REVIEW Mechanisms of antimicrobial defense in avian eggs Liliana D’Alba 1 • Matthew D. Shawkey 1 Received: 13 November 2014 / Revised: 8 April 2015 / Accepted: 10 April 2015 Ó Dt. Ornithologen-Gesellschaft e.V. 2015 Abstract One of the greatest threats to the survival of avian eggs is the risk of infection by microbes; as such, a large number of parental defense mechanisms have evolved in response to the decreased fitness imposed by microbial infection. The existing literature on this topic has focused largely on the mechanisms of microbial invasion through eggshells and the identification of molecules with antimicrobial properties in eggs of commercial species. However, little is still known about antimicrobial mechanisms in wild birds or how they vary with environ- mental pressures. This review concentrates on recent findings that shed new light on the role of parental be- haviors (including incubation and placement of vegetation with antifungal activity in the nest) and the physical properties of eggshells (including nanometer-scale spheres that prevent microbial attachment) that protect eggs from contamination in high-risk environments. In addition to presenting a summary of current information, we identify evident gaps in knowledge and highlight research avenues for the future. Keywords Antimicrobial defense Á Eggshell Á Infection Á Incubation Introduction Embryos from oviparous vertebrates are frequently ex- posed to environmental challenges that may reduce their viability and survival. Predation has been recognized as the major cause of egg mortality (Ricklefs 1969; Martin 1995). In a less traditional sense, predation could also be con- sidered as the consumption of egg components by heterotrophic bacteria (Board and Hornsey 1978). Indeed, the intricate structure of avian eggshells and albumen likely evolved from the ancestral amniotic egg at least partly as a result of intense predation by microbes (Packard and Packard 1980). Numerous studies over the past 30 years have examined the susceptibility of eggs to contamination by bacteria. However, most have focused on domestic species (pri- marily chickens), with the goal of improving egg hygiene and thereby reducing loss due to contamination (Board and Fuller 1994; Board and Tranter 1995; Hincke et al. 2011; Baron et al. 2011). Researchers have recently begun to expand upon the lessons learned in these studies and ap- plying them to natural bird populations. Broadly speaking, this research has shown that microbial infection affects eggs in the wild (e.g., Pinowski et al. 1994; D’Alba et al. 2011), that certain environmental conditions promote egg contamination (e.g., Berrang et al. 1999; Cook et al. 2005a), and that birds have evolved numerous behavioral, chemical and physiological adaptations to combat infection (e.g., Gwinner and Berger 2005; Martı ´n-Vivaldi et al. 2014; D’Alba et al. 2014). The goal of this review is to summarize what is cur- rently known about the factors that influence the risk of infection in avian nests and the defense mechanisms against it. Ultimately, we hope to stimulate more research on this important but understudied topic. Communicated by E. Matthysen. & Liliana D’Alba liliana@uakron.edu 1 Department of Biology and Integrated Bioscience Program, University of Akron, Akron, OH 44325-3908, USA 123 J Ornithol DOI 10.1007/s10336-015-1226-1