316 BIRD BEHAVIOUR 



Any water-bird also, when kept away from water 

 for a time, loses the waterproof property in its 

 plumage, this being most marked in the most 

 aquatic kinds such as Grebes and Petrels ; nor is 

 the plumage of dead birds waterproof. This being 

 so, it is difficult to see how the waterproofing 

 efficacy of the oil-gland over the tail, the only 

 important skin-gland birds possess, comes in, al- 

 though it is admittedly especially well developed 

 in waterfowl, and these may be seen repeatedly 

 rubbing their heads on it after bathing and when 

 pluming themselves. 



Another puzzle in the toilet arrangements of 

 birds is the comb-like edge on the inner side of 

 the third claw which some birds, such as Nightjars, 

 Barn-Owls, Herons, Grebes, and Cormorants, pos- 

 sess ; it is true that birds always scratch themselves 

 with this claw, though it is not the nearest to the 

 head, that place being held by the second toe or 

 inner front one (the hind toe being the first), nor, 

 in the case of Grebes and Cormorants, is it the 

 longest. But though this argues that the comb- 

 edge where it exists has some function, it does 

 not explain why it is so curiously distributed one 

 could hardly have two birds more dissimilar in 

 habits than a Barn-Owl and a Dabchick, for in- 

 stance. In the Frigate-bird, which is very ver- 

 minous, the claw-comb appears to act as a vermin- 

 catcher, but a verminous condition does not seem to 

 be universal among comb-clawed birds by any means, 

 and is often found in others, such as the Starling. 



