Physical properties of fungal rhizomorphs of marasmioid basidiomycetes used as nesting material by birds

The use of fungal rhizomorphs of marasmioid fungi ('horsehair fungi') as nesting material, especially by tropical bird species, is a widespread but poorly documented phenomenon. I show in this first experimental examination of the physical properties of 'horsehair fungi', that the fungal rhizomorphs utilized by Icterus pustulatus have a significantly higher tensile strength and reduced water uptake than alternative fibres used to build nests. These results suggest an adaptive advantage for the use of marasmioid fungi in nest construction.