Energy Requirements
Both the Great Horned Owl and Burrowing Owl have specific energy requirements for maintenance, growth and reproduction. Differences in body weight between the species will show in the energy required for day to day life.
Bubo virginianus perched in a tree (Gerrit Vyn, www.arkive.org)
Young Athene cunicularia standing in an open field (M. Watson, www.arkive.org)
Maintenance
Basal Metabolic Rate (BMR)
Wijnandts (1984) defines the basal metabolic rate (BMR) as "...the rate of energy utilization of fasting, inactive birds in the zone of thermoneutrality..." BMR will differ with body weight, so the Great Horned Owl (average 1000g) will have a higher metabolic rate than the Burrowing Owl (145g), who weighs much less than the prior species.
BMR = 15.62kJ/hour (mass 1000g) (McNab 2009)
or 89.6kcal/day
BMR = 2.80kJ/hour (mass 146.7g) (McNab 2009)
or 16.1kcal/day
Daily Energy Expenditure (DEE)
Both species will go through daily activities at a cost to their energy levels. A study on Long-eared Owls (Asio otus) allocated 24.2% of energetic costs to growth, 31.5% cost for daily activity, 6.5% cost for thermoregulation and 37.8% cost for basal metabolic rate (Wijnandts 1984). These values can be found below for the Great Horned Owl and Burrowing Owl. Robbins (1993) determined the DEE of owls to be 2.5 * the basal metabolic rate. Results below show that there is very small difference between these two methods.
DEE= 2.5 * BMR (Robbins 1993)
= 2.5 * 89.6kcal/day = 224kcal/day
DEE = growth + activity + thermoregulation + BMR
= 224*0.242 + 224*0.315 + 224*0.065 + 224
= 54.208 + 70.56 + 14.56 + 89.6kcal/day
= 228.9 kcal/day
DEE= 2.5 * BMR (Robbins 1993)
= 2.5 * 16.1kcal/day = 40.25kcal/day
DEE = growth + activity + thermoregulation + BMR
= (40.25kcal/day*0.242) + (40.25*0.315) + (40.25*0.065) + 16.1kcal/day
=9.7405 +12.67875 + 2.61625 + 16.1
= 41.1kcal/day
Activity
Each species will need to fly, perch, feed and moult throughout its lifetime. The energetic requirements to carry out these essential activities can be found below for both Bubo virginianus and Athene cunicularia. The values below were based off equations used in (Robbins 1993) for feeding and flying, (Wijnandts 1984) for moulting and (Aschoff and Pohl 1970) for perching.
Perching
0.8 * BMR = 0.8 *89.6kcal/day = 71.7 kcal/day
0.8 * BMR = 0.8 * 16.1kcal/day = 12.9 kcal/day
Feeding
0.11 * Perching = 0.11 * 71.7kcal/day = 7.89kcal/day
0.11 * Perching = 0.11 * 12.9kcal/day = 1.42kcal/day
Moulting
0.178 * BMR = 0.178 *89.6kcal/day = 15.95kcal/day
0.178* BMR = 0.178 * 16.1kcal/day = 2.87kcal/day
Flying (Gliding)
3.4 * BMR = 3.4 *89.6kcal/day = 304.64kcal/day
= 0.21kcal/minute
3.4 * BMR = 3.4 *16.1kcal/day = 54.74kcal/day
=0.04kcal/minute
Reproduction
Reproduction in owls requires more energy on top of the daily metabolic requirement (BMR). In a study on Long-eared Owls (Asio otus) approximately 29% more energy was required when laying eggs and 76% more energy during peak brooding period (Wijnandts 1984). Gonadal growth in owls has been found to require 4% more energy (than BMR) to grow female gonads (ovary/oviduct) and require 0.5% more energy to grow testes in males, with 0.8% more energy to produce sperm (Robbins 1993). These percents were applied to the basal metabolic rates of both the Great Horned Owl and Burrowing Owl to produce the amount of energy required in each species throughout the different stages of reproduction.
Egg-laying
0.29 * BMR = 0.29 *89.6kcal/day = 26.0 kcal/day
0.29* BMR = 0.29 * 16.1kcal/day = 4.7 kcal/day
Brooding
0.76 * BMR = 0.76 * 89.6 kcal/day = 68.1kcal/day
0.76 * BMR = 0.76 * 16.1kcal/day = 12.2kcal/day
Gonad Growth
Ovary/Oviduct = 0.04 * BMR= 0.04 * 89.6kcal/day = 3.6kcal/day
Testes = 0.005 * BMR = 0.005 * 89.6kcal/day = 0.448kcal/day
Sperm = 0.008 * BMR = 0.008 * 89.6kcal/day = 0.72kcal/day
Ovary/Oviduct = 0.04 * BMR= 0.04 * 16.1kcal/day = 0.64kcal/day
Testes = 0.005 * BMR = 0.005 * 16.1kcal/day = 0.08kcal/day
Sperm = 0.008 * BMR =0.008 * 16.1kcal/day = 0.13 kcal/day
Growth
In both Bubo virginianus and Athene cunicularia, the growth of nestlings requires more energy and therefore more food compared to a grown adult. Growth was calculated by Wijnandts (1984) as 24.2% of the daily energy expenditure (DEE), so this was adopted to determine the energy required for growth in each species of owl.
0.242 * DEE = 0.242 * 224kcal/day = 54.21kcal/day
0.242 * DEE = 0.242 * 40.25kcal/day = 9.74kcal/day
Summary of Energy Requirements
The values for the energetic cost of various activities for both species are summarized in the table below for convenience. Comparing these values, you can see that all activities, growth and reproduction are more energetically costly in the Great Horned Owl.
A Great Horned Owl in flight (Thomas Mangelsen, www.arkive.org).
Food Consumption Required to Meet Energetic Needs
Based on the nutritional value of food items on the previous page "Diet", the food consumption required to sustain either species can be determined. It was found that a small mammal (15g) provides 89.7kcal, a small bird (45g) provides 247kcal, an insect (0.35g) provides 0.48kcal and a reptile (50g) provides 273kcal. Looking at each species daily energy expenditure (DEE), it can therefore be determined how many of each prey item should be consumed per day (DEE/Prey kcal).
Bubo virginianus
Athene cunicularia
DEE = 229kcal/day
2.5 Small Mammals (229/89.7kcal)
or
1 Bird (229/247kcal)
DEE = 40.25kcal/day
84 insects (40.25/0.48kcal)
or
15% of 1 lizard (40.25/273kcal)
The number of prey items will increase during reproduction and growth, as both species will require more energy during these activities. In a study by Bozinovic and Medel (1988), they found that Bubo virginianus needed to consume approximately 204.2g/day of food. This would mean that the Great Horned Owl would be eating about 7 small mammals a day if each prey item weighed about 30g. 2.5 small mammals a day seems low, so it is possible that the calculated value for kcal/day in mammals is exaggerated, or the DEE is underexaggerated. The values for the Burrowing Owl seem more accurate.