Herr E. Hartert on the Caprimulgidse. 275 



simple coloration of the plumage of the Swifts, mostly very 

 constant and liable to little variation, and the beautiful and 

 richly coloured plumage of the Goatsuckers, varying in most 

 of the species to a certain extent, and, in some cases, more 

 than in almost any other family of birds. The colours of 

 the Caprimulgidae are, as a rule, highly protective, and, 

 as in other similar cases (for instance, Alauda, Galerita, &c.), 

 vary much according to the surroundings, and especially 

 in relation to the soil. So, as a rule, we find yellow, buff, 

 and isabelline-caloured species in sandy deserts and desert- 

 like localities, darker species in more wooded countries, and 

 richly coloured ones in tropical forests. But even the same 

 species varies much, according to the soil that it frequents, 

 and therefore several species readily form more or less well- 

 defined local races, often well worthy of subspecific rank. 

 In many cases, of course, as usual, we do not understand the 

 reasons why a certain form is differentiated, because we have 

 so little knowledge of the influence of climate, the amount of 

 rainfall, the surroundings, and the food. Still more puzzling 

 are several other forms, because they undoubtedly occur in 

 reddish and brownish or greyish phases, apparently indepen- 

 dent of locality and age, as is the case with certain Owls, 

 also birds of nocturnal habits. 



Before an author writes about any group of birds he should 

 make himself acquainted with their seasonal changes of 

 plumage ; and this I have tried to do as regards the Goat- 

 suckers, but have unexpectedly found it very difficult. C. L. 

 Brehm and also Naumann, whose statements generally stand 

 like stone and iron, both assert that the Common Goat- 

 sucker moults twice in the year — the first time before it 

 leaves Europe, and the second in its winter-quarters. All 

 subsequent writers that touch upon the question at all also 

 state that a double moult takes place. Until lately I had no 

 reason to doubt this assertion, as the specimens of Capri- 

 mulgus that I have shot in Germany during the migration in 

 autumn were, so far as I can remember, all without white 

 tips to the lateral rectrices — a character which, according to 

 the authors named above, is that of young specimens as well 



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