Fluted Bird's Nest
Cyathus striatus
Also called striate bird's nest, splash cup, fluted nest fungus
A tiny vase-shaped fungus, shaggy and brown outside, with a silvery-grey inner wall marked by lengthwise grooves and a clutch of disc-like 'eggs' inside. The grooving separates it from every other common bird's nest fungus.
Overview
Bird's nest fungi are among the most charming miniatures in the fungal world, and Cyathus striatus is the classic representative. Each fruiting body is a stiff little cup only about a centimetre tall, standing on woody debris and containing several flattened, seed-like packages called peridioles — the "eggs" that give the group its name. Each peridiole is a self-contained parcel of spores.
The species sits in the family Nidulariaceae within the Agaricales, so its nearest relatives are ordinary gilled mushrooms even though the form is wildly different. Instead of exposing spores on gills, Cyathus has abandoned wind release altogether and evolved a rain-powered splash mechanism, with the cup shaped to catch and redirect falling drops.
Cyathus striatus is distinguished from its relatives by the strong longitudinal fluting inside the cup, a feature visible with the naked eye or a hand lens once the covering membrane has ruptured. Combined with its dense, shaggy brown outer hairs, this makes it one of the easiest small fungi to name with confidence.
How to identify it
Fruiting body
A rigid inverted cone or vase, 7–15 mm tall and 6–10 mm wide at the flared mouth, narrowing to a small attachment point below. Fruiting bodies almost always occur in dense clusters or troops, sometimes dozens together on one piece of wood.
Outer surface
Coated in coarse, shaggy brown to reddish-brown hairs that stand out in tufts, giving a rough, felted appearance. The hairs weather away with age, leaving a duller, browner surface.
Inner surface
This is the key character: the inside of the cup is silvery grey to lead-grey or brownish and is marked with distinct lengthwise grooves or pleats running from rim to base, so it looks fluted. The inner wall is also shiny compared with the matt exterior.
Lid and peridioles
Young cups are sealed by a pale, felted membrane (the epiphragm) that tears and disappears as the cup opens. Inside sit roughly 5–15 peridioles, each 1.5–2 mm across, flattened-lens shaped, blackish to dark grey-brown, and often with a slight pale sheen. Each is anchored to the cup wall by a coiled elastic cord, the funiculus, which uncoils dramatically when the peridiole is launched. Spores are produced inside the peridioles, so a conventional spore print is not taken.
Where and when it grows
Cyathus striatus grows on dead wood and woody debris: fallen twigs, small branches, bark, rotting stumps, sawdust and — very commonly in gardens and landscaped beds — bark mulch and wood chips. It prefers damp, shaded, humid situations, and troops often appear where mulch has been laid thickly and stays moist.
It is widespread across temperate and subtropical regions worldwide, including Europe, North America, Asia, Africa and Australasia. Fruiting runs mainly from summer into autumn, extending later in mild, wet weather, and the dried cups can remain recognisable on the substrate well after spore dispersal has finished.
Ecological role
The species is a saprotroph, decaying lignin- and cellulose-rich woody litter and returning its nutrients to the soil. Its mycelium permeates twigs, chips and bark, and the cups are simply the reproductive stage pushed up from that hidden network.
Dispersal is a piece of natural engineering. The cup's angled walls funnel a falling raindrop so that it strikes the peridioles and flicks them out, sometimes more than a metre away. As a peridiole flies, its funiculus unravels into a long sticky thread with an adhesive pad at the end; if this catches a stem or leaf it wraps around and tethers the package there. Spores are released when the peridiole wall eventually breaks down, or after passing through the gut of a grazing animal that has taken the attached vegetation.
Similar species
Other bird's nest fungi are the obvious comparisons. Cyathus olla has a wider, more flaring, goblet-like cup with a smooth grey inner wall — no fluting — and larger peridioles. Cyathus stercoreus is hairy outside when young but has a smooth, dark inner surface and black peridioles, and it grows on dung and manured ground rather than woody debris.
Crucibulum laeve is separated by colour and shape: it is a shallower, more cylindrical crucible, ochre to tan and felty rather than shaggy dark brown, with a smooth pale inner wall and whitish, disc-shaped peridioles. Nidula species contain many small pale peridioles that lack a funiculus cord entirely and instead sit in a sticky matrix, and their inner surface is not grooved.