Microbial abundance on the eggs of a passerine bird and related fitness consequences between urban and rural habitats

Abstract

Urban environments present novel and challenging habitats to wildlife. In addition to well-known difference in abiotic factors between rural and urban environments, the biotic environment, including microbial fauna, may also differ significantly. In this study, we aimed to compare the change in microbial abundance on eggshells during incubation between urban and rural populations of a passerine bird, the Eurasian Magpie (Pica pica), and examine the consequences of any differences in microbial abundances in terms of hatching success and nestling survival. Using real-time PCR, we quantified the abundances of total bacteria, Escherichia coli/Shigella spp., surfactin-producing Bacillus spp. and Candida albicans on the eggshells of magpies. We found that urban magpie eggs harboured greater abundances of E. coli/Shigella spp. and C. albicans before incubation than rural magpie eggs. During incubation, there was an increase in the total bacterial load, but a decrease in C. albicans on urban eggs relative to rural eggs. Rural eggs showed a greater increase in E. coli/Shigella spp. relative to their urban counterpart. Hatching success of the brood was generally lower in urban than rural population. Nestling survival was differentially related with the eggshell microbial abundance between urban and rural populations, which was speculated to be the result of the difference in the strength of the interaction among the microbes. This is the first demonstration that avian clutches in urban and rural populations differ in eggshell microbial abundance, which can be further related to the difference in hatching success and nestling survival in these two types of environments. We suggest that future studies on the eggshell microbes should investigate the interaction among the microbes, because the incubation and/or environmental factors such as urbanization or climate condition can influence the dynamic interactions among the microbes on the eggshells which can further determine the breeding success of the parents.

Fig 1. Changes in microbial abundance.

Comparisons of changes in the microbial abundance on the naturally incubated magpie eggs between rural (n = 14 nests) and urban (n = 17 nests) populations; (a) total bacteria, (b) Escherichia coli/Shigella spp., (c) surfactin-producing Bacillus spp., and (d) Candida albicans. Paired responses of each nest (connected circles) are given along with the average aligned ranks (circles with error bars). Open circles (connected with solid lines) and closed circles (connected with dashed lines) denote the responses of the rural and urban populations respectively. Units of Y axis values for the microbial abundance are log₁₀ (copy numbers of microbes) per cm² of the eggshells for (a), (b) and (c), and log₁₀ (copy numbers+1) per cm² of the eggshells for (d). Error bars for the aligned ranks denote standard errors. Significance level of the comparison of aligned ranks is given as “*” for 0.01<P<0.05 and “**” for 0.001<P<0.01.

Fig 2. Microbial abundance, hatching success and nestling survival.

Relationship between microbial abundance and hatching success (a) and nestling survival (b)–(d). Open circles (fitted values marked with dashed lines) and closed circles (fitted values marked with solid lines) denote the responses of the rural and urban areas respectively. For microbial abundance, principal components (PCs) extracted from day 3 abundance, day 18 abundance, or day 3 and Δ abundance were used (see Statistical analyses).