Recent studies have revealed surprising role for bacteria in the ecology and evolution of animals. Although bacteria are of vital importance for animal fitness, we have however just made the first steps in discovering their complexity, diversity and function. In the “SexCoMonArc2” program, we will study the interactions between bacteria and bird behavior at different stages of reproduction. We will focus on the ultimate and proximate mechanisms of several behaviors that are potential strategies against bacteria. We will study an Alaskan population of black-legged kittiwakes nesting on an abandoned tower that enables us to easily capture and monitor breeders and chicks. Based on the knowledge and skills acquired from our previous IPEV projects, we will also develop new approaches to study offspring sex-ratio adjustments.