Food consumption and energy, water, and nitrogen budgets in captive great-horned owls (Bubo virginianus)

Comp. Biochem. Physiol., 1973, Vol. 44A, pp. 283 to 292. Pergamon Press. Printed in Great Britain

FOOD CONSUMPTION AND ENERGY, WATER, AND NITROGEN BUDGETS IN CAPTIVE GREAT-HORNED OWLS (BUBO VIRGINIANUS) G. E. DUKE,

J. G. CIGANEK

and 0. A. EVANSON

Department of Veterinary Physiology and Pharmacology, University of Minnesota, St. Paul, Minnesota 55101 (Receiwed 11 May 1972)

Abstract-l.

The metabolizability of diets of laboratory white mice and 1 -dayold domestic turkey poults by adult great-horned owls averaged about 67.9 and 71.2 per cent respectively. 2. The metabolizable energies of these two diets were 4304 and 4151 Cal/g respectively. 3. The owls ate about 26.5 g/kg (dry weight) of each diet per day. 4. The owls consumed an average of 4.4 per cent of their body weight in water per day. All water consumed was in their food. Sensible water losses amounted to slightly over 50 per cent of water consumed for both diets. INTRODUCTION

STUDIES on natural diets (e.g. Cunningham, 1960; Marti, 1969), pellet formation (Reed & Reed, 1928; Howard, 1958; Wilson & Niosi, 1961; Grimm & Whitehouse, 1963) and habits (e.g. Reed, 1925; Austing & Holt, 1966; Stewart, 1969) of the great-horned owl (Bubo airginiunus) are available, but little is known about the bioenergetics or water and nitrogen balances of this bird. Benedict & Fox (1927) determined the metabolic rate of one great-horned owl and metabolic rates have also been studied with other owls (e.g. Graber, 1962; Collins, 1963 ; Ligon, 1969; Gatehouse & Markham, 1970). Metabolic rate information alone, however, is not sufficient for the establishment of an energy budget. Bioenergetics information is available for long-eared (Asio ohs), shorteared (Asio jlammeus) and saw-whet owls (Aegolius acadicus) from the studies of Graber (1962). And, the metabolizability of natural foods has been determined for several species of wild birds (e.g. Kendeigh, 1949; Seibert, 1949; Davis, 1955; King & Farner, 19.56; Duke et al., 1968), but apparently not for owls. Water balance studies have been conducted on a few species of wild birds (e.g. Willoughby, 1968; Poulson, 1969; Anderson, 1970; Moldenhauer & Wiens, 1970; Ohmart et al., 1970; Carey & Morton, 1971) but not references to this type of study with owls can be found. Similarly, nitrogen balance studies on poultry are quite common (Mitchell, 1964) but, although a few studies are available on wild birds (e.g. Hill et al., 1968; Pendergast & Boag, 1971), none could be found which dealt with raptors.

283

284

G. E. DUKE, J. G. CIGANEKAND 0. A. EVANSON

The objectives of the present study were (1) to determine the average metabolizable energy available to great-horned owls from diets of turkey poults and mice, (2) to determine normal food consumption by these owls and (3) to gain some insight into the nitrogen and water balances of these owls. MATERIALS

AND METHODS

Four great-homed owls approximately 18 months old were used. These owls were raised in captivity from the age of about 2 months. They were fairly easy to handle and apparently very healthy. For about 5 months prior to this study, the owls were housed in a controlled environment room in which the temperature was maintained at 25-27”C, the relative humidity was kept at 45-50 per cent, and the photoperiod was adjusted weekly to simulate natural daylengths. During this S-month period and also subsequent to the experiments described herein, the owls were weighed frequently. The studies were performed within a S-week period in August and September 1971. Physiological changes associated with the annual cycle of the owls would probably not significantly affect the results of the study in this short period. The owls did not appear to be molting during the study. The owls were attached by jesses to an elevated perch and excreta and pellets were collected from sheets of plastic placed under them. Excreta and pellets were collected separately for each owl at the same time each day. Additionally, the plastic sheets were checked frequently during the day so that some “fresh” pellets and droppings could be collected and their wet weight determined. This was done so that the moisture content of these fresh excreta and pellets could be determined. Both fresh and daily samples were oven dried for 36-40 hr at approximately lOO”C, as recommended by Shannon & Brown (1969) for poultry excreta, and their dry weights were determined. Two diets were used during the tests, laboratory white mice (MUSmwculus) and 1 -day-old domestic turkey poults. The owls were fed one of the diets for 3-5 days prior to the start of collection of excreta and pellets. They were always given more than they chose to eat. All food was weighed before it was given to an owl and any parts of the food that were not eaten were weighed 24 hr later. The percentage weight loss due to 24 hr of desiccation of samples of the two diets was determined and the weight of the food not eaten was corrected for this factor. Thus, accurate records of food intake were obtained for each owl. Four mice and four poults were weighed, homogenized, dried and reweighed so that the moisture content of each diet could be determined. The owls were not given water to drink during the tests nor during any of the S-month period prior to the tests. However, freshly killed or fresh-frozen food was always provided for them. The dry weights of excreta and pellets were totaled for each day to determine total dry weight of waste materials. This total, and the calculated dry weight of the food consumed each day by each owl were used to calculate daily metabolizability coefficients for both diets for each owl using the following formula (Kleiber, 1961): total wt. of waste total wt of food

)Ix

100.

The caloric content of dried samples of mice and poults and of samples of excreta and pellets from each of these diets was also determined (Parr Oxygen Bomb Calorimeter, Model 1211) and the metabolizable energy of the food was calculated using the formula of Sibbald et al. (1960) : Metabolizable energy/g food = gross energy/g food-(non-metabolizability

coefficient

x gross

energy/g waste).

FOOD CONSUMPTION

IN GREAT-HORNED

285

OWLS

The energy metabolized per unit of time (e.g. kcal/kg per day) by the owls was determined by simply subtracting the gross energy output in excreta and pellets from the gross energy intake in food. This determination requires the assumption that body energy stores were not being metabolized unless the animals were losing weight. Slight weight gains were recorded for the owls during these tests (see beyond). The nitrogen content of dried samples of the diets and wastes was also determined using the Kjeldahl procedure. The weights of food and waste materials were determined on a Sartorius analytic balance (Model 2462). During the study period the owls were weighed daily on a Toledo Scale (Model 1070). The experiment in which the mouse diet was used involved the use of four owls in a S-day trial for a total of 20 owl days. In the experiment with poults, two owls were used for 5 days and two others were used for 8 days for a total of 26 owl days.

RESULTS

The body weights of the owls were fairly stable during the trials (Table 1). However, the weight of individual owls was observed to vary as much as 30 g in one 24-hr period. This variation was probably determined by how much the owl had eaten or whether it had caste a pellet just before it was weighed. The stable TABLE

1. AVERAGE

DAILY BODY WEIGHTS (g) OF GREAT-HORNED MICE OR POULTS

OWLS

No.

Average

S.D.

Range

No. of weighings

1 2 3 4

1629 1409 1630 1791

32.6 19.3 33.6 5.4

1590-1680 1380-1450 1580-1670 1780-1800

13 13 10 10

Owl

ON DIETS OF

Mean = 1615.

weights of the owls, their general appearance and their food consumption all indicated that they were healthy during these studies. The average body weights of the owls at 3 months prior to this study and at 3 months after were 1560 and 1685 g respectively. During this study their body weights averaged 1615 g indicating a gain of about O-7 g/day during this 6-month period. The average metabolizability coefficient was higher for the poult diet than for the mouse diet (Table 2). This higher digestibility was also apparent in the significantly lower weight of pellets (indigestible material) formed on the poult diet. However, as the owls often did not eat the heads of the poults, this would tend to slightly increase the digestibility of the poult and to slightly decrease the amount of indigestible material in the pellet. Approximately one pellet per day was caste with both diets (Table 2). The rate of one pellet per day agrees with most of the reports on pellet formation in great-horned owls (see review in Balgooyen, 1971).

G. E. DUKE,J. G. CIGANEKAND0. A. EVANSON

286 TABLE

~-FOOD

CONSUMPTION,

WASTE

OUTPUT,

AND

METABOLIZABILITY

INFORMATION

FOR

GREAT-HORNED OWLS ON DIETS OF MICE OR POULTS

Dry food intake (g/kg Per day)

Diet

Average Dry pellet metaboliz. Average No. Average No. output coefficient pellets cecals per day (g/kg Per day) Per day (%) per day

Dry excreta output (g/kg Per day)

Mice *

d S N

26.58 4.08 20

6.02 1.88 20

2.83-j. 2.01 20

67.88 9.30 20

1 .oo 0.69 181

1.20 0.77 20

Poults§

f S N

26.37 6.33 26

6.51 1.36 26

O-86? 0.67 26

71.21 6.54 26

0.84 0.62 26

1.36 0.49 26

$ One-day-old turkey poults. 3 Mean. S Standard * Laboratory white mice. N No. of owl days of experiments. deviation. t Means are significantly different at greater than the 1 per cent level (t = 4.606). (Other means were not significantly different.) 1 Pellets from one owl were broken so the number of pellets present could not be determined on two separate days. Excreta which we believed to be of cecal origin occurred about once daily (Table 2). These droppings looked very much like the cecal droppings of chickens, turkeys and pheasants (Duke, unpublished observations). There are apparently no other references available on the identification of cecal excreta in raptors. The dry matter consumed and excreted was nearly equal on the two diets (Table 2); however, the metabolizable energy of the mouse diet was slightly greater than that of the poult diet (Table 3). Thus, even though the owls ate similar amounts of each diet and the metabolizability of the poult diet was higher TABLE

3-METABOLIZABLE

NITROGEN

ENERGIES

(M.E.)

OF MOUSE

AND

POULT

DIBTS AND

AND CALORIC CONTENTS IN THE TWO DIETS AND IN EXCRETA AND PELLETS FROM THE DIETS FOR GREAT-HORNED OWLS

Total nitrogen (mg/g) I

Caloric content (Cal/g)

Range

N

3

Range

N

Mouse diet Excreta Pellets M.E.

79.75 17859 81.10 -

78.92-80.58 151.95-205.22 68.8493-36 -

2 2 2 -

6272 2439 3686 4304

6260-6284 2436-2441 3677-3695 -

2 2 2 -

Poult diet Excreta Pellets M.E.

96.55 178.53 116.14 -

95.64-97.45 176.79-180.26 113.22-119.06 -

2 2 2 -

6398 3139 4665 4151

6377-6419 3116-3161 4663-4667 -

2 2 2 -

287

FOOD CONSUMPTION IN GREAT-HORNED OWLS

than that of the mouse diet, the higher metabolizable energy of the mouse diet allowed the owls to obtain similar amounts of energy per g of food from the two diets. This conforms with previous studies on poultry which have indicated that feed consumption is, in part, regulated by the energy content of feeds (Gleaves et al., 1968). The total nitrogen per g in the mice and in the pellets from the mouse diet was less than in the poults and in the pellets from the poult diet (Table 3). The total nitrogen per g in the excreta was essentially the same from the two diets (Table 3). This would be expected in an adult homeostatic organism. The nitrogen intake with the mouse diet was 2.12 g/kg per day and the loss in excreta and pellets from the mouse diet was 1.30 g/kg per day. For the poult diet, nitrogen intake was 2.55 g/kg per day and loss was 1.25 g/kg per day. Nitrogen retention was greater with the poult diet. Knowing the average proportion of wet to dry mass in food, excreta or pellets (Table 4) and the average dry food intake or dry waste output (Table 2), one can TABLE ~-AVERAGE

WATER CONTENT (%)

OF FOODS AND WASTE MATERIALS OF GREAT-

HORNED OWLS ON DIETS OF MICE OR POULTS Average Diet White Turkey * Numberlin

Food

mice

62.2

poults

Excreta

(4)*

66.9 (4)

parentheses

y0 water

is the number

Pellets

76.6

(43)

61.3

(8)

808

(41)

62.9

(5)

of fresh samples

dried

and weighed.

calculate the average water intake with food and the average water loss with wastes, i.e. sensible loss (Table 5). The difference between water intake and sensible loss provides an estimate of the insensible loss. Sensible losses were 55.4 and 54.2 per cent of the water from the mouse and poult diets, respectively (Table 5). TABLE s--CALCULATED

AVERAGEWATER INTAKE AND LOSS FROM FOUR GREAT-HORNED

OWLS ON DIETS OF LABORATORY WHITE MICE AND l-DAY-OLD

TURKEY POULTS Calculated

Water Water Diet

(g/kg

intake per day)

loss

in excreta (g/kg

per day)

Water

loss

in pellets (g/kg

per day)

Total

sensible

water loss (g/kg

per day)

insensible water loss (g/kg

per day)

Mice

43.7

19.7

4.5

24.2

19.5

Poults

53.3

27.4

1.5

28.9

24.4

The calculation of the average water intake with food may be slightly in error if the food is not eaten immediately after it is weighed and presented to the owls. However the weight losses due to desiccation for 24 hr in the room in which the

288

G. E. DUKE, J. G. CIGANEK AND 0. A. EVANSON

owls were kept were 5.2 and 4.3 per cent for samples of the mouse and poult diets, respectively. Normally the food was eaten within 1-2 hr after it was presented. In view of this and the small loss due to desiccation, the error would be minimal

DISCUSSION

A higher metabolizability coefficient for the poult diet and a lower mass of pellets from that diet were evidence that poults were more digestible by owls than mice. Even though the heads of the poults were not always eaten, young poults were probably more digestible (including their heads) than adult mice because poults have less calcified bones. However, because avian bones are hollow and birds lack teeth, one would expect that a greater proportion of the mass of any bird would be digestible than of a comparable mass of mammal. For this reason one should be cautious in comparing physiological processes between birds and mammals on a “per g body weight” basis. Graber (1962) determined that the actual metabolized energy for two longeared owls under aviary conditions and eating M. musculus was 334 and 355 kcal/kg per day. By measuring gross energy intake per day and subtracting from it the energy lost through pellets and excreta in a day, he calculated metabolized energy to be about 87 per cent of gross energy intake. In the present study, the metabolized energy for great-horned owls eating M. musculus was 141.9 kcal/ kg per day or about 85 per cent of the energy consumed. The percentage of the gross energy metabolized by the two species of owls was very similar. Probably the large difference in estimated total daily metabolized energy between the two species of owls in the two studies was due to (1) differences in the activity levels of the owls during the two studies, (2) differences in the size (and thus the metabolic rates of the two species), (3) d i ff erent environmental temperatures during the two studies (1%19°C for the long-eared owls) or (4) interspecific variation. Factors such as these must be considered in an evaluation of the results of any study of animal energetics. The data obtained in the present study on inactive owls in a constant environment can, at best, be only a basis from which to estimate enerThese data certainly do not represent natural getics under natural conditions. conditions. For long-eared owls eating M. muscuZus, the caloric content of the pellets averaged 3159 Cal/g and of the excreta averaged 2739 Cal/g (Graber, 1962). These values are comparable to those reported herein (Table 3). Previously reported caloric values for small rodents differ, however, from those reported herein. Golley (1960) found that the average caloric content for three adult Microtus pennsylvanicus was 4597 Cal/g. Graber (1962) reported the following energy values for five species of small rodents : Blarina brevicauda-4340 Cal/g; Peromyscus maniculatus-4490 Cal/g; Peromyscus leucopus~321 Cal/g; Microtus ochrogaster4348 Cal/g; Mus musculus-3945 Cal/g. This difference cannot be explained. A lower total nitrogen content was found in mouse tissue than in the tissues of poults (Table 3). This may be because the adult mouse had more body fat than the

FOODCONSUMPTION IN GREAT-HORNED OWLS

289

l-day-old poults and thus the mice had a lower proportion of total body nitrogen. However, this is unlikely since the Cal/g of mice and poults were about equal. Pellets from the poult diet were found to have a higher nitrogen content than pellets from the mouse diet (Table 3). This might be expected since the poult was found to have more nitrogen in its tissues than the mouse. This can also be explained if we assume that pellets are mainly composed of bones and hair or feathers and that poult bones are more digestible than mouse bones. Then, feathers (which have a high nitrogen concentration) make up a greater proportion of the poult-diet pellet than hair (which also has a high nitrogen concentration) does in the mousediet pellet. Perhaps the Cal/g of the poult-diet pellet is also higher than that of the mouse-diet pellet because of the higher nitrogen concentration and lower mineral concentration of the poult-diet pellet. The normal daily water requirement of great-horned owls appears to be quite low as compared to other land birds. Great-horned owls in this study consumed the equivalent of 4.4 and 5.3 per cent of their body weight daily as moisture from the mouse and poult diets, respectively. Bartholomew & Cade (1963) list the mean water consumption as a percentage of body weight for twenty-one species of land birds. Only one species listed by them drank (ad lib.) less than the equivalent of 5 per cent of its body weight per day. California quail (Lophortyx californicus) and Gambel’s quail (Lophortyx gambelii) drank an average of 3*-S-4*1 per cent of their body weight (163 and 148 g respectively) daily (Carey & Morton, 1971). And road runners (Geococcyx californianus) consumed an average of 6.6 per cent of their body weight (290 g average) in moisture in their food daily (Ohmart et al., 1970). These are species living primarily in arid or somewhat arid environments. Greathorned owls live in arid environments and although the environment in which the owls were housed for the present experiments was not “arid”, the low water requirements of the birds in the present experiments would indicate that they may be well adapted for living with a restricted water intake. The data obtained in this study can be used to formulate an energy and nutrient budget. For example, with the mouse diet, the following data in Table 6 applies for 1 day. TABLE 6-SAMPLE DAILYENERGY,WATER,AND NUTRIENTBUDGETFOR GREAT-HORNED OWLS ON A MOUSEDIET Intake (kg) 26.6 g dry matter 166.8 kcal 2.1 g N 43.7 ml H,O

Output (kg) 2.8 6.0 10.3 14.6 0.2 1-l 4.5 19.7

g dry pellets g dry excreta kcal, pellets kcal, excreta g N, pellets g N, excreta ml, pellets ml, excreta

Difference 17.8 g/kg 141.9 kcal/kg 0.8 g N/kg 19.5 ml/kg

290

G. E. DUKE, J. G. CIGANEKAND 0. A. EVANSON

The dry food metabolized/kg per day was 178 g on the average. The metabolized energy, 141.9 kcal/kg per day, represents the average metabolic level of the owls during these studies. Benedict & Fox (1927) estimated the resting metabolic rate of a great-horned owl to be only 74 kcal/kg per day. During the present study, however, the owls were eating food and, while their activity level was much less than that of free-flying owls, they were definitely more active than the resting state. The budget above indicates a positive nitrogen balance. Although the owls in this study were gaining weight at the rate of 0.7 g/day on the average, this was probably mostly body fat increase. Reed (1925) found that the body weights of great-horned owls she raised stabilized at about 13-14 weeks of age and remained so until 58 weeks of age. Assuming that adult great-horned owls have about the same concentration of nitrogen in their tissues as turkey poults (O-097 g/g body weight) then the owls would average a total of about 157 g of nitrogen per owl. So, the average of 08 g of nitrogen retained per owl per day represents only a O-5 per cent increase in the total nitrogen in the owl tissues. Therefore, the amount of nitrogen retention found in this study period was fairly small and probably peculiar to this study period. Subsequent periods may have shown similar nitrogen losses since consistent positive nitrogen balance is only characteristic of growing animals. The period covered in this study was probably too short to accurately assess nitrogen balance. The difference between the water consumed and lost, 19.5 ml/kg per day, represents primarily insensible water loss since net assimilation of water in a nongrowing animal would be close to zero. SUMMARY

Studies on food, energy, water and nitrogen intakes and losses were conducted on four adult great-horned owls. The owls were housed in a controlled environment room. They were attached by jesses to perches and they were fed diets of either white mice or l-day-old turkey poults. They were not given drinking water. The body weights of the owls during this study averaged 1615 g. They ate an average of about 26.5 g/kg per day (dry weight) of either diet. The metabolizability of the poult diet, 71.2 per cent, was slightly higher than that of the mouse diet, 67.9 per cent, but the metabolizable energy of the mouse diet, 4304 Cal/g, was higher than that of the poult diet, viz. 4151 Cal/g. Similar amounts of energy were therefore available to the owls on both diets. On the mouse diet, the owls consumed an average of about 167 kcal/kg per day. They lost about 10 kcal/kg per day in the pellets and about 15 kcal/kg per day in the excreta. Thus, their metabolized energy was about 142 kcal/kg per day. Daily water consumption for the owls was about 44 and 53 ml/kg per day for the mouse and poult diets, respectively, or about 4.4 and 5.3 per cent of their body weight. Total sensible losses amounted to about 24 and 29 ml/kg per day for the mouse and poult diets. The total nitrogen concentrations in the mouse diet and the pellets from that diet were lower than in the poults and the pellets from the poult diet. The total

FOODCONSUMPTION IN GREAT-HORNED OWLS

291

nitrogen concentrations in the excreta were about the same for the two diets indicating a fairly constant nitrogen excretion rate. Since nitrogen intake in the two diets exceeded nitrogen losses in pellets and excreta for the two diets, the owls were in a positive nitrogen balance during this study. However, since they were not growing, this positive balance was believed to be coincidental. Acknowledgenzents-The assistance of Dr. R. D. Goodrich and Mr. J. G. Linn of the Animal Science Department, University of Minnesota, St. Paul, Minn. in the analyses for caloric and nitrogen values was greatly appreciated. REFERENCES ANDERSONS. H. (1970) Water balance of the Oregon junco. Auk 87, 161-163. AUSTINGG. R. & HOLT J. B., JR. (1966) The World of the Great Horned Owl, p. 158. J. B. Lippincott, Philadelphia. BALGOOYENT. G. (1971) Pellet regurgitation by captive sparrow hawks (F&o sparverius). Condor 73, 382-385. BARTHOLOMEW G. A. & CADE T. J. (1963) The water economy of land birds. Auk 80, 504-539. BENEDICTF. G. & FOX E. L. (1927) The gaseous metabolism of large wild birds under aviary life. Proc. Am. Phil. Sot. 66, 511-534. CAREYC. & MORTONM. L. (1971) A comparison of salt and water regulation in California quail (Lophortyx californicus) and Gambel’s quail (Lophortyx gambelii). Comp. Biochem. Physiol. 39A, 75-101. COLLINS C. T. (1963) Notes on the feeding behavior, metabolism, and weight of the sawwhet owl. Condor 65, 528-530. CUNNINGHAM J. D. (1960) Food habits of the homed and barn owls. Condor 62, 222. DAVIS E. A., JR. (1955) Seasonal changes in the energy balance of the English sparrow. Auk 72, 385-411. DUKE G. E., P~TRIDESG. A. & RINGERR. K. (1968) Chromium-51 in food metabolizability and passage rate studies with the ring-necked pheasant. Poult. Sci. 47, 1356-1364. GATEHOUSES. N. & MARKHAMB. J. (1970) Respiratory metabolism of three species of raptors. Auk 87, 738-741. GLEAVESE. W., TONKINSONL. V., WOLF J. D., HARMANC. K., THAYERR. H. & MORRISON R. D. (1968) The action and interaction of physiological food intake regulators in the laying hen. Poult. Sci. 47, 38-67. GOLLEY F. B. (1960) Energy dynamics of a food chain of an old field community. Ecol. Monogr. 30, 187-206. GRABERR. R. (1962) Food and oxygen consumption in three species of owls (Strigidae). Condor 64, 473-487. GRIMM R. J. & WHITEHOUSEW. M. (1963) Pellet formation in a great horned owl: a roentgenographic study. Auk 80, 301-306. HILL D. C., EVANSE. V. & LUMSDIINH. G. (1968) Metabolizable energy of aspen flower buds for captive ruffed grouse. J. Wildl. Mffmt. 32, 854-858. HOWARDW. E. (1958) Food intake and pellet formation of a horned owl. Wilson Bull. 70, 145-150. KIINDEIGHS. C. (1949) Effect of temperature and season on the energy resources of the English sparrow. Auk 66, 113-127. KING J. R. & FAIINERD. S. (1956) Bioenergetic basis of light-induced fat deposition in the white-crowned sparrow. Proc. Sot. exp. Biol. Med. 93, 354-359. KLEIBER M. (1961) The fire of life. In An Introduction to Animal Energetics, p. 454. Wiley, New York.

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G. E. DUKE, J. G. CIGANEKAND0. A. EVANSON

LIGON J. D. (1969) Some aspects of temperature relations in small owls. Auk 86, 458-472. MARTI C. D. (1969) Some comparisons of the feeding ecology of four owls in north central Colorado. Southeuest. Nut. 14, 163-170. MITCHELL H. H. (1964) Comparative Nutrition of Man and Domestic Animals, p. 840. Academic Press, New York. MOLDENHAWRR. R. & WIENS J. A. (1970) The water economy of the sage sparrow, Amphispiza belli nevadensis. Condor 72, 265-275. OHMARTR. D., CHAPMANT. E. & MCFARLANDL. 2. (1970) Water turnover in roadrunners under different environmental conditions. Auk 87, 787-793. PENDERGAST B. A. & BOAC D. A. (1971) Nutritional aspects of the diet of spruce grouse in central Alberta. Condor 73, 43743. POULSONT. L. (1969) Salt and water balance in seaside and sharp-tailed sparrow. Auk 86, 473-489. REED B. P. (1925) Growth, development, and reactions of young great horned owls. Auk 42, 14-31. REED C. I. & REED B. P. (1928) The mechanism of pellet formation in the great horned owl. Science, N.Y. 68, 359-360. SEIBERTH. C. (1949) Difference between migrant and non-migrant birds in food and water intake at various temperatures and photoperiods. Auk 66, 128-153. SHANNOND. W. F. & BROWNW. 0. (1969) Losses of energy and nitrogen on drying poultry excreta. Poult. Sci. 48, 41-43. SIBBALDI. R., SUMMERSJ. D. & SLINGERS. J. (1960) Factors affecting the metabolizable energy content of poultry feeds. Poult. Sci. 39, 544-556. STEWARTP. S. (1969) Movements, population fluctuations, and mortality among great horned owls. Wilson Bull. 81, 155-162. WILLOUGHBYE. J. (1968) Water economy of the Stark’s lark and gray-backed finch-lark from the Namib Desert of Southwest Africa. Comp. Biochem. Physiol. 27, 723-745. WILSON F. H. & NIOSI P. N. (1961) Some observations on gastric digestion in the horned owl. Am. Zoologist. 1, 399 (Abstr.). Key Word Index-Owl of owls.

metabolism; Bubo virginianus;

food consumption;

water budget