Iridescent structural colors produce some of the most vibrant colors in nature, particularly in birds – such as throats of many hummingbirds. These colors are produced by interactions between light and organic structures, such that optical effects from the nanoscale arrangement of melanin-containing organelles called melanosomes within bird feather barbules produces color variation. Uniquely to birds, these melanosomes can take four distinct forms: (i) rods (rod-shaped and solid); (ii) hollow rods (rod-shaped and hollow); (iii) platelets (flat and solid); and (iv) hollow platelets (flat and hollow). While their importance for production of colour variation is well explored, how these unusual melanosomes form during feather development in nearly unknown. In the MOlecular and DEvelopmental bases of Iridescent Feathers project (MoDeIF project) I propose a developmental genetics approach to identify developmental and molecular pathways by which melanosomes achieve hollowness and flatness. To explore hollowness, I will test two hypotheses: the phaeomelanin-core hypothesis and the air-filled-core hypothesis. To explore flatness, I will test the hypothesis that the pre-melanosome structural organizational gene Pmel enables the development of flat melanosomes by providing scaffolding for pigment deposition. Comparative transcriptomics and concurrent elucidation of the physical processes via electron microscopy will greatly improve our understanding of the mechanisms of development of these unusual melanosomes. Finally, I will focus on whether melanosome shape and nanostructuring are developmentally constrained for other colors once a species evolves iridescent coloration. Understanding the molecular basis of iridescent feather development is vital for comprehending its evolution. Furthermore, insights into melanosomes' optical packing could inspire innovative synthetic color production techniques.