White-egg Bird's Nest
Crucibulum laeve
Also called common bird's nest, crucible fungus, white-egg bird's nest fungus
A squat, crucible-shaped little cup, tawny-ochre and felty outside with a smooth pale interior, holding neat white disc-shaped 'eggs'. The white peridioles set it apart from the grey and black ones of Cyathus.
Overview
Crucibulum laeve is the only widespread member of its genus and the easiest bird's nest fungus to name once the cup has opened, because its peridioles are white rather than grey or black. The fruiting body is a small, thick-walled crucible — squatter and more cylindrical than the tapering vases of Cyathus — with a warm ochre to cinnamon, finely felted exterior.
The genus belongs to the family Nidulariaceae in the Agaricales. Like its relatives it has traded wind dispersal for a rain-powered splash mechanism, packaging its spores inside hard little discs that are fired out of the cup by falling drops and then tethered to vegetation by an elastic cord.
It is a common colonist of woody litter and garden mulch, often appearing in dense colonies of dozens or hundreds of cups across a single stick or chip bed. The young stage, sealed by a smooth whitish lid, looks quite unlike the open cup and is frequently mistaken for a tiny puffball.
How to identify it
Fruiting body
A squat crucible or shallow barrel 5–10 mm tall and 5–8 mm wide, with nearly straight or only slightly flaring sides and a fairly narrow, even mouth. Colonies are usually crowded, with many cups packed side by side.
Outer surface
When young, coated in a soft, fine, tawny to ochre-brown or cinnamon felt or fuzz. This wears off with age, leaving the outside smoother and paler buff to greyish. The texture is felty, never coarsely shaggy.
Lid
Young cups are closed by a conspicuous smooth, whitish to pale buff membrane (the epiphragm) stretched across the mouth like a drumskin. It ruptures and vanishes as the cup matures, and this closed stage is a good field character.
Inner surface
Smooth, without grooves, and pale — whitish, cream or pale buff — contrasting with the darker exterior.
Peridioles
Usually numerous, around 10–20 per cup, each 1–2 mm across, disc- or lens-shaped, and distinctly white to pale cream, sometimes dulling to buff with age. Each is attached by a coiled funiculus enclosed in a sheath. Spores form inside the peridioles, so an ordinary spore print is not obtained.
Where and when it grows
Crucibulum laeve fruits on all kinds of dead plant material: fallen twigs and small branches, bark, rotting stumps, wood chip and bark mulch, sawdust, and also dead herbaceous stems, straw, bracken litter and old fern rachises. It is very common in gardens, parks and landscaped beds where mulch is applied, as well as on woodland floor debris and hedge bottoms.
Damp, shaded conditions favour it, though it tolerates more open sites than the fluted bird's nest. It is widespread throughout temperate regions of the Northern Hemisphere and is also recorded from other continents. Fruiting occurs mainly from summer through autumn, continuing later in mild, wet weather; empty cups persist on the substrate for months.
Ecological role
This is a saprotrophic fungus that decomposes cellulose and lignin in dead wood and plant litter. Its mycelium spreads through twigs, chips and stems, and the crowded cups appear once the substrate is well colonised, which is why mulch beds can suddenly produce vast numbers of them in a single season.
Reproduction uses the same splash-cup mechanism as Cyathus: rain striking the cup ejects the white peridioles, and the funiculus pays out into a long sticky thread with an adhesive tip that catches on nearby vegetation and coils around it. Fixed there, the peridiole waits until its tough wall weathers away, or until the plant tissue is grazed by a herbivore and the spores pass through the gut and are deposited elsewhere.
Similar species
The Cyathus species are the closest look-alikes but differ clearly once open. Cyathus striatus has strong vertical grooves inside the cup, coarse shaggy dark brown hairs outside and dark grey to blackish peridioles. Cyathus olla is a larger, wider-flaring grey goblet with a smooth interior and a few big lead-grey peridioles. Cyathus stercoreus has black peridioles and grows on dung.
In Crucibulum the combination of tawny-ochre felty outside, pale smooth inside and white peridioles is unique. Nidula species can also look pale but their many small peridioles are set in a sticky matrix and lack the funiculus cord. Closed young cups may be confused with minute puffballs, but the flat, taut whitish lid across a cylindrical cup is a giveaway once examined with a lens.