Hummingbird iridescence: an unsuspected structural diversity influences colouration at multiple scales

Iridescent colours are colours that change depending on the angle of illumination or observation. They are produced when light is reflected by multilayer structures or diffracted by gratings. While this phenomenon is well understood for simple optical systems, it remains unclear how complex biological structures interact with light to produce iridescence. There are very few comparative studies at interspecific level (often focusing on a single colour patch for each species), resulting in an underestimation of structure diversity. Using an interdisciplinary approach combining physics and biology, we here quantify the colour and structure of 36 hummingbirds species evenly distributed across the phylogeny. We explore at least 2 patches per species, which are assumed to be under different selective regimes. For each patch, we measure structural features (number of layers, layer width, irregularity, spacing, etc.) of the feathers at different scales using both optical and electronic microscopy and we measure colour using a novel approach we developed to encompass the full complexity of iridescence, including its angular dependency. We discover an unsuspected diversity of structures producing iridescence in hummingbirds. We also study the effect of several structural features on the colour of the resulting signal, using both an empirical and modelling approach. Our findings demonstrate the need to take into account multiple patches per species and suggest possible evolutionary pressures causing the evolutionary transitions from one melanosome type to another.