A. Sexual maturity.
1. In general, smaller species begin breeding at an earlier age than larger species.
a. Small Accipiter hawks, falcons, and harriers usually start to breed at one to two years.
b. Buteo hawks, kites, and large falcons breed at two or three years. Ospreys begin breeding at three or more years.
c. Large eagles usually start to breed at four to nine years, and vultures up to 12 years.
2. First-time breeding may be delayed if competition for food is high or territories are unavailable.
B. Reproductive behavior.
1. Most diurnal birds of prey are monogamous. Some birds keep the same mate for several years, such as ospreys, while others have different mates each year, such as snail kites. In some groups of Harris' hawks, monogamous breeding pairs are assisted by a number of nonbreeding helpers, mostly previous offspring.
2. A few species are polygynous or polyandrous.
a. Polygyny is when a male mates with more than one female at a time. Many harriers are polygynous.
b. Polyandry is when a female mates with more than one male at a time. Polyandry has been observed in populations of Harris' hawks and Galapagos hawks (Buteo galapagoensis).
3. For migratory species, breeding territories are usually reestablished before pairs are formed.
a. Typically the males (American kestrels, Swainson's hawks) and occasionally the females (some Accipiter hawks) return to the breeding area first to reestablish a territory.
b. Once on the territory, the male (occasionally the female in some Accipiter hawks) begins to defend the territory with vocal and aerial displays. Aerial displays may also be used simultaneously, or sequentially, to attract new mates, and for bonding between established pairs.
(1) Individual aerial displays may involve shallow or deep undulations and dives, figure eights, and circles.
(2) Pair displays may include high speed dives, chases, mutual talon grabbing and cartwheeling, and food passing.
c. Other pair bonding behaviors include mutual roosting, cooperative hunting, courtship fights, mutual preening, nibbling, courtship feeding, nest inspections or nest building, and copulation. Copulation occurs when there is cloaca to cloaca contact.
4. For breeding pairs that stay on their territory year-round, the breeding season is marked by an increase in aerial displays and the male and female spending more time together around the nest-site.
5. For partial migrant populations, non-migrating males tend to hold their breeding territories during winter and initiate pair-forming in spring.
6. Most diurnal birds of prey establish territories and breed in the same general area where they were hatched and raised.
C. Breeding seasons.
1. Diurnal birds of prey breed during the time of the year when food is most available for raising young. They usually lay eggs in spring or early summer; but, the actual months differ depending on geographic location.
2. Each species usually lays its eggs at the same time each year.
D. Nesting.
1. Nest building varies between species.
a. New World vultures and falcons don't build nests. New World vultures scrape out a hollow usually in a tree or cliff cavity. Falcons use old nests of other species.
b. All other birds build stick nests either in trees, on cliffs, on humanmade platforms, or on the ground. Ospreys nest in dead trees up to 30 m (98 ft.) high and bald eagles nest in coniferous trees up to 25 m (82 ft.) high (Johnsgard, 1990). Many species line the nest with greenery before the eggs are laid.
2. Some pairs use the same nests year after year. Bald eagles reuse nests, adding sticks and greenery each year. The largest nest ever recorded was a bald eagle nest measuring 2.9 m (9.5 ft.) wide, 6 m (20 ft.) deep, and weighing more than 3,000 kg (2.9 tons) (Martin, 1987).
3. Pairs often maintain two or more nest sites per territory, alternating nests between years. The use of different nests may prevent the build up of nest parasites, and provide an alternative location if the first nest is disturbed or destroyed.
E. Clutch frequency and size.
1. Most species raise one brood per year. If the first clutch (group of eggs) fails, the pair may lay a replacement clutch. If the season is long, smaller species with short breeding cycles can sometimes raise two broods. A few species, such as the California condor, lay one clutch every two years.
2. In general, large species produce small clutches.
a. In temperate regions, small falcons, Accipiter hawks, and harriers usually lay four to six eggs in a clutch. Large falcons and Accipiter hawks lay three to four eggs.
b. Small kites lay three to five eggs. Large kites, Buteo hawks, caracaras, and ospreys lay two to three eggs.
c. Eagles and small vultures lay one to two eggs. Large vultures and condors lay only one egg per clutch.
3. The eggs are laid over several days. Small species lay an egg once every two days, while large species lay an egg once every three to five days.
F. Incubation.
1. Most females incubate the eggs while the male provides food for himself and the female. Both Old and New World vultures tend to share incubation between the male and female.
2. Incubation usually begins before the clutches are complete. As a result, the eggs hatch over two or more days.
3. Incubation lasts four to seven weeks according to species. Large species tend to have longer incubation periods than small species.
G. Chicks at hatching.
1. Chicks begin pipping (breaking their shells) about 24 to 48 hours before hatching.
2. The chick breaks through the shell using a growth on its bill called an "egg tooth." The egg tooth is not a true tooth and falls off soon after hatching.
3. Chicks hatch with their eyes open and are covered with a long, thin coat of white, buff, or grayish down.
4. Newly hatched chicks are relatively helpless and unable to stand.
H. Care of young.
1. In most species, the female continues to care for the young while the male continues to catch all the prey. One study found that a male osprey provided about two to three fish per day during incubation and up to six fish per day by the time the chicks left the nest (Newton, 1979).
2. When the chicks are first hatched, the female broods (sits on or over the young) almost continually. In most species, the female feeds the chicks by tearing off small pieces of flesh from prey provided by the mate. In a few species, such as Old and New World vultures and secretary birds, food is regurgitated to the young.
3. As the chicks get older the female broods less, but stays near the nest. The young are gradually fed more and more parts of the prey.
4. When the chicks are fully feathered (fledged) and can eat whole prey themselves, the female spends more time away from the nest hunting for herself and her young.
5. Once the young are able to fly, some parents show their young how to hunt. Cooper's hawk and peregrine falcon parents have been observed leading their young on rapid chases before giving up their prey. These same parents may also release still-living prey for their young to catch and kill.
I. Chick growth and development.
1. Because chicks of the same clutch hatch over several days, the older, larger chicks are better able to compete for food. As a result, the youngest chick(s) in the nest often dies of starvation or is killed by a larger nest mate. Scientists believe asynchronous hatching is an adaptation to unpredictable food supplies. If food is plentiful, all the chicks survive. If food is scarce, the brood is rapidly reduced.
2. The nestling period (the time from hatching to fledging, when a bird is able to fly) lasts from four to seven weeks. As with incubation, large species have proportionately longer nestling periods than small species.
a. Chicks gain weight slowly at first, then increase rapidly, leveling off around the time of fledging.
b. Small species grow more rapidly than large species. Small species may reach adult weight by fledging, while large species may not reach adult weight until sometime after leaving the nest.
c. As contour feathers replace downy ones, chicks begin to preen after meals.
d. Once wing feathers have developed, chicks spend a considerable amount of time flapping them at the nest.
3. Postfledging periods (the period between fledging and complete independence) range from two to three weeks in small falcons, five to ten weeks in Buteo hawks, and up to several months in large eagles and vultures.
a. After fledging, young birds begin to make short flights to nearby trees or perching sites. They usually return to the nest when a parent returns with food.
b. Later, the young may intercept a parent further and further away from the nest, sometimes screaming loudly or even attacking the parent for food.
c. When the fledglings can hunt independently, they may leave the nest area on their own or wait until the parents stop feeding them, drive them away, or leave to migrate.
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