HOST MICROBE INTERACTIONS Plumage Bacterial Assemblages in a Breeding Wild Passerine: Relationships with Ecological Factors and Body Condition Pauli Saag & Vallo Tilgar & Raivo Mänd & Priit Kilgas & Marko Mägi Received: 2 June 2010 / Accepted: 3 December 2010 / Published online: 15 January 2011 # Springer Science+Business Media, LLC 2011 Abstract Microorganisms have been shown to play an important role in shaping the life histories of animals, and it has recently been suggested that feather-degrading bacteria influence the trade-off between parental effort and self- preening behavior in birds. We studied a wild breeding population of great tits (Parus major) to explore habitat-, seasonal-, and sex-related variation in feather-degrading and free-living bacteria inhabiting the birds' yellow ventral feathers and to investigate associations with body condi- tion. The density and species richness of bacterial assemb- lages was studied using flow cytometry and ribosomal intergenic spacer analysis. The density of studied bacteria declined between the nest-building period and the first brood. The number of bacterial phylotypes per bird was higher in coniferous habitat, while bacterial densities were higher in deciduous habitat. Free-living bacterial density was positively correlated with female mass; conversely, there was a negative correlation between attached bacterial density and female mass during the period of peak reproductive effort. Bacterial species richness was sex dependent, with more diverse bacterial assemblages present on males than females. Thus, this study revealed that bacterial assemblages on the feathers of breeding birds are affected both by life history and ecological factors and are related to body condition. Introduction Wild animals harbor a diverse community of bacteria and fungi [reviewed in 1]. Considering the high impact of microorganisms as symbionts or parasites [2, 3], investigat- ing microbe–host interactions may help to explain behavioral and reproductive variation within and between host popula- tions. However, detailed research into the bacterial loads of wild animals and the influence of bacteria on hosts has been limited by the requirement for very complex methods. Recently however, molecular and microbiological techniques have developed rapidly, and avian ecologists have started to work in this field with great enthusiasm [e.g., 4–6]. Recent evidence suggests that plumage bacteria play an important role in shaping the life histories of wild birds [4, 7–9]. Birds make significant efforts to maintain plumage function and to control ectoparasite loads. Preening and other forms of grooming are critical for limiting the abundance of feather lice and other arthropods [10]. Behaviors such as anting, dusting, sunning, and the inclusion of green vegetation in nesting material may also defend against ectoparasites and bacteria [1]. Moreover, laboratory studies indicate that uropygial oil protects plumage either chemically [11] or physically [12] against damage by feather-degrading bacilli [see also 13]. Bird plumage can host various assemblages of bacteria and fungi, several of which are capable of degrading feather keratin [11, 14–16]. Several fitness consequences could result from bacterial damage to wild bird plumage. Plumage deterioration may result in decreased thermal insulation [17] and aerodynamic efficiency [18]. In the long-term, these negative effects might reduce parental survival and P. Saag (*) : V. Tilgar : R. Mänd : P. Kilgas : M. Mägi Department of Zoology, Institute of Ecology and Earth Sciences, University of Tartu, 46 Vanemuise Str., 51014 Tartu, Estonia e-mail: pauli@ut.ee Microb Ecol (2011) 61:740–749 DOI 10.1007/s00248-010-9789-0