Mechanics of Avian Mud Nests

How swallows build cantilevered homes from mud and saliva — granular composites tested to failure.

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Motivation

Barn swallows build nests from thousands of mud pellets bonded by self-secreted saliva, cantilevered off vertical walls, supporting many times the birds’ own weight. What makes this granular composite strong enough? (PNAS 2021)

What I did

• Field and lab characterization of real nest material: composition, pellet structure, and porosity
• Mechanical testing of saliva- and polymer-bonded granular composites, measuring strength as a function of binder content and moisture
• Physical modeling of the bonding mechanism — capillarity-assisted adhesion and the drying-induced strengthening of the pellet assembly
• Analysis connecting the nest’s construction sequence (pellet-by-pellet deposition) to its final load-bearing capacity

Why it matters

The nest is a natural example of additive manufacturing with wet granular matter: material deposition, curing, and structural function are inseparable. The same physics — capillary bridges, drying, particle-binder mechanics — appears in 3D printing of dense suspensions and in construction robotics. This work was featured in Harvard SEAS news and Korean national media.