Osprey

Common Name: Osprey, Fish eagle, Sea hawk, River hawk, Balbuzard pêcheur (French), Gavilán pescador (Spanish) – In the mid-15th Century, the term ospriet was first used in France to refer to the fish-eating raptor. This may have been derived from the Latin avis prede meaning ‘bird of prey’ or from the Latin ossifraga which was the name given to sea eagles, but which literally means ‘bone-breaking,’ presumably a descriptive epithet for its crushing talons.

Scientific Name: Pandion haliaetus – The genus is eponymously named for Pandion, the king of Athens in Greek mythology, in what seems to be a case of mistaken identity on the part of Savigny, the French scientist who is credited with the citation. The only avian connection is that Pandion’s daughters were transformed into birds to escape the wrath of King Tereus of Thrace. The species name is from the Greek hali meaning ‘sea’ and aietos, meaning ‘eagle.’

Potpourri: The Osprey is truly a fish eagle, sui generis among North American raptors for its preference of and aptitude for the capture of fresh and saltwater fish, which comprise over 90 percent of its wholly carnivorous diet. They are masters of the hunt, able to discern the shapes and shadows of near-surface prey, frequently hovering in place for confirmation before making the talon-first, Gadarene plunge that frequently includes complete immersion to a depth of up to three feet at the culmination of the hunt; they have long, slit-like nostrils that are closed to prevent water intrusion. They are successful about 70 percent of the time. The osprey is a sizable bird – about two feet in length with an even more impressive oversize wingspan of up to six feet; this provides the lift needed to pull a sizable fish out of the water. With allowances for visibility anddetection, the captured fish are hardly minnows, ranging in size from 6 to 12 inches and weighing upwards of two pounds, a pretty impressive feat for a bird that only weighs about twice that much. Their evolutionary adaptations that make them the masters of the sea hunt have been at the expense of other attributes; they lack the maneuverability of their cousin hawks and eagles of the Family Accipitridae so admirably employ in assaulting land mammals.

That Ospreys are consummate fish hunters is not coincidental – they achieved that distinction as a matter of evolution to occupy the survivable ecological niche of aerial fishing. As fish are slippery, a matter of the piscine evolution of scales and hydrodynamics that enhance speed for assault or escape in an aquatic habitat, the osprey evolved a superior biological weapon that is equal to the task – a talon-harpoon and a toehold-gaff. They have a reversible outer toe that can rotate to the back of the foot so that there are two talons in the front and two talons in the back, providing two formidable four-point vices from which there is no escape if fully engaged. The piercing talons are held firmly in place by barbed pads called spicules that extend from the underside of the toes. The impaled fish, its fate sealed, is held securely and, quite frequently, fore and aft aligned so that the fish is aerodynamic with the flight of the osprey to minimize drag. So prevalent and effective is this method that ospreys are known to embark on migration carrying a fish for sustenance, an occurrence for which the National Geographic Guide to Birds of North America provides the birdwatcher’s acronym OPAL, Osprey Packing a Lunch. Subsequent to the successful fish hunt and meal the claws and pads are frequently cleaned by flying low over the water with the feet extended to be ready for the next encounter.

The osprey is a global presence, the most widely distributed species of all of the hawks, eagles and kites of the Order Accipitriformes. They can be found on every continent except Antarctica and generally migrate from the northern reaches of Eurasia and the Americas to the warmer climates of South Asia, Central and South America and North Africa. In Australia and other relatively warm areas like the southern states, they are for the most part non-migratory. For those ospreys that do migrate, a hegira that can extend to more than 3,000 miles is not unusual – from a summer in the Northern Hemisphere to a summer in the Southern Hemisphere, a lifestyle once attributed only to peripatetic surfers. In one recorded migration from Massachusetts to French Guinea an average speed of 9 miles per hour was maintained for 13 days. As they are globally dispersed, ospreys are well known to ancient cultures, subject to quasi-scientific observations owing to their proximity to human habitation. The Roman naturalist Pliny the Elder provides in the encyclopedic Naturalis Historia that “the osprey, which has very keen eyesight, and which hovers at a great height and (sic) when it sees a fish in the sea drops on it with a swoop and cleaving the water with its breast catches it.” However, without the benefits of peer review, outlandish claims went uncontested, again Pliny “the sea-eagle only compels still unfledged chicks by beating them to gaze full at the rays of the sun and if it notices one blinking and with its eyes watering, flings it out of the nest as a bastard and not true to stock.” Other cultures had a more positive view; in the Jataka tales of Gautama Buddha in 4th Century BCE India, the osprey is the king of the birds.

Like many birds and most raptors, ospreys are monogamous for the extent of their 15 to 20 year life spans, a fact that is without doubt an evolutionary adaptation resulting from the necessity of brooding 1 to 4 eggs for over a month followed by feeding fledglings in a nesting phase that may last twice as long. This works only with the cooperation of two adult birds, one to hunt for fish and the other to tend the nestlings. This relationship is manifest in the design and construction of the nest, a bricolage affair comprised of sticks, sod, grass, and any other flotsam and jetsam gathered by the male and assembled by the female on any available elevated platform isolated from ground mammal predation. Starting as a rather limited affair, the nest is reused year after year by the same adult pair and can grow to gargantuan size through embellishments presumably added to enhance its strength and sustainability, but perhaps there is something eerily anthropomorphic going on with dwelling size and status. Males go to great lengths to attract females, another human-like trait, that is called the “fish-flight,” consisting of a shallow swooping over the nesting area while encumbered with an impaled fish in an acrobatic and athletic display intended to impress. Language skills are evident in the communication between and among ospreys, though their meanings are still being interpreted. Ospreys have five recognized distinctive calls that have been grouped according to function as guard, alarm, excited, screaming, and solicitation (presumably not the salacious type).

While not endangered (there are an estimated one million ospreys worldwide), ospreys are monitored closely by conservation minded organizations due to their importance as a phenological indicator species, one whose health, well-being and population are harbingers of environmental change. But this was not always the case. Like most raptors that are high in the food chain, the extensive use of long-lasting pesticides, notably DDT (dichloro-diphenyl-trichloroethane) in the decades following its development in World War II until it was banned by the EPA in 1972, wrought havoc on bird populations. This was noted in the osprey monitoring communities of the northeast by the disappearance of about 90 percent of breeding pairs between Boston and New York City by the early 1970’s. It should be duly noted that the widespread use by DDT was not without a great deal of research on its effects by the United States Government and other institutions including universities and the agrochemical industry – it is a toxin after all. Initial tests, such as those by the Sanitary Corps of the U. S. Army which focused on the efficacy of chlorinated hydrocarbons in the eradication of insect pests such as head lice, concluded that DDT was among the best treatments due primarily to the fact that it persisted for long periods of time which would provide better pest management outcomes. This became the argument of the economic entomologists.

However, the environmental persistence of DDT raised the specter of pernicious effects on other wildlife, including laboratory mammals, birds, fish and even humans. In an early study by the Public Health Service in 1944, which found that the LD50 of DDT for rats was 150 milligrams per kilogram of body weight, it was concluded that “The effects of DDT on experimental animals are cumulative, and small single doses given repeatedly lead to chronic poisoning.” (LD50 stands for the lethal dose to 50 percent of a population, a measure developed to minimize the number of animals whose demise is contingent on its calculation – which is, by the way, 5 dead out of 10 tested). However, tests on humans, such as one by the National Institutes of Health (NIH), found that DDT “produces no toxic effects in human subjects and should offer no serious health hazards if used under conditions required for its use as an insecticide.” Ultimately, DDT became the pesticide of choice absent any conclusive a priori evidence of serious environmental harm. The a posteriori long term effects on wildlife (notably fish and birds and therefore especially fish-eating birds like the osprey) had to await the decades- long experiment that was inadvertently conducted in the real world. According to F. Davis is the book Banned, a History of Pesticides and the Science of Toxicology, “Received wisdom marked the DDT ban as one of the great achievements of the environmental movement in the United States, and during the ensuing decades the gradual but pronounced recovery of populations of bald eagles, ospreys, peregrines, brown pelicans and other wildlife confirmed the sense of accomplishment. The effects of DDT were pernicious, and banning the chemical in the U. S mitigated the risks to wildlife.”

The protection of osprey populations requires the institution of regulatory laws; the United Kingdom provides a relevant case study. Osprey populations started to decline there in the late 19th Century due to human predation for eggs, plumes and skin. Even with the efforts of Queen Alexandra, who wrote a letter to the Royal Society for the Protection of Birds to eschew the use of osprey foliage in fashion, the osprey became extinct in the British Isles in 1916. Incidental recolonization started on northern Scotland in 1954 by ospreys migrating from Scandinavia; the reestablishment of nesting pairs in England required human intervention and is still tenuous with only several nesting pairs having been established. Queen Alexandra’s letter did have a positive, if transatlantic, effect. The president of the U. S. Audobon Society sent a copy to President Theodore Roosevelt who expressed “hearty sympathy which he and his wife feel with the work of the Audobon societies generally, and particularly in their efforts to stop the sale and use of so-called aigrettes.” The definitive Migratory Bird Treaty Act was signed into law in 1918.