Crawling on my hands and knees through the thick, stunted forest near treeline, a subalpine forest formation called krummholz, I stumbled upon some fine, black hairs wrapped around a dead Balsam Fir branch just above the mossy understory. I immediately recognized them. I had seen them woven into the lining of songbird nests I was studying. And remarkably at the very end of a few of these hairs was a tiny mushroom cap not much larger than the head of a pushpin.
George Wallace, born and raised in Vermont and later a great ornithologist, studied the life-history of Bicknell’s Thrush in the 1930s on Mount Mansfield, Vermont, not far from where I found these mushrooms. Wallace described in great detail the nests he had found, but he was unable to identify the inner lining of “fine, black rootlets” and commented that “they are unquestionably rootlets of some sort… resembling horsehair, but where the birds get them is a mystery.” I had a hunch I had solved that mystery.
I collected a sample as I lay hunched under the crooked trees. Later, when the birds were done breeding, I collected a few of the nests. Nearly all of them had copious amounts of the black, rootlet lining, which was a perfect match for my sample. But not one appeared to have a mushroom cap. I found the tiny mushrooms in the forest after a rainy period. Perhaps they needed moisture. I placed one of the nests in a plastic container with just a bit of water sprinkled on the bottom. Within just a few days some of the rootlets appeared to spring upward with caps on them. But what kind of mushroom grows in such a strange manner?
Samples of the hair-like structures and caps were sent to a North American expert, Dr. Dennis Desjardin, Director of H. D. Thiers Herbarium at San Francisco State University, for identification. The hair-like structures were identified as rhizomorphs of the Horsehair Fungus (Marasmius androsaceus). It’s considered common in North America, and can be found across the boreal zone and south along the Rockies, the Coastal Mountains and the Appalachians. It belongs to a group of closely related species, all of which produce numerous rhizomorphs.
The thread-like rhizomorphs are made up of parallel hyphae, branched tubular filaments that make up the body of a typical fungus, and absorb and transfer nutrients. Horsehair Fungus is saprotrophic, an organism that lives and feeds on dead organic material, and is found on dead needles, leaves and twigs and parasitic on some ericaceous plants in wet, boggy habitats. They penetrate solid materials such as dead wood and use acids and enzymes to digest it.
The use of rhizomorphs as nesting material, especially by tropical bird species, has been found to be widespread but poorly documented. It had not been reported for any birds breeding in North America prior to my discovery. From Bicknell’s Thrush to Yellow-bellied Flycatchers, we’ve found that most subalpine songbirds in the Northeast use these mushrooms in nest construction, but why?
A number of Marasmius species have been shown to produce antibiotic agents that inhibit the growth of Staphylococcus. However, Marasmius androsaceus, was practically inactive upon Staphylococcus cultures when tested years ago. While it is possible that this fungus may be an effective agent against nest pathogens and parasites of subalpine birds in the Northeast, this is not known. Bernd Freymann from the University of Groningen in the Netherlands examined the physical properties of Marasmius rhizomorphs used by Streak-backed Orioles in Central America. The rhizomorphs had significantly higher tensile strength and absorbed less water than alternative available fibers in the area. Alternatively, they may simply provide the best of most easily obtainable material in the subalpine forest for lining nests.
Whether the birds are shaman selecting a fungus for medicinal properties, engineers selecting the best construction materials, or just grabbing what is handy; I marvel at the intricate relationships of nature all woven in the lining of a nest.