INTRODUCTION 7 



they are likely to take the most easily obtained food that is at all suit- 

 able to their needs. Human beings do so, being much less particular 

 about what they eat when they are really hungry than when they eat 

 simply because the clock says it is mealtime. Consequently, statements 

 concerning the ordinary food habits of vertebrates must be taken with 

 due allowance for the possible taking of other foods at certain times ; 

 and the fact that an animal is once seen eating a particular kind of food 

 may not prove that to be a part of its ordinary diet. Thus, when insects 

 are particularly abundant many graminivorous birds temporarily be- 

 come decidedly insectivorous. Bears may not be particularly fond of 

 veal, but when hungry, if no other food is available, they do not hesi- 

 tate about attacking a calf. Foxes live largely upon rodents, but when 

 hungry they do not disdain game birds, poultry, young lambs or any 

 other meat within their reach. Furthermore, when their usual food is 

 scarce mammals cannot as readily migrate to other localities in search 

 of it as can the birds with their power of flight. 



The red squirrel of New England furnishes an excellent illustration 

 of a recent enforced change of food habit because of a failing supply of 

 its natural food, which consisted of nuts and conifer seeds. The large 

 conifers and nut-bearing trees having been cut from certain areas, the 

 squirrels have adopted the habit of feeding more extensively upon buds 

 and bud twigs, and thereby are said to be doing considerable damage to 

 the younger trees. 2 



The second fact is that often individual animals acquire a taste for 

 special kinds of food not in the normal diet of the species. Lions and 

 tigers do not ordinarily attack men, but occasionally one becomes a 

 man-eater, and, having acquired a taste for human blood, may kill 

 numbers of people before it pays the penalty with its life. Individual 

 birds have often been known to acquire a taste for food not usual in 

 their diet. In case of sheep-killing dogs, however, it is not certain that 

 their depredations are the result of a special liking for mutton. It 

 appears to be due to a lust for killing and the excitement of the chase, 

 as it is with the so-called "game hogs" among men. 



1 Hosley, Red squirrel damage to coniferous plantations and its relation to chang- 

 ing habits, Ecology, ix, 43-48, 1928. 



