Nesting Milan is a two phase implantation of biodiversity inside Milan. The first one is made through the railway system, considered as an experiment to know better how to deal with animals and nature, how to include them more deeply inside urban development. It is the starting point for the second step, spread within urban development. The nest infrastructure over the railways is a clear landmark in the urban environment, and tends to become an urban infrastructure : depending on the situation, the structure is linked to urban functions. It also becomes a connective tissue between the historical center of Milan and the new part in development, that the railway tended to separate. The light structure becomes a permeable and interactive membrane as well as a continuous green bubble. Building for animals inside cities is complex and experimental. The only indication that we can rely on, to design from a non- anthropocentric point of view, are scientific facts and observations.The project becomes a scientific approach itself, as we will rely on the results to improve our structure and merge it with urban development.