FOOD OF WARBLERS 25 



injurious on the "tree claims" of the prairie States before arboreal 

 birds became abundant there, are almost never numerous enough 

 to be destructive where such birds are plentiful. The fact that 

 Warblers do away with these insects while the caterpillars are still 

 very small and before they have had a chance to do any real injury, 

 is of great economic significance. It may yet place them on a par, 

 as regards usefulness, with the Cuckoo and other larger birds, 

 which are considered to be among the most useful caterpillar hunt- 

 ers, but which probably prefer the larger caterpillars ; for the 

 Warbler, notwithstanding its small size, may be able to destroy 

 more individual caterpillars in their infancy than even the Cuckoo 

 can devour after the same caterpillars have increased several hun- 

 dred times in size. 



Warblers are mainly insectivorous and most species cannot 

 live long without insect food. Hence their economic position is 

 quite different from that of the Vireos, Thrushes, or Sparrows, for 

 example, for these can live either largely or entirely for consider- 

 able periods on vegetable food. 



Warblers are obliged to spend a great part of their time in a 

 continual hunt for insects. Digestion in most small birds is con- 

 tinuous and the stomach is filled many times each day. It is some- 

 times so packed with food that when one is dissected the contents 

 will expand to twice, or, as Professor F. E. L. Beal tells me, nearly 

 three times, the size of that organ. 



It would seem impossible for digestion to go on under such 

 circumstances, but it nevertheless progresses so rapidly that, unless 

 the food supply is constantly replenished, the stomach is soon 

 empty. The capacity of Warblers for consuming the smaller 

 insects may be shown by the statement of a few facts. According 

 to Dr. S. D. Judd, Mr. Robert H. Coleman stated in a letter to the 

 Biological Survey, that he counted the number of insects caught 

 by a Palm Warbler and found that it varied from forty to sixty 

 per minute. He says "the bird spent at least four hours on our 

 piazza, and in that time must have gathered in about nine thousand, 

 five hundred insects."^ 



Of course the insects in this case must have been very small ; 

 but some of the greatest pests are small at maturity, as, for 

 instance, the Hessian fly and the wheat midge, — insects which 



^ The Birds of a Maryland Farm, by Sylvester D. Judd, Bulletin No. 17, U. S. 

 Department of Agriculture, Division of Biological Survey, p. 104. 



