Avian mud nest architecture by self-secreted saliva

Significance We provide a biomechanical explanation of how swallows and phoebes can construct strong nests of incohesive mud granules using saliva as a paste. The analysis leads to a hypothesis for why only 57 small light-weighted bird species (of approximately 10,000 species worldwide) can build mud nests on walls by utilizing their saliva. Our comprehensive study, combining experiments on natural and artificial mud nests and mathematical models on granular cohesion, not only elucidates the physical mechanism of this extraordinary animal architecture, but also provides inspiration to three-dimensional printing technology based on environmentally benign granular materials.