Bacterial communities within avian nests are considered an important determinant of egg viability, potentially selecting for traits that confer embryos with protection against trans-shell infection. A high bacterial density on the eggshell increases hatching failure, whether this effect could be due to changes in bacterial community or just a general increase in bacterial density. We explored this idea using intra- and interspecific comparisons of the relationship between hatching success and eggshell bacteria characterized by culture and molecular techniques (fingerprinting and high-throughput sequencing). We collected information for 152 nests belonging to 17 bird species. Hatching failures occurred more frequently in nests with higher density of aerobic mesophilic bacteria on their eggshells. Bacterial community was also related to hatching success, but only when minority bacterial operational taxonomic units were considered. These findings support the hypothesis that bacterial density is a selective agent of embryo viability, and hence a proxy of hatching failure only within species. Although different avian species hold different bacterial densities or assemblages on their eggs, the association between bacteria and hatching success was similar for different species. This result suggests that interspecific differences in antibacterial defenses are responsible for keeping the hatching success at similar levels in different species.

Center for Microbiome Innovation University of California San Diego La Jolla CA 92093 USA

Departamento de Ecología Funcional y Evolutiva Estación Experimental de Zonas Áridas C S 1 C E 04120 Almería Spain

Departamento de Microbiología Universidad de Granada Calle Fuentenueva s n E 18071 Granada Spain

Departamento de Zoología Universidad de Granada Campus de Fuentenueva s n E 18071 Granada Spain

Department of Botany University of British Columbia V6T 1Z4 Vancouver Canada

Department of Computer Science and Engineering University of California San Diego La Jolla CA 92093 USA

Department of Integrative Ecology Estación Biológica de Doñana C S 1 C Avda Américo Vespucio s n E 41092 Seville Spain

Department of Pediatrics University of California San Diego La Jolla CA 92093 USA

European Molecular Biology Laboratory European Bioinformatics Institute Hinxton Cambridge CB10 1SD UK

Faculty of Science University of South Bohemia Branisovska 1760 370 05 Ceske Budejovice Czech Republic

Groningen Institute for Evolutionary Life Sciences University of Groningen 9700 CC Groningen The Netherlands

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Temperature-induced changes in egg white antimicrobial concentrations during pre-incubation do not influence bacterial trans-shell penetration but do affect hatchling phenotype in Mallards