The Pellet Analysis Method of Raptor Food Habits Study

Abstract

15, 1930) of a series of experiments carried on under the quail research fellowship established July I, 1929, at the University of Wisconsin by the Sporting Arms and Ammunition Manufacturers' Institute and the United States Biological Survey.One of the objects of the fellowship is the determination of quail-predator relationships, and, in this connection, need was felt of testing the reliability of the pellet analysis method of studying raptor food habits.Th e experiments have not been wholly concluded, but recent discussions of this question make it timely to present a resume of the data already at hand.Experimental work was done with four Great Horned Owls, a Barred Owl, a Long-eared Owl, a Red-tailed Hawk, a Red-shouldered Hawk, and a Marsh Hawk.The aforementioned were young birds taken from this season' s nests and raised to full or to approximately full size.Their growth was normal as compared with that of the nest-reared young in the wild.No young falcons or Accipiters were available, although a Cooper Hawk nestling is now being kept for future experiments.Discussion of pellet formation by species follows.Great Horned Owl.Owlet aged 21 days (calculated), about one-fourth grown and with primaries just coming out, seemed to retain the residue of its meals (cottontail rabbit) so long in the stomach that the bones were digested and the fur passed out in the feces.At the age of 28 days the first creditable pellet was produced.Parts of a robin and a rat had been fed the day before, both of which were evident in the pellet.From this time on, pellets gradually improved in quality, bony material showing less and less digestion as growth of the owlet progressed.Acknowledgment is hereby made of the liability to error in the initial experiments with this three-weeks horned owl, for only the one bird was had at first, and the technique was by no means flawless.As grown juveniles, four horned owls were fed cottontails, rats, mice (Microtus, Peromyscus, and Mus) , ' mallard ducks, domestic chickens, domestic pigeons, pheasants, quail, flickers, English sparrows, miscellaneous birds, snakes, and frogs.Much of this material was taken directly from horned owl nests.Emphasis was laid upon offering them the game upon which they preyed in nature, allowing them to eat it in any manner that they wished.Later they were permitted to do their own killing..No differences in digestion were noted among the four horned owls when given the same type of food.Examples 1 to 5 might be considered representative of the horned owl experiments.(1) Owl fasted for 85 hours.Live English sparrows then put in cage and replenished at intervals to keep available for the owl a constant supply that he could catch as he wanted them.This he was able to do without difficulty.In 74 hours he had eaten 32 sparrows ; in 84 hours he had disgorged 6 pellets, in which remains of 28 out of the 32 sparrows were easily discernible.(2) Owl fed 6 sparrows and 15 house mice (some of the latter were very immature) in 13 hours.Pellet in 24 hours, in which pellet the six sparrows and the 12 largest of the 15 mice could be recognized.(3) Owl stuffed with 3 sparrows and 8 house mice (some very immature as 292