The function of lichen flakes and white spider cocoons on the outer surface of birds' nests

The predictions from two hypotheses for the adaptive significance of the application of lichen flakes and white silk cocoons to the outer surface of bird nests are compared. The hypotheses are: (a) concealment by resemblance to the branches to which the nest is attached, and (b) concealment by light reflection to make the nest dissolve into the background beyond the site of attachment. The predictions are tested with evidence obtained from a sample of 42 Long-tailed Tit (Aegithalos caudatus) nests and 64 Blue-grey Gnatcatcher (Polyoptila caerulea) nests, and from examination of single nests of over 50 other species. Little evidence is found to support branch matching although this hypothesis may partly or wholly explain the external application of lichen to the nests of some species. The hypothesis of concealment by light reflection is supported by the data, in particular by the general absence of lichen on branches to which lichen-covered nests are attached and the substitution in some species of pieces of white man-made materials for pieces of lichen or white silk cocoons. Thus concealment by light reflection is probably an important method of nest camouflage for a range of species of small bird.