生生活史权衡。本研究以家燕（Hirundo rustica）为研究对象，旨在探究繁殖巢址微生境与繁殖活动的细菌负荷代价之间的关联。家燕偏好于在大型家畜周边筑巢繁殖，尽管谷仓内家畜的存在会营造出温暖湿润的微生境，该环境可促进细菌增殖。为此，我们对栖息于有家畜与无家畜谷仓中的家燕开展窝雏数实验操控，并采集其羽毛样本，测定可培养总细菌（total cultivable bacteria, TCB）与羽毛降解细菌（feather-degrading bacteria, FDB）的丰度。研究结果显示，相较于在空谷仓中繁殖的同种个体，栖息有家畜的谷仓中繁殖的雌性家燕，其羽毛相关细菌（即TCB与FDB）丰度显著更高，而雄性家燕未表现出该差异。不过，无论雌雄个体，窝雏数操控均未对其羽毛细菌载量产生显著影响。此外，本研究还发现雌性家燕的TCB与FDB丰度均与孵化日期呈负相关关系，同时羽毛细菌丰度存在多项性别与季节差异。本研究首次证实，繁殖微生境（即与家畜共居）会对羽毛相关细菌的丰度产生显著影响。