Diet
Great Horned Owl
Bubo virginianus feeding on a mouse (Carson Baldwin Jr., www.arkive.org)
Bubo virginianus typically eats a high protein diet of small mammals, such as voles, mice and rabbits (Mikkola 2012). Other species of birds, frogs, reptiles, spiders and larger insects are also occasionally caught (Mikkola 2012).
Great Horned Owls will hunt by slowly gliding above the ground or from a perch in open areas (Mikkola 2012).
Seasonal Food Availability
Burrowing Owl
Athene cunicularia with crab prey (Brian Kenney, www.arkive.org)
Athene cunicularia feeds mainly on a variety of arthropods, like insects (especially beetles), spiders, scorpions and crabs. Their diet also includes amphibians, reptiles, small mammals and occasionally small birds (Mrykalo, Grigione and Sarno 2009).
Burrowing owls will hunt from a high perch, scouting the landscape for prey. They will also hunt on ground by walking or hopping, or during flight (Mikkola 2012).
Both species of owl are opportunistic predators, where the composition of its diet depends on the availability of prey items. Generally, an array of prey items are available year round for the Great Horned Owl and Burrowing Owl, but a specific rodent, insect, etc. may be fed on more frequently in the season its population is highest (Zimmerman, Stapp and Van Horne 1996; York, Rosenberg and Sturm 2002).
In a study on the diet of Bubo virginianus in Northern Colorado, it was found that species of rabbits (lagomorphs) comprised 70% of this owl's diet all year round, with small mammals being selected during periods of higher abundance (Zimmerman, Stapp and Van Horne 1996). For example the Kangaroo rat and Deer Mouse are more frequent in the Great Horned Owl's diet during summer and fall when their populations are high (Zimmerman, Stapp and Van Horne 1996).
Relatedly, a study on the Burrowing Owl's diet in California found that regardless of the season, Orthoptera insects (crickets, grasshoppers, etc.) were always available and dominated the diet (York, Rosenberg and Sturm 2002). Whereas, the consumption of Coleoptera (beetles) and Lepidoptera (butterflies) was dependent on their abundance year to year (York, Rosenberg and Sturm 2002).
Nutritional Value of Food Items
Nutrition Facts
Small Mammal
Protein
Fat
Ash
ME
Rats, deer mice, voles, small rabbits, kangaroo rats, etc.
498g/kg
347g/kg
75g/kg
5.98 kcal/g
89.7kcal/mammal(15g)
Nutrition Facts
Protein
Fat
Ash
ME
Passerines, chicks
674g/kg
210g/kg
54g/kg
5.49kcal/g
247kcal/bird (45g)
Small Bird
Nutrition Facts
Protein
Fat
Ash
ME
Earwigs, beetles, crickets, grasshoppers, etc.
165 g/kg
79g/kg
12.2 g/kg
1.375kcal/g
0.480
kcal/insect (0.35g)
Insect
Nutrition Facts
Protein
Fat
Ash
ME
Reptile
Lizards, snakes, etc.
236g/kg
40g/kg
12g/kg
5.47kcal/g
273kcal/reptile (50g)
The above labels show the nutrient breakdown of 4 food items used by either species of owl, with the Great Horned Owl feeding more on small mammals and birds, and the Burrowing Owl feeding more on insects/arthropods and reptiles. The metabolizable energy (ME) shows that small mammals are usually the food item with the highest amount of kilocalories per gram.
Nutrient information for the small mammal and small bird was derived from knowledge on the Syrian Golden Hamster (Mesocricetus auratus) and chicks of Gallus gallus (Tabaka et al. 1996). Information on nutrient breakdown of a typical insect came from data on the cricket (Acheta domestica) (Finke 2015). And finally, information regarding the nutrient content of a reptile came from a study on the Tegu lizard (Tupinambis merianae) (Caldironi and Manes 2006).
These values will change according to prey species and their corresponding weight, but provides a general depiction of the nutrient content in a typical small mammal, small bird, insect and reptile.
Limiting Nutrients
The Burrowing Owl's diet is deficient in Calcium due to being mainly composed of insects and this can be limiting to growing young and egg-laying females (York, Rosenberg and Sturm 2002). As a result, individuals will seek out calcium-rich foods, such as mollusc shells, during times of physiological need (York, Rosenberg and Sturm 2002). If this nutrient is not acquired during laying, smaller clutches with thinner shells will be produced and thus effect the reproductive rate (York, Rosenberg and Sturm 2002).
It can be assumed that Calcium is not a limiting nutrient in the Great Horned Owl's diet, as the main prey for this species is Calcium-rich rodents and other small mammals (York, Rosenberg and Sturm 2002).