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Correction Factors for Digestion Rates for Prey Taken by Snow Buntings

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Correction Factors for Digestion Rates for Prey Taken by Snow Buntings 210 SHORT COMMUNICATIONS KIWGEEY, H. E. 1973. The nesting season: June 1, of sympatry of Great-tailed and Boat-tailed 1973-July 31, 1973. Amer. Birds 27:897-902. Grackles. Condor 63 : 29-86. LIGOX, J. S. 1926. Nesting of the Great-tailed SKUTCH, A. F. 1958. Boat-tailed Grackle. In A. C. Grackle in New Mexico. Condor 28#:93-94. Bent [ed.] Life histories of North American black- PHILLIPS, A. R. 1950. The Great-tailed Grackles of birds, orioles, tanagers, and allies. U.S. Natl. the Southwest. Condor 52:78-81. Mus. Bull. 211. PHILLIPS, A. R., J, MARSHALL, AXI C. Memos. TUTOR, B. M. 1962. Nesting studies of the Boat- 1964. The birds of Arizona. Univ. Arizona Press, tailed Grackle. Auk 79:77-84. Tucson. SELANDER, R. K., ANU D. R. GLLER. 1961. Analysis Accepted for publication 1 February 1974. CORRECTION FACTORS FOR are so similar. Both species are primarily granivorous DIGESTION RATES FOR PREY on wintering grounds, but take both seeds and in- vertebrates during the breeding and molting seasons. TAKEN BY SNOW BUNTINGS Reviews of longspur and bunting diets are given by (PLECTROPHENAX NIVALIS) Williamson (1968) and Parmelee (1968), respec- tively. Because the diet of the finches studied is THOMAS W. CUSTER relatively catholic, we hope that the results will have AND significance also for studies of stomach contents of fringillids in general. FRANK A. PITELKA Museum of Vertebrate Zoology University of California METHODS Berkeley, California 94720 This study was carried out at the Naval Arctic Re- search Laboratory, Barrow, Alaska, during June and A major bias in studies of stomach contents of birds July 1973. Twenty male buntings were trapped, results from differences in digestion rates of various using four-celled Glenhaven sparrow traps baited with kinds of prey, such that proportions of some food commercial bird seed. The birds were immediately items are underestimated or overestimated (Hartley brought into the laboratory and force-fed known 1948, Dillery 1965, Goss-Custard 1969, Swanson and numbers and sizes of prey items collected near the Bartonek 1970). To account for such differences, laboratory (fig. 1). General background on the in- some investigators have applied correction factors in vertebrate fauna of the Barrow area is given by their assessments of diet composition. Goss-Custard MacLean and Pitelka (1971) and on the seed-pro- (1969) adjusted values of two dietary items (a soft ducing flora by Wiggins and Thomas ( 1962). crustacean, Corophium volutator, and a shelled gastro- Types of prey used were selected on the basis of pod, Hydrobia ulvae) found in stomachs of the Red- availability and known occurrence in the diet of the shank (Tringa totanus) according to their relative bunting. Except for tests involving millet seeds occurrence in stomach versus esophagus. The where pure samples of a single item were used, each method appears useful for the most abundant prey bird was fed a variety of items. Prey items in the items; however, its applicability to uncommon or rare stomachs of experimental birds not part of the force- items is weak unless very large samples of birds are fed meal were ignored. Force-feeding proceeds easily taken. Mook and Marshall (1965) attempted to find and does not lead to bias in results since intact bodies differences in digestion rates of spruce budworm of larvae and other invertebrates are regularly to be (Choristoneuru fumiferuna) larvae and pupae fed to found in both esophagus and stomach of these finches. Olive-backed Thrushes ( Cuthurus ustulutus) by After force-feeding, the birds were confined to a sacrificing birds at specified time intervals after they darkened chamber and sacrificed after selected five- had been fed known food items. Swanson and Bar- minute intervals. The stomachs were removed im- tonek (1970), studying the Blue-winged Teal (Anus mediately and analyzed for contents. An item was discors), concluded that the solution to the problem recorded as “present” when remains allowed a count of differential digestion of stomach contents is simply of the original number of individuals ingested (e.g., to use only esophageal samples taken from actively by wings, head capsules, or tarsi); an item was feeding birds. Because of the large esophageal recorded as “trace” when remains revealed occurrence capacity of ducks, this seems reasonable for that but not number of individuals ingested (e.g., frag- group. However, for small passerine birds, whose ments of elytra). stomachs hold relatively few items and whose In this study we did not use a standard combina- esophagi are often empty, investigation of correction tion of food items; that is, we thereby assumed that factors by Mook and Marshalls’ method is made the diet composition does not influence the digestion necessary to arrive at an adequate sample size. Exist- rate. This may be dubious, but because stomachs of ing data on this problem are good for ducks (Swan- each experimental bird, when force-fed, already con- son and Bartonek 1970 ), but otherwise they are tained some seeds and occasionally invertebrates con- meager in both variety of avian species studied and sumed before experimentation, we consider that the types of prey represented in their diets. experimental diets were exposed to the usual vari- In this paper we attempt to develop correction ability of stomach contents. factors for different prey items in the diet of the Snow Bunting ( Plectsophenux nivulis), using the technique of Mook and Marshall ( 1965). The pri- RESULTS mary motive for this study was to obtain correction Results are summarized in figure 1. Different food factors that could be used in dietary studies already items fed buntings remained distinguishable in the completed for the Lapland Longspur ( CaZcu~ius stomach for variable times. The small seeds of a native lupponicus). Buntings were chosen to avoid conflict biennial crucifer, Cochlearia officinulis, could be with ongoing studies of local longspurs and because recognized over 150 min after ingestion, and large the diets of these closely related emberizine finches commercial millet seeds remained in the stomach well SHORT COMMUNICATIONS 211 TIME IN WNUTES TABLE 1. Disappearance times and correction fac- FOOD TYPE 2 tors for prey items of the Snow Bunting. Spider Collembola Correction factors Corabld Lore Disappear- ance time Cochlearia Cawbid Food type _(inmin) seeds beetles Small ( Cochlearia) seeds 150 1 &rabid adults 38 3.9 1 Tipn2a adults 35 4.3 1.1 Collembola 30 5.0 1.3 Carabid larvae 30 5.0 1.3 Chlronomsd Larva Muscid larvae 30 5.0 1.3 Cochleoria Seeds Muscid adults 30 5.0 1.3 Ti&z larvae 30 5.0 1.3 Staphylinid adults 25 6.0 1.5 Peclicia larvae 23 6.5 1.6 Chironomid larvae 20 7.5 1.9 FIGURE 1. Summary of food types and their dis- Spiders 18 8.3 2.1 appearance rates. Symbols denote results as follows: ___ X alone, all individuals of particular prey type still recognizable; X circled, only some individuals rec- ognizable; solid circle, only trace recognizable; and witnessed in a few of the experimental birds sacrificed open circle, no trace. after short digestion intervals. DISCUSSION over 300 min; but all invertebrate material became Data in the literature for different bird species are uncountable, or even unrecognizable, after 40 min. conflicting with regard to minimal time for digestion Numbers of carabid beetles were usually determined of specific types of prey. In this study, soft-bodied by head parts, which remained recognizable up to items remained in the gut at least 15 min and up to 35 min. Though elytra remained qualitatively rec- 30 min. Similarly, in one Jackdaw (Corvus moneclula), ognizable for over 180 min, they began to fragment an earthworm, leather jacket, and mealworm were relatively quickly, while head parts were still count- distinguishable after 20 min (Koersveld 1951). On able; they could not be used to estimate numbers of the other hand, Dillery (1965) found that after five individuals ingested. Spiders disappeared after 20 minutes, there was no trace of a beetle larva and min. A detailed summary of results with individual harvestman fed to a Savannah Sparrow (Passerculus birds and their food intake is obtainable from the .sunduichensis). Also, Turcek ( 1956) reported, in a authors upon request. review of work by Zverov, that mealworms, cater- As figure 1 shows, for each food type there was a pillars, and earthworms fed to “small insectivorous regular pattern in time span taken by the digestive birds” ( presumably passerines ) were often unrec- breakdown. It is encouraging that the only conflicting ognizable in three to five minutes. Swanson and result among the arthropods is that shown by chirono- Bartonek (1970) found that after 10 min 100% of mid larvae. Three larvae after 15 min were not the amphipods, 82% of the snails, and 24% of present in the gut; however, after 20 min, one of two dipterous larvae fed Blue-winged Teal were digested larvae was detected in another bird. We believe the beyond distinction. discrepancy in this case results simply from the size Variability in digestion rates within a given species, of larvae used: the o;,e undigested larva was large we believe, is due mainly to the state of the stomach (8 mm), whereas the other larvae were small (5 mm before experimentation. It appears that digestion is gone after 15 min, 6 mm after 20 min). Swanson and more rapid in starved birds. Thus, two spiders fed to Bartonek (1970) also found a relative consistency a starved bunting were gone from the gut after 10 in digestion rates. minutes, whereas they nomally remain at least 15 To arrive at correction factors for the various prey minutes in nonstarved individuals. Additionally, in types, we use “disappearance time,” which is defined feeding experiments utilizing starved longspurs, some here as the time period at the close of which a par- food items were totally unrecognizable after five ticular kind of item is barely detectable, with some minutes.
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