Peregrine Falcon: Back from the Dead, Description, Ecology, Feeding and Reproduction

An adult of either the subspecies pealei or tundrius by its nest in Alaska

Peregrine falcons were on the brink of extinction in the early 1970’s. This magnificent bird of prey, considered a bird of royalty because their use for falconry was often relegated only to kings, was a victim of DDT poisoning.

DDT was a widely used insecticide from the discovery of its effectiveness against mosquitoes in 1939 until its ban in the US in 1972. DDT was used nearly indiscriminately until the publication of Silent Spring by Rachel Carson in 1962 brought to light the dangers of widespread DDT and insecticide use. DDT was considered such an important insecticide that Paul Hermann Muller was awarded the Nobel Prize in Physiology or Medicine in 1949 for the discovery of DDTs high effectiveness as a poison to several arthropods.

As the population of several bird species plummeted precipitously in the 1950-1970’s it was discovered that birds high in the food chain who preyed on and ate birds, fish, and other animals that ingested large amounts of DDT and concentrated it in their tissues were having widespread reproductive failure. DDT in high concentrations led to fragile eggs in these birds, so that the egg shells were not strong enough to effectively allow reproduction. Peregrine falcons, Bald Eagles, Brown and Pelicans were some of the species most affected by this problem.

After the ban on DDT use all of these species have made dramatic recoveries. The Bald Eagle and the Brown Pelican managed to recover with primarily protection from hunting and some habitat protection. Peregrine falcons required more help. Captive breeding and release programs were highly successful, and now we again can find these magnificent falcons, who can reach speeds up to 250 miles per hour when diving on prey, hunting and soaring in our skies again.

Description

The peregrine falcon has a body length of 34 to 58 cm (13–23 in) and a wingspan from 74 to 120 cm (29–47 in). The male and female have similar markings and plumage, but as in many birds of prey the peregrine falcon displays marked sexual dimorphism in size, with the female measuring up to 30% larger than the male. Males weigh 330 to 1,000 g (0.73–2.20 lb) and the noticeably larger females weigh 700 to 1,500 g (1.5–3.3 lb). In most subspecies, males weigh less than 700 g (1.5 lb) and females weigh more than 800 g (1.8 lb), with cases of females weighing about 50% more than their male breeding mates not uncommon.

The standard linear measurements of peregrines are: the wing chord measures 26.5 to 39 cm (10.4–15.4 in), the tail measures 13 to 19 cm (5.1–7.5 in) and the tarsus measures 4.5 to 5.6 cm (1.8–2.2 in).

Ecology and behaviour

The peregrine falcon lives mostly along mountain ranges, river valleys, coastlines, and increasingly in cities. In mild-winter regions, it is usually a permanent resident, and some individuals, especially adult males, will remain on the breeding territory. Only populations that breed in Arctic climates typically migrate great distances during the northern winter.

The peregrine falcon reaches faster speeds than any other animal on the planet when performing the stoop, which involves soaring to a great height and then diving steeply at speeds of over 320 km/h (200 mph), hitting one wing of its prey so as not to harm itself on impact. The air pressure from such a dive could possibly damage a bird’s lungs, but small bony tubercles on a falcon’s nostrils are theorized to guide the powerful airflow away from the nostrils, enabling the bird to breathe more easily while diving by reducing the change in air pressure.

To protect their eyes, the falcons use their nictitating membranes (third eyelids) to spread tears and clear debris from their eyes while maintaining vision. The distinctive malar stripe or ‘moustache’, a dark area of feathers below the eyes, is thought to reduce solar glare and improve contrast sensitivity when targeting fast moving prey in bright light condition; the malar stripe has been found to be wider and more pronounced in regions of the world with greater solar radiation supporting this solar glare hypothesis. Peregrine falcons have a flicker fusion frequency of 129 Hz (cycles per second), very fast for a bird of its size, and much faster than mammals. A study testing the flight physics of an “ideal falcon” found a theoretical speed limit at 400 km/h (250 mph) for low-altitude flight and 625 km/h (388 mph) for high-altitude flight.

In 2005, Ken Franklin recorded a falcon stooping at a top speed of 389 km/h (242 mph).

Feeding

The peregrine falcon feeds almost exclusively on medium-sized birds such as pigeons and doves, waterfowl, songbirds, and waders. This falcon tends to nest on tall buildings or bridges, and these urban dwelling birds subsist mostly on different pigeons. Worldwide, it is estimated that between 1,500 and 2,000 bird species (up to roughly a fifth of the world’s bird species) are predated somewhere by these falcons.

In North America, prey has varied in size from 3 g (0.11 oz) hummingbirds (Selasphorus and Archilochus ssp.) to a 3.1 kg (6.8 lb) sandhill crane (killed in Alaska by a peregrine in a stoop), although most prey taken by peregrines weigh from 20 g (0.71 oz) (small passerines) to 1,100 g (2.4 lb) (such as ducks and gulls).

Illustration by John James Audubon

The peregrine falcon takes the most diverse range of bird species of any raptor in North America, with more than 300 species having fallen victim to the falcon, including nearly 100 shorebirds. Smaller hawks and owls are regularly predated, mainly smaller falcons such as the American kestrel, merlin and sharp-shinned hawks.

In urban areas, the main component of the peregrine’s diet is the rock or feral pigeon, which comprise 80% or more of the dietary intake for peregrines in some cities. Other common city birds are also taken regularly, including mourning doves, common wood pigeons, common swifts, northern flickers, common starlings, American robins, common blackbirds, and corvids (such as magpies or carrion, house, and American crows).

Other than bats taken at night, the peregrine rarely hunts mammals, but will on occasion take small species such as rats, voles, hares, shrews, mice and squirrels. Coastal populations of the large subspecies pealei feed almost exclusively on seabirds. In the Brazilian mangrove swamp of Cubatão, a wintering falcon of the subspecies tundrius was observed while successfully hunting a juvenile scarlet ibis. Insects and reptiles make up a small proportion of the diet, which varies greatly depending on what prey is available.

Reproduction

The peregrine falcon is sexually mature at one to three years of age, but in larger populations they breed after two to three years of age. A pair mates for life and returns to the same nesting spot annually. The courtship flight includes a mix of aerial acrobatics, precise spirals, and steep dives. The male passes prey it has caught to the female in mid-air. To make this possible, the female actually flies upside-down to receive the food from the male’s talons.

During the breeding season, the peregrine falcon is territorial; nesting pairs are usually more than 1 km (0.62 mi) apart, and often much farther, even in areas with large numbers of pairs. The distance between nests ensures sufficient food supply for pairs and their chicks. Within a breeding territory, a pair may have several nesting ledges; the number used by a pair can vary from one or two up to seven in a 16-year period.

The peregrine falcon nests in a scrape, normally on cliff edges. The female chooses a nest site, where she scrapes a shallow hollow in the loose soil, sand, gravel, or dead vegetation in which to lay eggs. No nest materials are added. Cliff nests are generally located under an overhang, on ledges with vegetation. South-facing sites are favoured.

In some regions, as in parts of Australia and on the west coast of northern North America, large tree hollows are used for nesting. Before the demise of most European peregrines, a large population of peregrines in central and western Europe used the disused nests of other large birds.

In remote, undisturbed areas such as the Arctic, steep slopes and even low rocks and mounds may be used as nest sites. In many parts of its range, peregrines now also nest regularly on tall buildings or bridges; these human-made structures used for breeding closely resemble the natural cliff ledges that the peregrine prefers for its nesting locations.

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