Hairy Caterpillars 149 



trees entirel\- to the lack of their normal food-supply at this time of 

 the year, the birch-bud. The absence of this food was in the main 

 due to the severe frost which killed the buds, and they, therefore, 

 believe that a recurrence of this very extensive damage is not likely 

 to take place, as long as the birch supply is sufficiently plentiful : 

 that supply is not likely to fail, except under such entirely abnormal 

 conditions as prevailed in llu' two winters. 



IV. Hairy Caterpillars. Cuckoos and Their Food. 



It is a well recognised fact that hairy or spiney caterpillars are, 

 in the great majority of cases, distasteful to birds, and are untouched. 

 The same is true of some of the smooth-skinned caterpillars, which, 

 though smooth, are highly and gaudily ornamented. 



But the bulk of the plain smooth-skinned larvae — green, brown, 

 or otherwise protectively coloured — are eagerh' sought after and 

 greedily devoured. 



It may, I think, be fairly assumed that the body of the hairy 

 larva is as valuable a food as the body of the smooth larva. The 

 sole reason that they escape attack from birds is the protection they 

 acquire by their hairy covering. I am not dealing with the highly- 

 coloured, smooth-skinned larvte, only with hairy ones. 



The hairs of some of these caterpillars are exceedingly irritating 

 to human beings, others may be handled with impunity : but birds 

 do not discriminate between the irritating and non-irritating larvae, 

 they leave them both alone. 



It seems a fair deduction that their dislike of these hairy larvae 

 is due to the fact that the body is partly or entirely clothed with 

 hair, and does not depend on whether the hairs themselves are 

 poisonous or not. 



I imagine that in the case of most birds, the efficient digestion 

 of these larvae is difficult or impossible, either because the hairy 

 coN'ering prevents the gastric juices of the bird reaching the body 

 of the caterpillar, or because the larval hairs, as they are shed in the 

 process of digestion, adhere to the mucous lining of the stomach, 

 hindering the glandular secretion, and at the same time forming a 

 mechanical covering which prevents the food in the stomach being 

 brought into direct contact with the mucous membrane. 



This latter theory seems to me the probable explanation of 

 the distaste birds evince for hairy caterpillars. I can picture a 

 bird starving in the midst of plenty, after a full meal of hairy cater- 

 pillars. The hairs would be left in the stomach, matted over the 

 mucous surface ; these in themselves might lead to mechanical 

 obstruction. They would certainly interfere very greatly with the 

 normal process of digestion. 



Take, for example, an ordinary game-bird, such as the Red 

 Grouse. The diet consists, to a large extent, of young heather- 



