Climate change is one of the largest threats to biodiversity of our times. Determining the rate of climate change that populations can cope with is, therefore, information that is urgently needed. About half of the 10,000 bird species in the world are members of the order Passeriformes. A changing environment can influence the physiology and reproductive success of passerine bird species, yet nothing is known about the molecular mechanisms underlying the onset of egg-laying in any passerine species. This realization is quite remarkable when one considers that the "decision" of whether or not a female bird begins to lay eggs is the moment that defines the whole breeding season for that individual and her mate(s), with profound current and future fitness consequences. This project will uncover specific mechanisms regulating the initiation of breeding in female songbirds. One of the most intensely studied species with respect to ecology and reproductive timing, the European great tit (Parus major) will be used. This is an international collaboration that will combine the expertise of the PI, George Bentley (UC Berkeley, USA), whose lab studies mechanisms underlying physiology of reproduction, Marcel Visser (The Netherlands Institute of Ecology) who is at the forefront of ecological research and the effects of climate change, and Michaela Hau (Max-Planck Institute for Ornithology, Germany) who is an expert in development of theoretical frameworks that integrate endocrine and evolutionary theory. The broader impacts include training of minority graduate students in laboratories in different countries, encouragement of undergraduate participation in research and scientific meetings, and involvement of undergraduates in the publication process. This project fosters collaboration between premier ecology of climate-change and evolutionary biology laboratories in Europe and physiology of reproduction in the USA. Furthermore, the intellectual merit of the results obtained includes advancement our understanding of the mechanisms underlying how wild female birds time their "decision to lay eggs" something which we know almost nothing about, despite almost a century of research on similar mechanisms in male birds.