Page:The British Warblers A History with Problems of Their Lives - 2 of 9.djvu/54

 are built up and washed away a number of times during the same season, and each time the same position on the wall is chosen: only in an occasional instance are the young successfully reared. These cases are. I think, sufficient to make clear my meaning; but no doubt a number of similar ones will suggest themselves.

Their food is principally insects and their larvæ. I have not seen them feeding on berries, although they spend considerable time in the autumn hopping about amongst the elder bushes (Sambucus nigra) when the fruit is ripe. On their arrival in March and during the first part of April their principal food is Chironomidæ. In pursuit of these insects they search minutely the under side of the leaves of different evergreens, such as holly, ivy, &c, and the trunks of the larger trees, upon which these flies frequently cluster, almost the only species sufficiently numerous, during the cold east winds, to afford food. In April also they search the bare branches and trunks of the different trees for the young stages of Psocidæ, minute white insects, which can only be identified by a very close examination. It is insects, probably Chironomidæ, that are the cause of their searching the grass on lawns, where they hop about hunting the ground very quickly. They can sometimes be seen clinging to the trunks of apple trees in order to pick off the small moth-like flies of the genus Psychoda. In May and June they take quantities of the larvæ of Chimatobia brumata and Tortrix viridana. This latter insect, the oak leaf roller moth, causes very great destruction in some years to the oaks. If you stand under one of these trees some warm evening in May you will notice that the foliage is everywhere riddled, blighted, and partially destroyed by the larvæ, and that the undergrowth is covered with their excrement. Now watch these little Warblers, together with Whitethroats, Willow-Warblers and Blackcaps, at work; notice their insatiable appetite and the energy with which they seek their food; thus we come to understand the danger