Host-microbiome interactions during embryonal and early phase of life is critical point in microbiome formation and assemblage in neonates. In birds, transmission of microbes from the outer environment into the egg interior was found to shape embryo viability and hatchlings phenotype. Microbes' transmission may be modulated by egg white antimicrobial proteins (AMPs) whose concentration and antimicrobial action are temperature-modulated. As partial incubation and clutch covering with nest-lining

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... feathers during pre-incubation period may both significantly alter temperature conditions acting on eggs, we experimentally investigated effects of these behavioural mechanisms on the concentrations of primary egg white AMPs - lysozyme and avidin using Mallard (Anas platyrhychos) eggs. Moreover, we studied in vivo if concentrations of egg white AMPs reduced probability and intensity of bacterial trans-shell infection and hatchlings phenotype. We found significantly higher egg white lysozyme concentration, while avidin concentration tended to be higher in partially incubated eggs. Clutch covering with nest-lining feathers had no effect on egg white AMPs concentrations. Neither probability nor intensity of bacterial trans-shell infection was associated with concentrations of egg white AMPs. Finally, increased egg white lysozyme was associated with decreased scaled body mass index of hatchlings. These outcomes demonstrate that incubation prior to clutch completion in precocial birds may modulate concentrations of particular egg white AMPs, yet without any effect on transmission of bacteria into the egg in vivo. Furthermore, increased egg white lysozyme may compromise body condition of hatchlings supporting growth-regulating role of lysozyme during embryogenesis in precocial birds.