Intraspecific Variation and Interspecific Differences in the Bacterial and Fungal Assemblages of Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major) Nests

Although interest in the relationship between birds and microorganisms is increasing, few studies have compared nest microbial assemblages in wild passerines to determine variation within and between species. Culturing microorganisms from blue tit (Cyanistes caeruleus) and great tit (Parus major) nests from the same study site demonstrated diverse microbial communities with 32 bacterial and 13 fungal species being isolated. Dominant bacteria were Pseudomonas fluorescens, Pseudomonas putida, and Staphylococcus hyicus. Also common in the nests were the keratinolytic bacteria Pseudomonas stutzeri and Bacillus subtilis. Dominant fungi were Cladosporium herbarum and Epicoccum purpurascens. Aspergillus flavous, Microsporum gallinae, and Candida albicans (causative agents of avian aspergillosis, favus, and candidiasis, respectively) were present in 30%, 25%, and 10% of nests, respectively. Although there were no differences in nest mass or materials, bacterial (but not fungal) loads were significantly higher in blue tit nests. Microbial species also differed interspecifically. As regards potential pathogens, the prevalence of Enterobacter cloacae was higher in blue tit nests, while Pseudomonas aeruginosa—present in 30% of blue tit nests—was absent from great tit nests. The allergenic fungus Cladosporium cladosporioides was both more prevalent and abundant in great tit nests. Using discriminant function analysis (DFA), nests were classified to avian species with 100% accuracy using the complete microbial community. Partial DFA models were created using a reduced number of variables and compared using Akaike’s information criterion on the basis of model fit and parsimony. The best models classified unknown nests with 72.5–95% accuracy using a small subset of microbes (n = 1–8), which always included Pseudomonas agarici. This suggests that despite substantial intraspecific variation in nest microflora, there are significant interspecific differences—both in terms of individual microbes and the overall microbial community—even when host species are closely related, ecologically similar, sympatric, and construct very similar nests.