Field Bird's Nest Identification Guide

Where you find it and what it looks like

Cyathus olla is a fungus of open, disturbed, nutrient-poor ground rather than shaded woodland. The classic setting is the bare edge of an arable field, a gravel path, a sandy track, a stabilised dune, a garden border, or a fresh spread of wood chip. Look at the ground where old herbaceous stems, straw, twigs and crop debris lie half rotted on soil, and get down to eye level with it. The fungus is small, dull coloured and easily mistaken for grit.

The first impression, once you have your eye in, is unmistakable. Little grey-brown vases, each between 5 and 15 mm tall, stand in colonies with their mouths turned upward, and inside each one lies a handful of flattened grey discs like miniature eggs in a nest. Nothing else in the fungal world looks quite like a bird's nest fungus, so the question in the field is almost never which group but which species.

Reading the cup

Measure the cup first. It is 5 to 15 mm high and 6 to 12 mm wide across the mouth, and its shape is the key character: it is a flaring goblet or jar, narrow at the base and widening steadily upward, with a mouth that is distinctly wider than the body and a rim that often turns outward and becomes wavy. Hold it side on and the silhouette should be a trumpet, not a straight-sided cone.

Now examine the outside with a lens. It is dull grey-brown to ochre-brown, and in young specimens it is finely woolly or felted. As the fruit body ages and weathers, the wool wears off and the surface becomes smooth and slightly shining. It is never shaggy, never rusty red, and never strongly hairy.

The inside is equally diagnostic. It is smooth, silvery to lead grey, and often slightly shining, without any grooves, ribs or vertical fluting whatsoever. Run a fingernail up the inner wall and it should feel like polished card.

The lid and the eggs

Young fruit bodies are closed by an epiphragm, a pale whitish membrane stretched across the mouth like a drumskin. At this stage the fungus looks like a tiny fuzzy barrel and can be missed entirely. As the cup expands the membrane tears and flakes away, and the remnants may persist as a ragged collar on the rim.

Inside lie the peridioles. In this species they are relatively few, typically five to a dozen per cup, and they are large for the group at 2 to 3.5 mm across. Each is a flattened lens, grey to grey-brown or blackish, sometimes with a paler silvery skin when fresh. Take a pin and tease one gently away from the wall: it is anchored by a coiled funicular cord which unwinds into a fine elastic thread with a sticky pad at the end. That cord is the dispersal mechanism, and finding it confirms both the genus and the identification.

Dispersal itself is worth watching for. Raindrops striking the flared cup splash the peridioles outward, the funicular cord trails behind, and the sticky pad snags on a stem, wrapping the peridiole around it.

Flesh, texture and colour change

There is no fleshy tissue in the usual sense. The cup wall is thin, tough and leathery, and it does not break cleanly. It does not bruise, does not change colour on cutting, and produces no latex or juice. Old cups persist through winter, bleaching pale and losing their peridioles, and empty weathered cups are often the first sign that a colony exists.

Spores and why a print is impossible

You cannot take a spore print from a bird's nest fungus. There is no exposed hymenium beneath a cap from which spores fall: the spore-bearing tissue is sealed inside each peridiole, and the peridiole is the unit that travels. Standing a cup on card overnight will produce nothing at all.

The substitute is direct microscopy. Lift out one mature peridiole, crush it in a drop of water on a slide under a coverslip and examine the contents. The spores are large, thick-walled, smooth, colourless and broadly ellipsoid, roughly 10 to 14 by 6 to 8 microns, which is big enough to be obvious even at moderate magnification. In the field, substitute a careful count and measurement of the peridioles instead: few and large, at 2 to 3.5 mm, is a strong pointer to this species.

Substrate and season

The fungus is saprotrophic, living on dead plant material rather than partnering a tree. Old herbaceous stems, straw and crop residue, wood chip mulch, twigs, dune litter and bare compacted soil are all colonised, and it is one of the few fungi that thrives on poor, open, frequently disturbed ground. Fruiting runs from summer into late autumn, and cups commonly persist over winter in a weathered state, so an empty colony found in spring still records the species.

Separating it from look-alikes

Cyathus striatus is the commonest confusion. Its inner wall is conspicuously fluted with vertical grooves, its exterior is shaggy and rusty red-brown, its cup is a straight-sided cone rather than a flaring trumpet, and its peridioles are numerous and small. It also prefers shaded woody debris rather than open ground.

Crucibulum laeve is shallower and more crucible shaped with a straighter rim, its exterior is velvety ochre-yellow to tan, its epiphragm is bright yellowish, and its peridioles are white and numerous, at only 1 to 2 mm.

Nidula species have cinnamon, more cylindrical cups and peridioles embedded in mucilage with no funicular cord at all; tease one out and if there is no thread, it is not a Cyathus.

Nidularia and Mycocalia produce poorly formed, irregular, almost sac-like fruit bodies packed with many tiny peridioles and no properly built cup wall.

Sphaerobolus stellatus is much smaller, under 2 mm, orange, and splits into a star to eject a single glebal ball rather than holding several discs.

Finally, rule out imitations that are not fungi at all: some seed capsules, moss capsules and snail egg clusters look similar at a glance. If a lens shows lens-shaped peridioles attached by a coiled cord inside a felted grey trumpet, the identification is secure.