BRITISH WARBLERS
only handle and examine these fragile organisms in order to realize how prejudicial prolonged exposure must be to them. So that even if we could not verify it by direct observation, we should still have general reasons for believing that much of the parents' time would be passed in brooding. Since warmth and consequently freedom from exposure are of such vital importance, does it seem unreasonable to suggest that the covered nest would contribute towards that end, and by freeing the parents of part of their task give rise to the habit we observe in the case of some males? We require evidence from the lives of many species, whose young are nidicolous and whose nests are protected, to show whether there is really such a modification of behaviour as is here suggested. If a covered nest does afford protection to the young and thus allow greater freedom to the parents, it is clear that they will have more time to seek the necessary food and will therefore be in a position to rear a larger number of offspring; I have not, however, had sufficient time to investigate the matter closely. Some facts relating to brooding will be found in the lives of the Willow Warbler, Chiff-chaff, Whitethroat, Reed Warbler and Marsh Warbler.
The methods adopted for feeding the young and for ensuring the cleanliness of the nest involve a complicated series of activities showing wonderful nicety of adaptation, and suggest to my mind that the behaviour is purely instinctive. All the species dealt with are very much alike in the treatment of their offspring. I see no reason why a Garden Warbler should not rear Whitethroats, or a Whitethroat Blackcaps, as successfully as their respective progeny. In fact, experiment proves that such really is the case, for I have placed eggs of the Whitethroat in a nest of the Garden Warbler and have seen the young successfully reared. Some particular aspects of the parental instinct are discussed in the histories of the Whitethroat, Reed Warbler and Marsh Warbler. The normal routine of tending the young is as follows: the parent arrives at the nest with food and sometimes feeds one, more often two,
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