ill some si)ecies but not in otliers. Tlu- walls of the 

 stomach are more muscular in seed-eating birds than 

 ill flesh-eaters. Owls and some other species form 

 and regurgitate from the stomach pellets of indi- 

 gestible matter. Gallinaceous birds are able to retain 

 or shift the supply of gravel or grit in the stomach 

 as an aid to grinding seeds. The length of the in- 

 testine varies with the type of food consumed ; the 

 caeca are longer in browsing and seed-eating gal- 

 linaceous birds for digesting cellulose. 



Associated with these anatomical and histological 

 adaptations are adjustments in function and behavior. 

 Obviously, if an animal has mor])liological adapta- 

 tions for ingesting and digesting flesh, it must be- 

 have as a carnivore and not as an herbivore. The 

 possession of these adaptations and adjustments 

 means that animals are generally restricted to the 

 particular types of food that they can use most effi- 

 ciently. The kinds of food eaten by animals is of 

 fundamental ecological and economic importance. 



METHODS OF STUDY 



A common procedure for analyzing kinds 

 of food consumed by organisms is to identify the con- 

 tents of the crop, stomach, cheek pouches, or other 

 parts of the digestive tract (Hartley 1948). 



The diet may be described in terms of number of 

 items of each kind of food found in one specimen, 

 or percentage of specimens containing a particular 

 item, but it is usually more satisfactory to measure 

 in one specimen the percentage volume of each food 

 item against the total contents (McAtee 1912). 



This procedure has the advantage of showing ac- 

 curately what an animal has actually ingested, but 

 has the disadvantage that the animal must usually be 

 killed : thus, tlie information obtained is on only one 

 meal, or portion of a meal. It also gives no informa- 

 tion on where or how the food was obtained. Never- 

 theless, considerable information on the food habits 

 of animals has been obtained in this manner (Hen- 

 derson 1927, McAtee 1932, Davison 1940, Martin 

 et al. 1951). Improved techniques make it possible 

 to secure the stomach or crop contents without kill- 

 ing the animal. This is done by manual manipula- 

 tion of the crop (Errington 1932), or by use of 

 flushing tubes (Vogtman 1945, Robertson 1945). 

 Artificial beaks with open gapes placed as decoys 

 among nestlings have been used to collect food 

 brought by the parents (Betts 1954). 



There are usually indigestible parts in all kinds 

 of food, and these indigestible or undigested parts are 

 eliminated from the body. The contents of fecal 

 droppings or regurgitated pellets can often be identi- 

 fied by differences in shape, size, color and texture. 



or by histological techni(|ues (I)usi 1949). Collection 

 of dr()i)i)ings is not ])ractical)le for acjuatic animals, 

 or for more than a few of the terrestrial invertebrates. 

 Moreover, the droppings must be relatively fresh, 

 as they (|uickly disintegrate in wet weather. The 

 analysis of owl pellets is very fruitful, for owls swal- 

 low all parts of their mammalian or avian prey, and 

 then regurgitate the hair, feathers, and skeleton. 

 Hawks, gulls, and shrikes also produce pellets. The 

 considerable advantage of pellet analysis is the possi- 

 bility of continuous diet analysis on the same indi- 

 vidual, or species, through long periods of time, with- 

 out disturbance to its normal behavior (Dalke 1935, 

 I'.rrington 1932). 



Whatever method is used, field observation of the 

 feeding behavior of animals in the natural environ- 

 ment is desirable. For instance, one series of stom- 

 achs of the house sparrow contained a large number 

 of May beetles, which would suggest that the bird 

 was important for the control of this insect pest. 

 Observations disclosed, however, that the sparrows 

 were picking up dead beetles littering the pavement 

 under street lights. Field observation itself often fur- 

 nishes considerable information concerning the kinds 

 of food consumed, but the results are usually not 

 quantitative, do not disclose the less conspicuous 

 kinds of food taken, and may be inaccurate if not 

 carefully formulated. A hawk visiting a game farm 

 may take not game animals but undesirable rodents 

 that are also present (Kalmbach 1934). 



Food chains can be determined by correlating the 

 food eaten by different species in the community. 

 Radioactive elements incorporated metabolically into 

 an organism are taken into the predator of that or- 

 ganism ; radioactivity-tracing technique gives promise 

 of more direct tracing of how matter flows through 

 the ecosystem. A number of radioactive elements 

 may be used ; among them, phosphorus-32 and iodine- 

 131 (Odum 1959). Interesting studies in this con- 

 nection are being conducted at the Oak Ridge Na- 

 tional Laboratory in Tennessee. 



CHOICE OF FOOD 



The kinds of food eaten by animals depend 

 on factors of their genetic heritage, parental train- 

 ing, or conditioning while young. Involved in the 

 evolution of the food habits of a species are the ani- 

 mal's physical adaptations for ingesting and digesting 

 particular types of food, the nutritional values of the 

 food, its palatability, the size of it, its availability or 

 abundance, and its ease of procurement which de- 

 pends in large part on the various protective devices 

 that it possesses. 



Food and feeding relationships 1 



