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Food consumption, energy, nutrient and mineral balances in a eurasian kestrel and a screech owl
Camp. Biochem. Physiol. Vol. 83A, No. 2, pp. 249-254,
1986
0300-9629/86$3.00+ 0.00 c 1986Pergamon Press Ltd
Printed in Great Britain
FOOD CONSUMPTION, ENERGY, NUTRIENT AND MINERAL BALANCES IN A EURASIAN KESTREL AND A SCREECH OWL ERICK G. CAMPBELL* and JAMES R. KOPLIN Department
of Wildlife Management, College of Natural Resources, Humboldt CA 85521, USA. Telephone: 707-826-3658 (Receioed
21 May
State University,
Arcata,
1985)
Abstract-l. The food consumption, energy, nitrogen, fat, ash, calcium, phosphate and magnesium balances of the Eurasian kestrel (F&o finnunculus), and screech owl (Olus kennicotri) are compared. 2. Eurasian kestrel sample values were significantly greater than screech owl sample values in the energy, nitrogen, calcium, phosphate and magnesium content in pellets, and calcium content in excrement. Phosphate content in excrement was significantly greater in screech owl samples than in Eurasian kestrel samples. 3. The Eurasian kestrel ingested significantly greater amounts of calcium and phosphate, egested via excrement significantly greater amounts of ash. excreted or egested significantly greater amounts of energy, nitrogen, calcium and phosphate via excrement and pellets, and excreted significantly greater amounts of magnesium via excrement than did the screech owl. The screech owl assimilated significantly greater amounts of ash than did the Eurasian kestrel. 4. The metabolizability coefficient averaged 48.32% for the Eurasian kestrel, and 49.62% for the screech owl.
INTRODUCTION Studies on life histories (Audubon, ca, 1840; Bent, 1937, 1938), natural food habits (Craighead and Craighead, 1956; Cunningham, 1960; Marti, 1969; Earhart and Johnson, 1970; and many others), gastric digestion (Wilson and Niosi, 1961; Duke et al., 1975), pellet formation (Reed and Reed, 1928; Howard, 1958; Grimm and Whitehouse, 1963; Yapp, 1969; Balgooyen, 1971; Smith and Richmond, 1972; Marti, 1973), ecology (McGahan, 1968; Coulombe, 1971; Smith et al., 1974; Balgooyen, 1976; and many others), avian urine (Willoughby, 1970) and energy metabolism (Benedict and Fox, 1927; Giaja and Males, 1928; Herzog, 1930; Wing and Wing, 1939; Graber, 1962; Collins, 1963; Lasiewiski and Dawson, 1967; Ligon, 1969; Coulombe, 1970; Gatehouse and Markham, 1970; Gessamen, 1972; Duke et al., 1973; Calder and King, 1974; Kirkwood, 1979, 1980; Koplin et al., 1980; Cooper and Greenwood, 1981; Hamilton and Neill, 1981; Stalmaster and Gessamen, 1982) are available for raptors. Studies on general avian physiology (Sturkie, 1954; Marshall, 1963) are available, but the literature is notable for its scarcity of information on the digestive physiology and nutrient requirements of carnivorous vertebrates, particularly the birds of prey. Duke (1978) reviewed existing information on raptor physiology. Research on the nutrient requirements (nitrogen, fat, ash, magnesium, calcium and phosphorus) of domestic poultry is extensive (National Academy of Sciences, 1971; and many others), but few studies have dealt with raptors (Duke et al., 1973; Kirkwood, 1981; Koplin, unpublished data). Hawks and owls represent an example of convergent evolution. Investigators have studied several *Present post and address: District Biologist, Bureau of Land Management, (602) 428-4040.
Safford, AZ 85546, USA. Telephone:
aspects of digestive physiology of these orders of birds. Clark (1972) compared the osseous material assimilated by the short-eared owl (Asia jfammeus) and marsh hawk (Circus cyaneus). Gatehouse and Markham (1970) compared the metabolism of hawks and owls and determined that nocturnal owls had higher metabolic rates during night and that hawks had higher metabolic rates during daylight. Kirkwood (1979) compared the existence metabolism of the Eurasian kestrel (F&o tinnunculus) and the barn owl (Tyto ulbu). Koplin (unpublished data) investigated differential ash assimilation between these two orders. The objective of this study was to determine differences in energy, nitrogen, fat, ash, calcium, phosphate and magnesium balances between the two similarly sized representatives of the orders Falconiformes and Strigiformes, viz., Eurasian kestrel and screech owl (Otus kennicotti). MATERIALS AND METHODS Two similarly sized individuals, one male Eurasian kestrel and one screech owl, sex undetermined, were used in this study, Other specimens were not available during the course of this study. The Eurasian kestrel was an adult at least 3 years old and the screech owl was in its first year, but in an apparent adult condition when the study commenced. This study was conducted from 29 October 1974 to 29 January 1976. Ten trials quantifying the ingesta (food consumption) and egesta (egestdd pellkts -and excrement) were conducted on each individual. The Eurasian kestrel was fed for a total of 73 days and the screech owl for 87 days. A modified “nutritional balance technique” (Kendeigh, 1949) was used in this study. Digestion trials on each bird were conducted for 6 days or until the body weight of the bird was within + I% of its body weight at the beginning of the digestion trial. Since we were interested only in the comparative nutritional aspects of nongrowing, nonmolting, nonreproducing, weight-stable birds, digestion trials were conducted during 249
ERICK G. CAMPBELL and JAMES R. KOPLIN
250
the late fall and early winter at a time of the year when north temperate birds normally are not molting, are reproductively quiescent, and birds of the year have attained adult body size. The raptors were housed in separate flight rooms (3 x 3 x 3 m) at prevailing outside ambient temperatures and photoperiods in Arcata, Humboldt County, California (appoximately 40 52’ latitude). A hygrothermograph was placed in each enclosed flight room to record air temperatures and relative humidity. The birds were acclimated to the flight room and a diet of ground rdt (Rur~us norw~icu.r) (minus stomach and intestines) fed twice daily for 72 hr prior to the beginning of each digestion trial. The food was offered in a fresh condition, and no free water was supplied. Birds were fed whatever prey was available between dtgestion trials. During the acclimation period the weight of each bird was stabilized by controlled feeding at the maintenance level ( + 0.1 g). The birds were weighed ( + 0. I g) at the begtnning of each trial, every other day thereafter, and at the termination of the trial. All weighings occurred at the same time of day. Weight was lost due to human handling at the onset of a number of trials, but was regained before termination of the trial. Heavjy plastic was used to cover the floor and lower walls of the enclosed flight rooms to facilitate collection of pellets and excrement egested during each digestion trial. Pellets were collected daily, and excrement was collected at the termination of each trial. An aliquot of the homogenized rat diet was dried for chemical analysis. All samples of egested pellets and excrement and the homogenized rat were dried in a drying oven at 55 C for 48 hr. Weights of dried pellets and excrement were measured to the nearest 0.01 g on a Metlcr analytical balance. Gross energy (Parr adiabatic calorimeter), nitrogen (macro Kjeldahl), fat (anhydrous ether extraction), and ash (Parr adiabatic calorimeter byproduct) content of dried samples of egested pellets and excrement, and of homogenized rat were measured according to methods of the Association of Official Agricultural Chemists (Horwitz, 1965). Remaining sample materials were forwarded to Chem-met Associates (San Diego, California) for determination of magnesium, calcium and phosphate content according to methods of the American Public Health Association (1956). No attempt was made to correct for nitrogen lost from excrement, pellets or food during the drying process. Manoukas r/ ui. (1964) found that nitrogen lost from poultry excrement during the drying process ranged from ~ 7. I to I5.2”,, with a mean of 5.45%. Shannon and Brown ( 1969) reported nitrogen losses of 5.05 and 4.17% for two samples of chicken excrement dried at 60°C. Equipment for making wet sample analyses was not available during this study nor has a universal correction factor been developed to account for these losses, but, since wastes were handled similarly for the two species, similar losses probably occurred. Nutrient balance (retention) values of the digestion trials were determined by the following formula (Gessamen, 1973): A=I-E, where A = assimilated component, I = ingested component, and E = egested component. For example: A.E. = G.E.I. - P.E. - E.E., A.E. = assimilated energy, G.E.I. = gross energy intake, P.E. = pellet energy and E.E. = excrement energy.
where
These determinations assumed that body nutrient reserves were not being metabolized and body weights were stable during the trials. The assimilation coefficients over the whole study were computed using the following formula from
Table I. Companwn excrement
of mean dally weights of food ingested and and pellets egested by a Eurasian kestrel and a screech owl* Eurasian
Food consumed Mean dry weight &kg body wt day Excrement egeated Mean dry weight @kg body wt day) Pellets egested Mean dry weight @kg body wt day)
kestrel
Screech owl
58.1X”
50.89”
?h.?lh
?I 3Oh
3 Hh
4.34
*Values followed by the same letters are significantly difierent (P 5 0.05); all other values are not significantly different.
Kleiber
(1961):
metabolizability
coefficient
=
total wt wastes
-~~~--~
total wt food
x 100. >
Data for each trial were compared between the falcon vs the owl, using Student’s /-tests. All data were scaled to weight or energy per kilogram of body weight. RESULTS
The body weights of the Eurasian kestrel and screech owl averaged 178.8 g and 169.0 g, respectively, for the IO trials. The mean ambient temperature averaged 12.03 ‘C for the Eurasian kestrel trials and 1 I .63 ‘C for the screech owl trials. Temperature differences are the result of trials being conducted during the same time period, but not necessarily on the same days. The mean dry weights of pellets egested daily by the falcon and owl were not statistically significantly different, although the falcon consumed significantly more food and eliminated significantly more excrement daily than did the screech owl (Table I). Metabolizability coefficients averaged 48.32% for the Eurasian kestrel, and 49.62?/, for the screech owl. Significant differences were measured in the nutrient content of the following sample values: energy content of the pellets, nitrogen content of the pellets, calcium content of the excrement, calcium content of the pellets, phosphate content of the excrement, phosphate content of the pellets and magnesium content of the pellets of the falcon and owl (Table 2). All other samples were not statistically significant (Table 2). The mean daily energy and nitrogen ingested and mean energy and nitrogen assimilated by the Eurasian kestrel and screech owl were not statistically significant; however, the Eurasian kestrel excreted significantly more energy and nitrogen daily via excrement and pellets than did the screech owl (Table 3). The mean daily amounts of fat ingested, fat excreted in excrement and pellets, and fat assimilated by the kestrel were higher than that of the owl, but not statistically significant (Table 3). The mean daily ash ingested by the kestrel was higher than that of the owl; whereas, the mean daily ash egested in the pellets by the owl was higher than that of the kestrel (Table 3). Neither was statistically significant. However, the mean daily ash egested by the kestrel was significantly higher than that of the owl, but the mean daily ash assimilated by the owl
Food consumption in kestrel and owl Table
2. Mean energy and
excrement,
and pellets
nutrient
Sample
content
of samples
Eurasian kestrel
of a Food
Excrement Eurasian
Energy
contentt
94.91
Pellets
5.145” 125.63”
181.16
Fat:
242.0
55.6
25.1
Ash:
113.4
194 6
281.8
3.7
4 6’
4.7”
53.3
58.8’
78.1’
Calcium: Phosphate’: hlagneslum$
I.28
1.2
I.1
Screech Energy
contentt
5.959
hitrogen:
owl 3.877”
3.004
95.14
188.80
93.0lh
Fat:
249.0
60.3
14.8
Ash:
116.2
180.7
290.3
Calcium: Phosphatef
3.5
4.0’
51.2
61.2’
Magnesiumt: ‘Values
I.1
followed
between
the hawk
significantly tin
kcal/g
iln
mg/g
by
dry
food,
kestrel 3.064
5.975
Nitrogenf
of
and a screech owl*
the
3.7d 54.5’
I.0
same
and owl
letters (P I
are
0.05);
I .O” significantly
all other
different
values
are not
diKerent. wt.
dry wt.
was significantly higher than that of the kestrel (Table 3). The mean daily amounts of calcium and phosphate ingested and egested via excrement and pellets were significantly higher in the kestrel than that of the owl (Table 3). The differences in the daily amounts of calcium and phosphate assimilated by the kestrel and the owl were not statistically significant (Table 3). The mean daily amounts of magnesium ingested and excreted via pellets by the kestrel were higher than those of the owl, but the owl assimilated more mean daily magnesium (Table 3). None of the previous differences were significant. The mean daily magnesium excreted via excrement was significantly higher for the kestrel (Table 3).
DISCUSSION
Information from this study indicates that the falcon digested its food more poorly than did the owl; Table
3. Mean
and oellets
daily
quantities
and of enerw
Nutrient
of energy
and nutrients
251
that is, the kestrel consumed more food and produced more pellet matter (Table I), removed less energy from its food (Table 2), removed less nitrogen from its food (Table 2), and had a lower metabolizability coefficient (48.32 vs 49.62%) than did the owl. The energy assimilated-i.e. metabolic energy (Kendeigh, 1949)-by the falcon was also not significantly higher than that of the owl (Table 3), suggesting that at least some of the higher rate of food intake of the falcon compared to that of the owl must be related to a higher metabolic rate of the falcon. Such a conclusion is supported by studies on basal or standard rates of metabolism of falconiforms and strigiforms (Zar, 1968). The mean dry weights of food ingested and excrement and pellets egested by the Eurasian kestrel and the screech owl in the present study were similar to those obtained by Kirkwood (1979) for the Eurasian kestrel and barn owl, but were greater than similar values measured by Duke et al. (1973) for the greathorned owl (Bubo virgin&us). Duke et al. (1975) found that food intake was less in the owls and buteos than it was in the falcons and the bald eagle (Haliaectus leucocephalus). The metabolizability coefficients of the greathorned owl averaged 67.88% for the mouse diet, and 71.21% for the poult diet (Duke et al., 1973). Kirkwood’s (1979) metabolizability coefficients averaged 51.27% for the Eurasian kestrel, and 54.36y0 for the barn owl. Metabolizability coefficients averaged much lower in this study: 48.32% for the Eurasian kestrel, and 49.62% for the screech owl. The differences in mean dry weights of food ingested, excrement and pellets eliminated, and metabolizability coefficients between the various raptors could be explained by: (1) interspecific variation; (2) differences in body size; (3) differing environmental temperatures or experimental technique between studies, and/or (4) differing levels of physical activity. The major factor involved in differences in food, excrement and pellets is probably the tremendous size difference between the kestrel, barn owl or screech owl on the one hand, and the great-horned owl
and nutrients metabolized
Food
Excrement
ingested
egested
in ingested
food
by a Eurasian
egested
347.45 5.524.98
4,717.w
483.9@
14,1 19.2
1.467.7
100.0
6.58
Phosphatej
3.07’
Nitrogen:
64.05”
Fatf
I
12.712.7 5.86
Ash:
179.0”
Calciumff
2.57’
PhosohateT
56.2
Magnesiu&j *Values
followed
by the same letters
tin
not significantly different. kcal/(kg body wt day).
$In
mg/(kg
#In pg/(kg
body body
wt day). wt day).
I .23
4.6
302.96 4.848.3
76.5
0.30’
1.54’ 30.5” Screech
Energyt
0.33’
18.5’
61.5
Magnesium5
323.42 12.551.6
I .09
5.16’
1l9.0h
214.0”
245.58
l9.94h
81.93”
Energy’r
Ash:
Metabolized _
kestrel
Nitrogen:
Cdlcium$
and a screech owl’
Pellets
Eurasian
Fat:
and egested excrement
kestrel
owl 16.42h
4.038.49’
398.39d
I ,306.9
69.0
222 50 411.44 11,336.9 0.76’
I .27
3.83’
78.8
16.1’
84.5h
I .02
0.24’
1.3oL
4.4
21.2m are significantly
26.4
different
(P < 0.05);
30.7 all other
values
are
252
ERICK G. CAMPBELLand JAMESR. KOPLIN
( 1,615 g) on the other. Since the surface-volume ratio of the great horned owl is relatively small compared to that of the falcon and screech owl, it is to be expected that the great-horned owl would ingest relatively less food and eliminate relatively less excrement and pellets than the falcon and screech owl. However, we are unable to account for the differences in metabolizability coefficients between the greathorned owl and the falcon, barn owl and screech owl.
by an average of 1415% for the Eurasian kestrel, and by an average of 25-26% for the screech owl. Studies by Kirkwood (1979) found the Eurasian kestrel and barn owl exceeded Kendeigh’s (1970) equation relating existence metabolism to body weight of nonpasserine birds at 30°C by 28 and 16x, respectively. The birds in this study probably became progressively less physically active the longer they were maintained in captivity.
Energy
Nitrogen
The mean daily quantities of energy ingested, excreted via excrement and pellets, and metabolized was lower in Kirkwood’s (1979) study than they were in this study. The Eurasian kestrel, in both studies, ingested and metabolized more energy per unit of body weight than did the owls. The efficiency of energy assimilation ((gross energy-excrement energy-pellet energy)/gross energy) averaged 87% for the long-eared owl (Ask otus) (Graber. 1962) 85% for the great-horned owl (Duke et al., 1973) 72.54O/, for the barn owl (Kirkwood, 1979) 74.1x for the broad-winged hawk (Mosher and Matray, 1974) 71.327: for the Eurasian kestrel (Kirkwood, 1979) compared to 70.5% for the Eurasian kestrel and 73.2% for the screech owl found in this study. The birds in this study used progressively less energy daily as the study progressed. The birds apparently became less physically active the longer they were maintained in captivity and/or as they grew older their metabolic rates decreased.
Protein retention and storage have been reported for the chicken (Fisher, 1967), but not birds of prey. Two digestion trials conducted during this study resulted in negative metabolism values (i.e. more nitrogen was excreted than was ingested), suggesting that raptors, like chickens, also may store protein. The percentage of nitrogen in domestic rats used in this study was comparable to that for rats measured by Bird and Ho (1976) values only slightly greater than that of mice and turkey poults found by Duke et ul. (1973). The egested pellets of the Eurasian kestrel were found to contain significantly higher amounts of nitrogen than egested pellets of the screech owl (Table 3). The higher nitrogen content of the Eurasian kestrel pellets was expected because of the higher hair content of their pellets compared to the pellets of the screech owl (Clark, 1972). These values were comparable to the nitrogen content in pellets of greathorned owls fed turkey poults, but were higher than the nitrogen in pellets of great-horned owls fed mice (Duke et al., 1973). The nitrogen content of the excrement of the Eurasian kestrel and screech owl in this study were comparable to excrement of great-horned owls fed turkey poults and mice (Duke et al., 1973). Quantities of gross nitrogen ingested by the Eurasian kestrel and screech owl in this study averaged higher than quantities ingested by the great-horned owl on the mice and turkey poult diet (Duke et al., 1973). Nitrogen eliminated through excrements and pellets in this study were much higher than those reported for the great-horned owl on the mouse and turkey poult diet (Duke et al., 1973). Quantities of nitrogen assimilated in this study were higher than that reported by Kirkwood (1981) for the Eurasian kestrel, but less than that of the great-horned owl (Duke et u/., 1973).
Energy content of food, excrement
and pellets
The energy content of domestic mice and poults used by Duke et al. (1973) the cockerels used by Kirkwood (1979) and domestic rats used in this study were considerably higher than the energy content of five species of small rodents measured by Graber (1962). The caloric content of pellets of the screech owl used in this study was comparable to that of the barn owl (Kirkwood, 1979) great-horned owl (Duke et al., 1973) and long-eared owl (Graber, 1962) all of which were lower than the energy content of the pellets of the Eurasian kestrel measured by Kirkwood (1979) and this study. The higher caloric content of the pellets of the Eurasian kestrel must be explained by a higher caloric content of hair than of bone. Clark (1972) found that the pellets of Falconiformes have a significantly higher hair-tobone ratio than do the pellets of Strigiformes. The caloric content of excrement of the screech owl and Eurasian kestrel used in this study were comparable to the energy content of excrement of the Eurasian kestrel and barn owl (Kirkwood, 1979). great-horned owl (Duke ef a/., 1973) and long-eared owl (Graber, 1962) thus far studied. The existence metabolic values measured in this study were compared with existence metabolic values predicted by linear interpolation to mean ambient temperature of Kendeigh’s (1970) allometric equations for predicting existence metabolism of nonpasserine birds during winter at 0 and 30 C, according to a model developed by Koplin (personal communication). The theoretical existence metabolic rates exceeded the observed existence metabolic rates
Fur The crude fat content of the domestic rat used in this study was only slightly higher than that reported for the rat by Bird and Ho (1976). There are no other fat assimilation data available for raptors with which to compare our results. Ash Differential ash assimilation had been expected, the screech owl assimilated more than twice as much ash as did the Eurasian kestrel; however, we had expected the difference to be due to pellet ash egested rather than the excrement ash egested. We are unable to explain these findings. The higher njtrogen content in the falcon pellets than in the owl pellets is related to the higher ratio of hair to bone in the falcon pellets than in the owl
Food
consumption
pellets. The higher ash content in falcon excrement than in the owl excrement is also related to the ratio of hair to bone in pellets of both birds. Both findings indicate that the falcon more readily digested bone than did the owl, an interpretation consistent with other findings in the literature (Clark 1972; Duke et al., 1975). In his study on short-eared owls (Asio flummeus) and marsh hawks (Circus cyaneus) Clark (1972) reported that “considerably less osseous material was found to comprise the hawk pellets (17% of the total by weight), than the owl pellets (44%)” Similarly, high osseous content of egested pellets has been reported for the eagle owl (B&o bubo) (Hiiglund, 1966), and the great gray owl (Striw nebuloss) (Hoglund and Lansgren, 1968). The higher ash content of owl pellets compared to falconiform pellets is to be expected because falconiform gastric digestion is more thorough (i.e. corrosion of bones) due to the much higher hydrogen ion concentrations in the gastric juices of Falconiformes (Duke et al., 1975). In their studies on the tawny owl (Strix &co), Raczynski and Ruprecht (1974) found that the degree of digestion of osseous material is also dependent upon age of the bird. Adult tawny owls were less efficient than were young tawny owls. Furthermore, Raczynski and Ruprecht (1974) found that variation in degree of digestion of osseous material depended both on the age of the bird and age of the prey species. C&urn,
phosphate
and magnesium
There are no comparable data in the literature to compare with the calcium, phosphate and magnesium metabolism data reported in this study. Acknowledgement-We are grateful to Dr Gary E. Duke for his advice and comments on the draft. REFERENCES
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254
ERI~K G. CAMPBELL and JAMES R. KOPLIN
during growth in the kestrel (Fulco finnunculus). Proc. Nutr. Sot. 40, 6A. Kleiber M. (1961) The Fire of Life. Wiley, New York. Koplin J. R., Collopy M. W., Bamman A. R. and Levenson H. (1980) Energetics of two wintering raptors. Auk 97, 795-806. Lasiewiski R. C. and Dawson W. R. (1967) A reexamination of the relation between standard metabolic rate and body weight in birds. Condor 69, 13-23. Ligon J. D. (1969) Some aspects of temperature relations in small owls. Auk 86, 458472. Manoukas A. G., Colovos N. F. , and Davis H. A. (1964) Losses of energy and nitrogen in drying excreta of hens. Poultry Sci. 43, 547-549. Marshall A. J. (1963) Biology und Comparatiw Physiology of Birds. Academic Press. New York. Marti C. D. (1969) Some comparisons of the feeding ecology of four owls in north central Colorado. Sourhwest Not. 14, 163-170. Marti C. D. (1973) Food consumption and pellet formation rates in four owl species. Wilson Bull. 85, 178-181. McGahan J. (1968) Ecology of the golden eagle. Auk 85, l-12. Mosher J. A. and Matray P. F. (1974) Size dimorphism: A factor in energy savings for broad-winged hawks Auk 91, 325-341. National Academy ofsciences (1971) Nutrient requirements of poultry. Washington, D.C. Raczynski J. and Ruprecht A. L. (1974) The effect of
digestion on the osteological composition of owl pellets. Acra Omitho. 14, l-37. Reed C. I. and Reed B. P. (1928) The mechanism of pellet formation in the great horned owl (Bubo virginianus). Sci. Wash. 68, 359-360. Shannon D. W. F. and Brown W. 0. (1969) Losses of energy and nitrogen on drying poultry excreta. Poultr) Sci. 48, 41-43. Smith C. R. and Richmond M. E. (1972) Factors influencing pellet egestion and gastric pH in the barn owl. Wilson Bull. 84, 179-186. Smith D. G., Wilson C. R. and Frost H. H. (1974) History and ecology of a colony of barn owls in Utah. Condor 76, 131-136. Stalmaster M. V. and Gessaman J. A. (1982) Food consumption and energy requirements of captive bald eagles, J. Wild. Mgt 46, G&654. Sturkie P. D. (1954) Avian Physiology. 2nd ed. Comstock, Ithaca. Willoughby E. J. (1970) Composition of avian urine. Science 169, 1230-1231. Wilson F. H. and Niosi P. N. (1961) Some observations on gastric digestion in the horned owl. Am. Zool. I, 399. Wing L. and Wing A. H. (1939) Food consumption of a sparrow hawk. Condor 41, 168%170. Yapp W. B. (1969) The composition of raptor pellets Ihis 111, 613. Zar J. H. (1968) Standard metabolic comparisons between orders of birds. Condor 70, 278.
1986
0300-9629/86$3.00+ 0.00 c 1986Pergamon Press Ltd
Printed in Great Britain
FOOD CONSUMPTION, ENERGY, NUTRIENT AND MINERAL BALANCES IN A EURASIAN KESTREL AND A SCREECH OWL ERICK G. CAMPBELL* and JAMES R. KOPLIN Department
of Wildlife Management, College of Natural Resources, Humboldt CA 85521, USA. Telephone: 707-826-3658 (Receioed
21 May
State University,
Arcata,
1985)
Abstract-l. The food consumption, energy, nitrogen, fat, ash, calcium, phosphate and magnesium balances of the Eurasian kestrel (F&o finnunculus), and screech owl (Olus kennicotri) are compared. 2. Eurasian kestrel sample values were significantly greater than screech owl sample values in the energy, nitrogen, calcium, phosphate and magnesium content in pellets, and calcium content in excrement. Phosphate content in excrement was significantly greater in screech owl samples than in Eurasian kestrel samples. 3. The Eurasian kestrel ingested significantly greater amounts of calcium and phosphate, egested via excrement significantly greater amounts of ash. excreted or egested significantly greater amounts of energy, nitrogen, calcium and phosphate via excrement and pellets, and excreted significantly greater amounts of magnesium via excrement than did the screech owl. The screech owl assimilated significantly greater amounts of ash than did the Eurasian kestrel. 4. The metabolizability coefficient averaged 48.32% for the Eurasian kestrel, and 49.62% for the screech owl.
INTRODUCTION Studies on life histories (Audubon, ca, 1840; Bent, 1937, 1938), natural food habits (Craighead and Craighead, 1956; Cunningham, 1960; Marti, 1969; Earhart and Johnson, 1970; and many others), gastric digestion (Wilson and Niosi, 1961; Duke et al., 1975), pellet formation (Reed and Reed, 1928; Howard, 1958; Grimm and Whitehouse, 1963; Yapp, 1969; Balgooyen, 1971; Smith and Richmond, 1972; Marti, 1973), ecology (McGahan, 1968; Coulombe, 1971; Smith et al., 1974; Balgooyen, 1976; and many others), avian urine (Willoughby, 1970) and energy metabolism (Benedict and Fox, 1927; Giaja and Males, 1928; Herzog, 1930; Wing and Wing, 1939; Graber, 1962; Collins, 1963; Lasiewiski and Dawson, 1967; Ligon, 1969; Coulombe, 1970; Gatehouse and Markham, 1970; Gessamen, 1972; Duke et al., 1973; Calder and King, 1974; Kirkwood, 1979, 1980; Koplin et al., 1980; Cooper and Greenwood, 1981; Hamilton and Neill, 1981; Stalmaster and Gessamen, 1982) are available for raptors. Studies on general avian physiology (Sturkie, 1954; Marshall, 1963) are available, but the literature is notable for its scarcity of information on the digestive physiology and nutrient requirements of carnivorous vertebrates, particularly the birds of prey. Duke (1978) reviewed existing information on raptor physiology. Research on the nutrient requirements (nitrogen, fat, ash, magnesium, calcium and phosphorus) of domestic poultry is extensive (National Academy of Sciences, 1971; and many others), but few studies have dealt with raptors (Duke et al., 1973; Kirkwood, 1981; Koplin, unpublished data). Hawks and owls represent an example of convergent evolution. Investigators have studied several *Present post and address: District Biologist, Bureau of Land Management, (602) 428-4040.
Safford, AZ 85546, USA. Telephone:
aspects of digestive physiology of these orders of birds. Clark (1972) compared the osseous material assimilated by the short-eared owl (Asia jfammeus) and marsh hawk (Circus cyaneus). Gatehouse and Markham (1970) compared the metabolism of hawks and owls and determined that nocturnal owls had higher metabolic rates during night and that hawks had higher metabolic rates during daylight. Kirkwood (1979) compared the existence metabolism of the Eurasian kestrel (F&o tinnunculus) and the barn owl (Tyto ulbu). Koplin (unpublished data) investigated differential ash assimilation between these two orders. The objective of this study was to determine differences in energy, nitrogen, fat, ash, calcium, phosphate and magnesium balances between the two similarly sized representatives of the orders Falconiformes and Strigiformes, viz., Eurasian kestrel and screech owl (Otus kennicotti). MATERIALS AND METHODS Two similarly sized individuals, one male Eurasian kestrel and one screech owl, sex undetermined, were used in this study, Other specimens were not available during the course of this study. The Eurasian kestrel was an adult at least 3 years old and the screech owl was in its first year, but in an apparent adult condition when the study commenced. This study was conducted from 29 October 1974 to 29 January 1976. Ten trials quantifying the ingesta (food consumption) and egesta (egestdd pellkts -and excrement) were conducted on each individual. The Eurasian kestrel was fed for a total of 73 days and the screech owl for 87 days. A modified “nutritional balance technique” (Kendeigh, 1949) was used in this study. Digestion trials on each bird were conducted for 6 days or until the body weight of the bird was within + I% of its body weight at the beginning of the digestion trial. Since we were interested only in the comparative nutritional aspects of nongrowing, nonmolting, nonreproducing, weight-stable birds, digestion trials were conducted during 249
ERICK G. CAMPBELL and JAMES R. KOPLIN
250
the late fall and early winter at a time of the year when north temperate birds normally are not molting, are reproductively quiescent, and birds of the year have attained adult body size. The raptors were housed in separate flight rooms (3 x 3 x 3 m) at prevailing outside ambient temperatures and photoperiods in Arcata, Humboldt County, California (appoximately 40 52’ latitude). A hygrothermograph was placed in each enclosed flight room to record air temperatures and relative humidity. The birds were acclimated to the flight room and a diet of ground rdt (Rur~us norw~icu.r) (minus stomach and intestines) fed twice daily for 72 hr prior to the beginning of each digestion trial. The food was offered in a fresh condition, and no free water was supplied. Birds were fed whatever prey was available between dtgestion trials. During the acclimation period the weight of each bird was stabilized by controlled feeding at the maintenance level ( + 0.1 g). The birds were weighed ( + 0. I g) at the begtnning of each trial, every other day thereafter, and at the termination of the trial. All weighings occurred at the same time of day. Weight was lost due to human handling at the onset of a number of trials, but was regained before termination of the trial. Heavjy plastic was used to cover the floor and lower walls of the enclosed flight rooms to facilitate collection of pellets and excrement egested during each digestion trial. Pellets were collected daily, and excrement was collected at the termination of each trial. An aliquot of the homogenized rat diet was dried for chemical analysis. All samples of egested pellets and excrement and the homogenized rat were dried in a drying oven at 55 C for 48 hr. Weights of dried pellets and excrement were measured to the nearest 0.01 g on a Metlcr analytical balance. Gross energy (Parr adiabatic calorimeter), nitrogen (macro Kjeldahl), fat (anhydrous ether extraction), and ash (Parr adiabatic calorimeter byproduct) content of dried samples of egested pellets and excrement, and of homogenized rat were measured according to methods of the Association of Official Agricultural Chemists (Horwitz, 1965). Remaining sample materials were forwarded to Chem-met Associates (San Diego, California) for determination of magnesium, calcium and phosphate content according to methods of the American Public Health Association (1956). No attempt was made to correct for nitrogen lost from excrement, pellets or food during the drying process. Manoukas r/ ui. (1964) found that nitrogen lost from poultry excrement during the drying process ranged from ~ 7. I to I5.2”,, with a mean of 5.45%. Shannon and Brown ( 1969) reported nitrogen losses of 5.05 and 4.17% for two samples of chicken excrement dried at 60°C. Equipment for making wet sample analyses was not available during this study nor has a universal correction factor been developed to account for these losses, but, since wastes were handled similarly for the two species, similar losses probably occurred. Nutrient balance (retention) values of the digestion trials were determined by the following formula (Gessamen, 1973): A=I-E, where A = assimilated component, I = ingested component, and E = egested component. For example: A.E. = G.E.I. - P.E. - E.E., A.E. = assimilated energy, G.E.I. = gross energy intake, P.E. = pellet energy and E.E. = excrement energy.
where
These determinations assumed that body nutrient reserves were not being metabolized and body weights were stable during the trials. The assimilation coefficients over the whole study were computed using the following formula from
Table I. Companwn excrement
of mean dally weights of food ingested and and pellets egested by a Eurasian kestrel and a screech owl* Eurasian
Food consumed Mean dry weight &kg body wt day Excrement egeated Mean dry weight @kg body wt day) Pellets egested Mean dry weight @kg body wt day)
kestrel
Screech owl
58.1X”
50.89”
?h.?lh
?I 3Oh
3 Hh
4.34
*Values followed by the same letters are significantly difierent (P 5 0.05); all other values are not significantly different.
Kleiber
(1961):
metabolizability
coefficient
=
total wt wastes
-~~~--~
total wt food
x 100. >
Data for each trial were compared between the falcon vs the owl, using Student’s /-tests. All data were scaled to weight or energy per kilogram of body weight. RESULTS
The body weights of the Eurasian kestrel and screech owl averaged 178.8 g and 169.0 g, respectively, for the IO trials. The mean ambient temperature averaged 12.03 ‘C for the Eurasian kestrel trials and 1 I .63 ‘C for the screech owl trials. Temperature differences are the result of trials being conducted during the same time period, but not necessarily on the same days. The mean dry weights of pellets egested daily by the falcon and owl were not statistically significantly different, although the falcon consumed significantly more food and eliminated significantly more excrement daily than did the screech owl (Table I). Metabolizability coefficients averaged 48.32% for the Eurasian kestrel, and 49.62?/, for the screech owl. Significant differences were measured in the nutrient content of the following sample values: energy content of the pellets, nitrogen content of the pellets, calcium content of the excrement, calcium content of the pellets, phosphate content of the excrement, phosphate content of the pellets and magnesium content of the pellets of the falcon and owl (Table 2). All other samples were not statistically significant (Table 2). The mean daily energy and nitrogen ingested and mean energy and nitrogen assimilated by the Eurasian kestrel and screech owl were not statistically significant; however, the Eurasian kestrel excreted significantly more energy and nitrogen daily via excrement and pellets than did the screech owl (Table 3). The mean daily amounts of fat ingested, fat excreted in excrement and pellets, and fat assimilated by the kestrel were higher than that of the owl, but not statistically significant (Table 3). The mean daily ash ingested by the kestrel was higher than that of the owl; whereas, the mean daily ash egested in the pellets by the owl was higher than that of the kestrel (Table 3). Neither was statistically significant. However, the mean daily ash egested by the kestrel was significantly higher than that of the owl, but the mean daily ash assimilated by the owl
Food consumption in kestrel and owl Table
2. Mean energy and
excrement,
and pellets
nutrient
Sample
content
of samples
Eurasian kestrel
of a Food
Excrement Eurasian
Energy
contentt
94.91
Pellets
5.145” 125.63”
181.16
Fat:
242.0
55.6
25.1
Ash:
113.4
194 6
281.8
3.7
4 6’
4.7”
53.3
58.8’
78.1’
Calcium: Phosphate’: hlagneslum$
I.28
1.2
I.1
Screech Energy
contentt
5.959
hitrogen:
owl 3.877”
3.004
95.14
188.80
93.0lh
Fat:
249.0
60.3
14.8
Ash:
116.2
180.7
290.3
Calcium: Phosphatef
3.5
4.0’
51.2
61.2’
Magnesiumt: ‘Values
I.1
followed
between
the hawk
significantly tin
kcal/g
iln
mg/g
by
dry
food,
kestrel 3.064
5.975
Nitrogenf
of
and a screech owl*
the
3.7d 54.5’
I.0
same
and owl
letters (P I
are
0.05);
I .O” significantly
all other
different
values
are not
diKerent. wt.
dry wt.
was significantly higher than that of the kestrel (Table 3). The mean daily amounts of calcium and phosphate ingested and egested via excrement and pellets were significantly higher in the kestrel than that of the owl (Table 3). The differences in the daily amounts of calcium and phosphate assimilated by the kestrel and the owl were not statistically significant (Table 3). The mean daily amounts of magnesium ingested and excreted via pellets by the kestrel were higher than those of the owl, but the owl assimilated more mean daily magnesium (Table 3). None of the previous differences were significant. The mean daily magnesium excreted via excrement was significantly higher for the kestrel (Table 3).
DISCUSSION
Information from this study indicates that the falcon digested its food more poorly than did the owl; Table
3. Mean
and oellets
daily
quantities
and of enerw
Nutrient
of energy
and nutrients
251
that is, the kestrel consumed more food and produced more pellet matter (Table I), removed less energy from its food (Table 2), removed less nitrogen from its food (Table 2), and had a lower metabolizability coefficient (48.32 vs 49.62%) than did the owl. The energy assimilated-i.e. metabolic energy (Kendeigh, 1949)-by the falcon was also not significantly higher than that of the owl (Table 3), suggesting that at least some of the higher rate of food intake of the falcon compared to that of the owl must be related to a higher metabolic rate of the falcon. Such a conclusion is supported by studies on basal or standard rates of metabolism of falconiforms and strigiforms (Zar, 1968). The mean dry weights of food ingested and excrement and pellets egested by the Eurasian kestrel and the screech owl in the present study were similar to those obtained by Kirkwood (1979) for the Eurasian kestrel and barn owl, but were greater than similar values measured by Duke et al. (1973) for the greathorned owl (Bubo virgin&us). Duke et al. (1975) found that food intake was less in the owls and buteos than it was in the falcons and the bald eagle (Haliaectus leucocephalus). The metabolizability coefficients of the greathorned owl averaged 67.88% for the mouse diet, and 71.21% for the poult diet (Duke et al., 1973). Kirkwood’s (1979) metabolizability coefficients averaged 51.27% for the Eurasian kestrel, and 54.36y0 for the barn owl. Metabolizability coefficients averaged much lower in this study: 48.32% for the Eurasian kestrel, and 49.62% for the screech owl. The differences in mean dry weights of food ingested, excrement and pellets eliminated, and metabolizability coefficients between the various raptors could be explained by: (1) interspecific variation; (2) differences in body size; (3) differing environmental temperatures or experimental technique between studies, and/or (4) differing levels of physical activity. The major factor involved in differences in food, excrement and pellets is probably the tremendous size difference between the kestrel, barn owl or screech owl on the one hand, and the great-horned owl
and nutrients metabolized
Food
Excrement
ingested
egested
in ingested
food
by a Eurasian
egested
347.45 5.524.98
4,717.w
483.9@
14,1 19.2
1.467.7
100.0
6.58
Phosphatej
3.07’
Nitrogen:
64.05”
Fatf
I
12.712.7 5.86
Ash:
179.0”
Calciumff
2.57’
PhosohateT
56.2
Magnesiu&j *Values
followed
by the same letters
tin
not significantly different. kcal/(kg body wt day).
$In
mg/(kg
#In pg/(kg
body body
wt day). wt day).
I .23
4.6
302.96 4.848.3
76.5
0.30’
1.54’ 30.5” Screech
Energyt
0.33’
18.5’
61.5
Magnesium5
323.42 12.551.6
I .09
5.16’
1l9.0h
214.0”
245.58
l9.94h
81.93”
Energy’r
Ash:
Metabolized _
kestrel
Nitrogen:
Cdlcium$
and a screech owl’
Pellets
Eurasian
Fat:
and egested excrement
kestrel
owl 16.42h
4.038.49’
398.39d
I ,306.9
69.0
222 50 411.44 11,336.9 0.76’
I .27
3.83’
78.8
16.1’
84.5h
I .02
0.24’
1.3oL
4.4
21.2m are significantly
26.4
different
(P < 0.05);
30.7 all other
values
are
252
ERICK G. CAMPBELLand JAMESR. KOPLIN
( 1,615 g) on the other. Since the surface-volume ratio of the great horned owl is relatively small compared to that of the falcon and screech owl, it is to be expected that the great-horned owl would ingest relatively less food and eliminate relatively less excrement and pellets than the falcon and screech owl. However, we are unable to account for the differences in metabolizability coefficients between the greathorned owl and the falcon, barn owl and screech owl.
by an average of 1415% for the Eurasian kestrel, and by an average of 25-26% for the screech owl. Studies by Kirkwood (1979) found the Eurasian kestrel and barn owl exceeded Kendeigh’s (1970) equation relating existence metabolism to body weight of nonpasserine birds at 30°C by 28 and 16x, respectively. The birds in this study probably became progressively less physically active the longer they were maintained in captivity.
Energy
Nitrogen
The mean daily quantities of energy ingested, excreted via excrement and pellets, and metabolized was lower in Kirkwood’s (1979) study than they were in this study. The Eurasian kestrel, in both studies, ingested and metabolized more energy per unit of body weight than did the owls. The efficiency of energy assimilation ((gross energy-excrement energy-pellet energy)/gross energy) averaged 87% for the long-eared owl (Ask otus) (Graber. 1962) 85% for the great-horned owl (Duke et al., 1973) 72.54O/, for the barn owl (Kirkwood, 1979) 74.1x for the broad-winged hawk (Mosher and Matray, 1974) 71.327: for the Eurasian kestrel (Kirkwood, 1979) compared to 70.5% for the Eurasian kestrel and 73.2% for the screech owl found in this study. The birds in this study used progressively less energy daily as the study progressed. The birds apparently became less physically active the longer they were maintained in captivity and/or as they grew older their metabolic rates decreased.
Protein retention and storage have been reported for the chicken (Fisher, 1967), but not birds of prey. Two digestion trials conducted during this study resulted in negative metabolism values (i.e. more nitrogen was excreted than was ingested), suggesting that raptors, like chickens, also may store protein. The percentage of nitrogen in domestic rats used in this study was comparable to that for rats measured by Bird and Ho (1976) values only slightly greater than that of mice and turkey poults found by Duke et ul. (1973). The egested pellets of the Eurasian kestrel were found to contain significantly higher amounts of nitrogen than egested pellets of the screech owl (Table 3). The higher nitrogen content of the Eurasian kestrel pellets was expected because of the higher hair content of their pellets compared to the pellets of the screech owl (Clark, 1972). These values were comparable to the nitrogen content in pellets of greathorned owls fed turkey poults, but were higher than the nitrogen in pellets of great-horned owls fed mice (Duke et al., 1973). The nitrogen content of the excrement of the Eurasian kestrel and screech owl in this study were comparable to excrement of great-horned owls fed turkey poults and mice (Duke et al., 1973). Quantities of gross nitrogen ingested by the Eurasian kestrel and screech owl in this study averaged higher than quantities ingested by the great-horned owl on the mice and turkey poult diet (Duke et al., 1973). Nitrogen eliminated through excrements and pellets in this study were much higher than those reported for the great-horned owl on the mouse and turkey poult diet (Duke et al., 1973). Quantities of nitrogen assimilated in this study were higher than that reported by Kirkwood (1981) for the Eurasian kestrel, but less than that of the great-horned owl (Duke et u/., 1973).
Energy content of food, excrement
and pellets
The energy content of domestic mice and poults used by Duke et al. (1973) the cockerels used by Kirkwood (1979) and domestic rats used in this study were considerably higher than the energy content of five species of small rodents measured by Graber (1962). The caloric content of pellets of the screech owl used in this study was comparable to that of the barn owl (Kirkwood, 1979) great-horned owl (Duke et al., 1973) and long-eared owl (Graber, 1962) all of which were lower than the energy content of the pellets of the Eurasian kestrel measured by Kirkwood (1979) and this study. The higher caloric content of the pellets of the Eurasian kestrel must be explained by a higher caloric content of hair than of bone. Clark (1972) found that the pellets of Falconiformes have a significantly higher hair-tobone ratio than do the pellets of Strigiformes. The caloric content of excrement of the screech owl and Eurasian kestrel used in this study were comparable to the energy content of excrement of the Eurasian kestrel and barn owl (Kirkwood, 1979). great-horned owl (Duke ef a/., 1973) and long-eared owl (Graber, 1962) thus far studied. The existence metabolic values measured in this study were compared with existence metabolic values predicted by linear interpolation to mean ambient temperature of Kendeigh’s (1970) allometric equations for predicting existence metabolism of nonpasserine birds during winter at 0 and 30 C, according to a model developed by Koplin (personal communication). The theoretical existence metabolic rates exceeded the observed existence metabolic rates
Fur The crude fat content of the domestic rat used in this study was only slightly higher than that reported for the rat by Bird and Ho (1976). There are no other fat assimilation data available for raptors with which to compare our results. Ash Differential ash assimilation had been expected, the screech owl assimilated more than twice as much ash as did the Eurasian kestrel; however, we had expected the difference to be due to pellet ash egested rather than the excrement ash egested. We are unable to explain these findings. The higher njtrogen content in the falcon pellets than in the owl pellets is related to the higher ratio of hair to bone in the falcon pellets than in the owl
Food
consumption
pellets. The higher ash content in falcon excrement than in the owl excrement is also related to the ratio of hair to bone in pellets of both birds. Both findings indicate that the falcon more readily digested bone than did the owl, an interpretation consistent with other findings in the literature (Clark 1972; Duke et al., 1975). In his study on short-eared owls (Asio flummeus) and marsh hawks (Circus cyaneus) Clark (1972) reported that “considerably less osseous material was found to comprise the hawk pellets (17% of the total by weight), than the owl pellets (44%)” Similarly, high osseous content of egested pellets has been reported for the eagle owl (B&o bubo) (Hiiglund, 1966), and the great gray owl (Striw nebuloss) (Hoglund and Lansgren, 1968). The higher ash content of owl pellets compared to falconiform pellets is to be expected because falconiform gastric digestion is more thorough (i.e. corrosion of bones) due to the much higher hydrogen ion concentrations in the gastric juices of Falconiformes (Duke et al., 1975). In their studies on the tawny owl (Strix &co), Raczynski and Ruprecht (1974) found that the degree of digestion of osseous material is also dependent upon age of the bird. Adult tawny owls were less efficient than were young tawny owls. Furthermore, Raczynski and Ruprecht (1974) found that variation in degree of digestion of osseous material depended both on the age of the bird and age of the prey species. C&urn,
phosphate
and magnesium
There are no comparable data in the literature to compare with the calcium, phosphate and magnesium metabolism data reported in this study. Acknowledgement-We are grateful to Dr Gary E. Duke for his advice and comments on the draft. REFERENCES
American Public Health Association (1956) Standard methods for the examination of water, sewage and industrial wastes. 1Ith edition. APHA. AWWA. FFIWA. Audubon J. J. ca. (1840) The Birds of America, Vol. 1. Dover, New York. Balgooyen T. G. (1971) Pellet regurgitation by captive sparrow hawks. Condor 73, 382-385. Balgooyen T. G. (1976)Behavior and ecology of the American kestrel (F&o sparoerius) in the Sierra Nevada of California. Univ. Calif. Pub]. Zoo]. 103. University of California Press, Berkeley. Benedict F. G. and Fox E. L. (1927) The easeous metabolism of large wild birds under aviary IifelProc. Am. Phil. Ser. 66, 51 l-534. Bent A. C. (1937) Life histories of North American birds of prey, part 1: order Falconiformes. Bull. US Nat. Mus. 167. Washington, DC. Bent A. C. (1938) Life histories of North American birds of prey, part 2: orders Falconiformes and Strigiformes. Bull. US Nat. Mus. 170. Washington, D.C. Bird D. M. and Ho S. K. (1976) Nutritive values of whole-animal diets for captive birds of prey. Raptor Res. 10, 45-49. Calder W. A. and King J. R. (1974) Thermal and caloric relations of birds. In Aoiun Biology (Edited by Farner D. S. and King J. R.), Vol. 4, pp. 260-413. Academic Press, New York. Clark R. J. (1972) Pellets of the short-eared owl and marsh hawk compared. J. Wildl. Mgt 36, 962-964.
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