Red-cockaded woodpeckers, Picoides borealis, are the only birds that excavate cavities in living pine trees. Each woodpecker might spend years excavating its own roost in the live tree – which is a lot harder than making a hole in a dead tree. But by partnering with wood-decaying fungi, these cavity-excavating woodpeckers may have found a way to speed up the construction of their nest. The findings are published in Proceedings of the Royal Society B this week.

Some cavity excavators depend on decayed wood in living or dead trees, while others – such as the endangered red-cockaded woodpecker – require relatively healthy, living trees. These woodpeckers live in family groups in longleaf pine ecosystems in southeastern North America. They’re the only birds that exclusively excavate through the sapwood and into the heartwood of living pine trees – a long process that can range from months to decades. But once the cavity is complete, the woodpeckers can use it for many years, during which they maintain active resin wells in the wood. It takes many other woodpecker species just weeks to excavate in dead trees, though they might use that cavity for just one year.

Researchers have long suspected that red-cockaded woodpeckers utilize fungi that soften healthy wood to create potential excavation sites. In return, excavators likely help facilitate fungal dispersal. However, this relationship hasn’t been experimentally verified. 

To test this facilitation hypothesis, a team led by Michelle Jusino of the U.S. Forest Service used cotton to swab the bills, wings, and feet of 11 adult woodpeckers living on Marine Corps Base Camp Lejeune on the central coast of North Carolina. They wanted to compare fungal communities on the birds’ bodies to those in their complete and incomplete excavations.

The team also conducted a 26-month field experiment to see if these woodpeckers facilitate fungal infection in longleaf pine trees during cavity excavation. They drilled holes (called cavity starts) into 60 trees, and by restricting access to half of them using steel screens, the team could compare the change in fungal communities in excavations accessible and inaccessible to the woodpeckers.

The researchers discovered a complex symbiotic association between these woodpeckers and the multiple species of Basidiomycota fungi that they carried with them from existing cavities to future excavation sites. 

^{RCW = red-cockaded woodpecker. M.A. Jusino et al., Proc. R. Soc. B 2016}

Red-cockaded woodpeckers directly altered the colonization and composition of fungus communities: Accessible, human-made excavations contained fungal communities similar to natural, woodpecker-made excavations, whereas inaccessible excavations contained different fungi. The woodpeckers even made wounds in tree trunks to give the fungi an access point.

Woodpeckers like these are ecosystem engineers who construct cavities used by many species of birds, small mammals, reptiles, and invertebrates. Turns out, wood-decaying fungi are also ecosystem engineers too. 

^{Image in the text: A red-cockaded woodpecker being swabbed. M.A. Jusino et al., Proc. R. Soc. B 2016}