BRITISH WARBLERS
now find that, in some years, the immature birds are the first to arrive. There is little difficulty in deciding this, since, compared to the fully developed males, these immature individuals are inferior in beauty; their plumage is dull, the feathers appearing to lack sufficient colouring matter, and in some cases the black feathers on the head are still tipped with brown. The difference also in the quality of the feathers is very striking, which, while lacking the finish and gloss of the older birds, resemble more nearly the undeveloped plumage of the young on leaving the nest. The inferiority with regard to the song is even more marked, the immature birds not possessing so great a variation nor the same richness of tone. But some recognised definition of the term "mature" is really needed. We cannot judge by any one particular phase of plumage, since there is little doubt that the colour increases in intensity and the growth of the feathers continues for some years, possibly throughout the life of many species. It is probable that it will ultimately be found that the quality of the plumage and the richness of the song develop correspondingly. There is only one logical definition of the term, i.e., the age at which an individual attains to the power of sexual reproduction.
Wooded banks and dingles, coppices, gardens with plenty of evergreens, and woods in which the timber is annually felled, and in which, consequently, there is an abundance of undergrowth, are the favourite resorts of these birds, provided that there are sufficient trees overgrown with the common climbing ivy (Hedera helix) close at hand, for they live on the fruit of this ivy until there is sufficient insect food to maintain them; which is probably for two or three weeks after their arrival, but the time varies according to the condition of the season and the development of insect life.
There are few more spring-like sounds than the song of the male; it has, in fact, a peculiar cheerfulness, which alone seems to transform winter into spring. Arriving during the night, he commences to sing soon after dawn, but the energy
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