A. Food preferences and resources.
1. All diurnal birds of prey eat some type of animal flesh, including reptiles, insects, fish, birds, mammals, molluscs, and carrion.
2. Groups of birds favor certain foods.
a. Most Old and New World vultures prefer to eat carrion. Many diurnal birds of prey include carrion in their diet; but in vultures, carrion tends to be the main food source.
b. Larger falcons and Accipiter hawks eat mostly birds.
(1) Peregrine falcons feed on doves, pigeons, grouse, shorebirds, and jays.
(2) Sharp-shinned hawks eat sparrows, robins, and warblers.
c. Buteo hawks tend to feed on mammals, such as mice, voles, ground squirrels, rats, rabbits, and gophers.
3. Some species have specialized diets.
a. Ospreys feed almost exclusively on fish.
b. Palmnut vultures (Gypohierax angolensis) feed mainly on the fruit and husks of palms, though they occasionally eat fishes or insects.
c. Honey buzzards (Pernis apivorus) prefer wasps and their larvae or pupae.
d. Bat hawks (Macheiramphus alcinus) eat bats.
e. Snail kites dine on apple snails.
f. Lesser kestrels (Falco naumanni) feed on grasshoppers and locusts.
4. In general, diurnal birds of prey feed on game that average 12% to 50% of their own body weight. Occasionally, larger species will catch prey their own weight or larger (Johnsgard, 1990).
a. A harpy eagle of South America was recorded carrying a sloth weighing 5.9 kg (13 lb.) (Martin, 1987).
b. A bald eagle was seen carrying a mule deer fawn weighing 6.8 kg (15 lb.) (Martin, 1987).
B. Food intake.
1. The amount of food a diurnal bird of prey eats depends largely on the species, size, and sex of the bird.
a. Smaller birds have a larger surface area to body volume ratio than larger birds, causing them to lose body heat more rapidly. To compensate, smaller birds usually eat proportionately more food than larger birds to maintain their metabolism and body temperature.
(1) A nonbreeding black-shouldered kite (Elanus caeruleus) weighing on average 0.25 kg (.55 lb.) eats about 20% of its body weight per day (Pickford and Tarboton, 1989).
(2) On the other hand, a cape vulture weighing on average 9 kg (20 lb.) eats about 6% of its body weight per day (Pickford and Tarboton, 1989).
b. In many species the female is larger than the male and therefore captures and consumes larger prey. For instance, the average female sharp-shinned hawk captures prey weighing up to 300 g (0.7 lb.), while the average male captures prey weighing up to 100 g (0.2 lb.). Scientists believe the size difference between mated pairs reduces competition for food during the breeding season (Johnsgard, 1990).
2. Food consumption is also influenced by season. Food intake increases in cold weather, before migration, and before egg laying (in females).
C. Methods of collecting and eating food.
1. The majority of diurnal birds of prey hunt independently, and capture and kill game with their talons. Falcons use both their talons and beak to subdue prey.
2. Group hunting has been observed in wintering Harris' hawks (Parabuteo unicinctus), where individuals cooperate to flush and ambush prey.
3. Birds that feed on invertebrates tend to have higher hunting success rates than those that feed on vertebrates. In one study, attacks on invertebrates by American kestrels had a 85% success rate. Attacks on vertebrates had a 23% success rate (Cade, 1982).
4. The capture and killing of vertebrate prey is a highly developed skill that varies from species to species.
a. Ospreys capture fish at shallow depths after a dive of 5 to 70 m (16-230 ft.) (Johnsgard, 1990). Dives are usually initiated from a flapping, gliding, or hovering flight. At the final stage of the dive, the feet are thrust forward and the wings are held directly back. The birds may disappear into a spray of water before emerging with a fish (typically carried head forward).
b. When attacking from a stoop, the peregrine falcon will strike its prey (a bird) with a hind talon, knocking it to the ground. If the prey is not sufficiently stunned, the falcon will attack again. Peregrines are the world's fastest moving bird, reaching speeds up to 180 kph (112 mph) when stooping (Martin, 1987).
5. In addition to capturing their own prey, many birds, including peregrine falcons and bald eagles, steal prey from other birds.
6. Once the prey is subdued, the bird usually "mantles," spreading its wings out and over the prey. Mantling shields the prey from theft by other birds. Many bird-eating species pluck the larger feathers before tearing and consuming the prey.
7. Some Old World vultures, whose main diet is carrion, have unique ways to obtain food.
a. Many African species dine on the same carcass, but they arrive and feed at different times. The powerful whiteheaded vultures usually arrive first, cautiously approaching the carcass before tearing open the tough hide. They're soon joined by whitebacked vultures (Gyps africanus) that assist in quickly exposing the carcass' entrails. Ruppell's griffon vultures (Gyps rueppellii) may arrive next, thrusting their long necks into the open carcass to obtain pieces of soft flesh. Lappetfaced vultures typically feed last, eating the remaining skin, tendons, and ligaments.
b. Bearded vultures, who feed on bone marrow, break open large carcass bones by lifting them into the air and dropping them onto rocks.
c. Egyptian vultures (Neophron percnopterus) drop rocks onto ostrich eggs to break them open for eating.
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