The layer of eggs once deposited, the two valves would produce the foam required to envelop the eggs.

It is extremely difficult, however, to guess what occurs beneath the veil of foam-like secretion.
In a recent nest the zone of issue is surrounded by a layer of finely porous matter, of a pure matt, almost chalky white, which contrasts distinctly with the remainder of the nest, which is of a dirty white.

It resembles the icing composition made by confectioners with whipped white of egg, sugar, and starch, for the ornamentation of cakes.
This snowy border is easily crumbled and easily detached.

When it disappears the zone of issue is clearly defined, with its double series of leaves with free edges.

Exposure to the weather, wind, and rain result in its disappearance, fragment by fragment, so that old nests preserve no trace of it.
At first sight one is tempted to regard this snowy substance as of a different material to the rest of the nest.