314 THE BIOLOGY OF BIRDS 



a lengthening out or a telescoping down of particular arcs on 

 the normal life-curve. In the mound-birds there is a 

 lengthening out of the ante-natal period and there is a 

 telescoping down of the nestling period to a vanishing point. 

 Both in cuckoo and mound-bird there is on the parent's 

 part a suppression of incubation, but could there be a greater 

 divergence in the associated tactics which secure the welfare 

 of the young ? 



Biologically regarded, the facts of parental care among 

 birds are in three ways of surpassing interest, (i) They 

 illustrate a subtle mingling of instinctive routine and in- 

 telligent plasticity. (2) They show how economised repro- 

 ductivity may be made feasible by an elaboration of parental 

 care. (3) They convince us that in the struggle for existence 

 the collecting of feathers to line the nest — to take a diagram- 

 matic instance — may have survival value as real as beak and 

 claws. When we appreciate the nest-making and the brood- 

 ing, when we think of the feeding, the education, and the 

 defence of the young, when we inquire into aberrant cases 

 like that of the mound-birds (surely balancing cuckoos !), 

 must we not admit that birds spend a large fraction of their 

 time and energy in activities not directly self-preservative, 

 but in the interests of their offspring. If so, animal evolution 

 has its ethical note. 



§ 8. Social Life 



Many birds are gregarious, but not many are social. 

 When suitable breeding-places are not very numerous, 

 those that are available may be congested, as in the case of 

 shelved cliffs which are densely crowded with gulls, guille- 

 mots, razorbills, and the like, or in the case of swampy ground 

 near a loch which is thickly covered with the nests of black- 

 headed gulls. But there is no corporate life in such cases, 

 though it must be admitted that the mere force of numbers 

 may have a protective value against birds of prey even when 

 there is no concerted action. 



A social note is struck when there is some measure of 



