A Builder, A
Collector, A Scratcher, and A Thief
By Julie Collier
From DVRC Journals Spring/Summer 97 and Fall/Winter 97
As I write the first draft of this article, New England is still reeling from a disastrous April Fool's Day nor'easter that hammered Boston to a standstill and dumped as much as three feet of snow on some areas of Massachusetts. Calamitously for the region's trees, the snow was wet and clinging. The Innuit, who are said to have dozens of words to describe different snow types, may well have a special word for this heavy, hard-to-shovel, sloppy snow. Certainly the people of my area came up with an impressive, very descriptive list of terms for the white stuff, none of them printable. Trees everywhere groaned under the extraordinary weight. Many toppled, uprooted by the unusual combination of thawed-out soil and snow-laden branches. Those that remained standing lost crowns and major branches as overburdened wood snapped. Vast swaths of forest now look as though a gigantic bowling ball was rolled through them, leaving a trail of broken branches, fallen trees and saplings bowed to the ground. The sounds of spring this year, instead of the calls of nesting songbirds, have been the whine of the chain saw and the roar of the wood chipper.
Besides the clean-up companies, there is one Massachusetts resident who is happy about the nor'easter's damage. Lakota, the female golden eagle that has lived with me for several years, is the recipient of the branches that broke off my white pine tree. There were a lot of them, and I've chopped them up so she gets the smaller boughs with plenty of twigs and needles. These I either carry into her flight cage or poke partway through the chain-link outer wall of her cage so she can pull them into her compound herself. This she does for an hour at a time, carrying each branch into her inside sleeping compound. Here her bounty is carefully being woven into a large and somewhat inelegant nest. It's not Lakota's fault that the nest isn't a study in symmetry; of necessity it's being built on the ground. With no tree or cliff to give the construction a supporting framework, the nest sprawls in a rough circle about four feet across. Using her chest to push out from the nest's center, Lakota has gradually molded a central depression where, if she had a normal life in the wild with a mate, she would lay and incubate her eggs. Unfortunately for both Lakota and me she does not have a mate; at least, not an eaglish one. What she does have is me. She's been with me so long she has bonded with me as she would bond with a male in the wild. Deeply complimented as I am by her friendship, being her surrogate mate has its snags. For starters, I am anatomically incorrect by both gender and species. That's a source of considerable frustration and confusion for Lakota. My frustration stems from the fact that she expects me to share the nest with her. I am, after all, enacting the part of a male golden eagle by supplying her with nesting material. Why can't I fulfill the rest of the duties that instinct tells her her mate should be performing?
As I've watched and participated in this tragicomic nesting I've begun thinking about our other North American eagle species, the bald eagle. How would its nest differ from a golden's? And what about the nests of our other raptors? We have a breeding pair of peregrine falcons right here in Springfield, one of only four nesting pairs in Massachusetts. Area residents have become quite familiar with this particular pair's home life, because the local cable company, with the permission of wildlife officials, has installed a camera that gives us a bird's-eye view of the nest box and its inhabitants. Channel 19B is now known as the Falcon Channel. The nestbox, which is simply a large shallow pan filled with gravel, was supplied by wildlife officials. It was installed on an outside ledge of a downtown high rise because peregrines had been observed checking out this particular building. Since 1989 peregrines have been raising young in this barren, unadorned box more than twenty stories above Springfield. The falcon eggs are laid directly onto the gravel surface, and the young are brooded on this uncomfortable-looking material. The peregrine falcon's box is about as different from Lakota's comfy, stick-and-greenery nest as one can imagine. What are other raptor nests like? Do most birds of prey build stick structures like Lakota, or do they opt to simply appropriate a handy cliff of skyscraper ledge and without fanfare lay their eggs and brood their young directly on its hard, uncompromising surface? Let's take a closer look at the nests and nesting behavior of four North American birds of prey.
The "builder" of our title is the bald eagle, which in fact constructs the world's largest nest made by a single pair of birds (some communal nesters such as Old World Weaverbirds make group nests that can be larger). Our national bird, which lives nowhere but in North America, shows strong fidelity to nest sites, which may be used (and added to) year after year. An Ohio nest that was used and refurbished every year for more than three decades ultimately weighed two tons and toppled its host tree. That nest, which fell in 1925, is outdone, at least dimensionally, by a Florida nest that measures nine and one-half feet across and 20 feet deep. While these two behemoths are spectacular example's of one species' drive to build, it should be pointed out that such constructions are probably the labor of more than one eagle generation, piling up material for decade after decade. A more typical, newer nest would measure something like five, six or seven feet across and roughly the same in depth.
The huge size and weight of the bald eagle's nest is due to the twin facts that eagles return to the same home range to breed every year and that the big birds, if they are fortunate, are long-lived. And since the eagles always add new material to an old nest - usually the refurbishment consists of a roughly foot-high layer of branches and smaller vegetation woven around the existing nest's rim - a nest that has been in use for many years may reach proportions that are a hazard to the support tree.
Which is usually what kind? And do bald eagles always nest in trees? The answer to the latter question is no. Bald eagles that breed in the far north where trees are large enough to house a nest are hard to come by raise their young in ground nests. That's unusual, however, and the results of circumstance, not the eagle's preference. The usual nest site is a tall tree. This may be a conifer, like a white pine, or a deciduous species such as an oak. Bald eagles nesting in Florida Bay rely heavily on mangroves, which are usually the tallest trees in their environment. Still, mangroves are only tall relative to other trees in their area, and bald eagle nests on these islands in Florida Bay are often no more than 30 feet up. Such low-riding nests can't really be called eyries. That distinction belongs to the northern birds. Tree shape and location matter more than tree type. Most important is the host tree's position near water. Bald eagles are primarily fish eaters; another major food source is waterfowl (add to this water-related diet water turtles for the Florida bald eagles, which snatch diamondback terrapin from the surface as the turtles come up to breath). You will notice that I did not use the word "hunter" in that sentence. Balds are just as apt to scavenge a meal as they are to hunt for it. Indeed, the places in Alaska where large numbers of eagles congregate are the places where dying salmon collect in the shallows of rivers. Thousands of bald eagles assemble annually at the Chilkat River in Alaska. The salmon, having laid and fertilized their eggs, have fulfilled their purpose. Now they float, dead or dying, an easy target for a foraging bald eagle. All the bird has to do is wade into the river, pluck out a helpless victim, and retire to the bank to feast. Even amid this bounty frequent squabbles break out, for bald eagles are dedicated thieves as well as scavengers, and find robbing their neighbor easier than doing their own fishing. Scenes like this occur after the breeding season. Eagles do not nest in huge groups like this because there would be constant competition and harassment from other eagles. So bald eagles defend a nesting territory, which is always some distance, perhaps a mile, from another bald eagle's nesting territory. This strategy isn't to prevent other bald eagles from preying on another pair's nestlings, but rather to cut down on competition for food.
The nest is rarely placed in the
topmost branches of the host tree because thin upper boughs can't support a structure that
even unexceptionally is four or five feet across, the same in depth, and weighs several
hundred pounds. A new nest usually starts with a major branch a dozen or more feet from
the top of the tree. A fork in the branch helps hold the first sticks in place.
Photographer Scott Nielson writes in A Season with Eagles that this can be ". . . the
most difficult stage of nest building. I've seen potential nest trees littered with
branches at their bases and climbed them only to find the unfinished nests, aborted simply
because the first few sticks would not stay in place." If the initial stage of stick
placement is successful, both sexes will continue to bring in sticks, frequently snapping
them off with their powerful feet while in flight. An old nest being reused is added to
with the same degree of enthusiasm. With some of the sticks being brought in measuring six
feet or more in length. Smaller branches form the rim of the inside bowl; spaces between
the branches is filled in with smaller vegetation such as grasses, rushes, moss, cattails
and conifer sprigs. Dr. Nielson calls bald eagles "compulsive nest builders"
that simply must rework last year's nest, no matter how complete it looks to a human
observer. It may well be that the mutual building helps cement the pair bond and bring the
female into breeding condition. A new nest takes several days to construct, while
revamping a used one can be done in a few days. The work isn't constant; both sexes spend
a great deal of time in those early nesting-season days perched near the nest and closely
watching the surrounding area. Disturbance at this point, whether caused by a human
intruder or by an animal perceived as a threat can result in the eagle's deserting the
nest. This pre-egg-laying scrutiny is crucial. Once the eggs are laid the eagles are
obliged to settle in for the long haul. Incubation takes five weeks, and then the eaglets
don't fledge until they're ten weeks old. They won't be independent of their parents until
they're four months old or more. So the nest had better be safe, and it had better be
sturdy, because it's going to take a pounding from the growing eaglets. Bald eagles usually lay two
eggs, and both eaglets usually survive into adolescence. They help to keep the nest
hygienic as they grow older by defecating over the rim. The central hollow that the adults
created by wriggling their bodies from side to side begins to fill up with prey remains,
pellets ejected by the young, and with green sprigs brought more or less constantly to the
nest site by the adults. Some researchers theorized that the greenery, some of which
contains natural insecticides, may help keep the nest free of insect pests. I also think
the adults are strongly attracted to greenery as part of their nesting/breeding cycle, and
have a strong need to handle them during this period. The adult birds may also snap off
twigs from the top of the nest tree in order to get a better view of their surroundings. At the finish of the nesting
season the young leave to seek their own fortunes and their own home range. The adult
birds may split up for a time, but if all goes well they will return to the original home
range the following spring. The home range covers between 10 and 15 square miles and the
nest or nests the adults use are always located within this area. The nesting territory
defended by the paired adults is much smaller; perhaps it is only the square mile directly
surrounding the nest. Nests may be located wherever a favorable site is discovered within
the home range.
The nest is rarely placed in the topmost branches of the host tree because thin upper boughs can't support a structure that even unexceptionally is four or five feet across, the same in depth, and weighs several hundred pounds. A new nest usually starts with a major branch a dozen or more feet from the top of the tree. A fork in the branch helps hold the first sticks in place. Photographer Scott Nielson writes in A Season with Eagles that this can be ". . . the most difficult stage of nest building. I've seen potential nest trees littered with branches at their bases and climbed them only to find the unfinished nests, aborted simply because the first few sticks would not stay in place."
If the initial stage of stick placement is successful, both sexes will continue to bring in sticks, frequently snapping them off with their powerful feet while in flight. An old nest being reused is added to with the same degree of enthusiasm. With some of the sticks being brought in measuring six feet or more in length. Smaller branches form the rim of the inside bowl; spaces between the branches is filled in with smaller vegetation such as grasses, rushes, moss, cattails and conifer sprigs. Dr. Nielson calls bald eagles "compulsive nest builders" that simply must rework last year's nest, no matter how complete it looks to a human observer. It may well be that the mutual building helps cement the pair bond and bring the female into breeding condition. A new nest takes several days to construct, while revamping a used one can be done in a few days. The work isn't constant; both sexes spend a great deal of time in those early nesting-season days perched near the nest and closely watching the surrounding area. Disturbance at this point, whether caused by a human intruder or by an animal perceived as a threat can result in the eagle's deserting the nest. This pre-egg-laying scrutiny is crucial. Once the eggs are laid the eagles are obliged to settle in for the long haul. Incubation takes five weeks, and then the eaglets don't fledge until they're ten weeks old. They won't be independent of their parents until they're four months old or more. So the nest had better be safe, and it had better be sturdy, because it's going to take a pounding from the growing eaglets.
Bald eagles usually lay two eggs, and both eaglets usually survive into adolescence. They help to keep the nest hygienic as they grow older by defecating over the rim. The central hollow that the adults created by wriggling their bodies from side to side begins to fill up with prey remains, pellets ejected by the young, and with green sprigs brought more or less constantly to the nest site by the adults. Some researchers theorized that the greenery, some of which contains natural insecticides, may help keep the nest free of insect pests. I also think the adults are strongly attracted to greenery as part of their nesting/breeding cycle, and have a strong need to handle them during this period. The adult birds may also snap off twigs from the top of the nest tree in order to get a better view of their surroundings.
At the finish of the nesting season the young leave to seek their own fortunes and their own home range. The adult birds may split up for a time, but if all goes well they will return to the original home range the following spring. The home range covers between 10 and 15 square miles and the nest or nests the adults use are always located within this area. The nesting territory defended by the paired adults is much smaller; perhaps it is only the square mile directly surrounding the nest. Nests may be located wherever a favorable site is discovered within the home range.
Osprey share many nesting traits and techniques with bald eagles. Like the eagles the osprey builds a stick nest and returns to it year after year. But there are major differences between what the eagle chooses for a nest site and what the osprey deems appropriate. Ospreys like dead trees, and often build their nests in the very topmost branches. They get away with this latter location because their nests are smaller than those of bald eagles. The nests are also more symmetrical, more rounded and altogether daintier than the gargantuan, sloppy constructions of the bald eagle.
Well, perhaps it would be more accurate to say that the osprey's nest is daintier on the outside, because the inside can get pretty cluttered. This bird is the "collector" of our title, and will pick up all manner of flotsam and jetsam and cart it back to the nest. Unlike bald eagles, which do the same thing with greenery, ospreys frequently nest out in the open, using exposed trees or man-made platforms. Lacking greenery in their surrounding area but having, as eagles do, a strong desire to carry items back to the nest, the osprey resorts to beachcombing. The results can tell how the world has changed. Writing in Ospreys: A Natural and Unnatural History, Alan Poole lists the oddities found in osprey nests in the nineteenth century (a rag doll; a small doormat; a toy sailboat complete with sail; a feather duster; barrel staves and hoops; a bootjack; 20 feet of hem pen rope; a blacking brush; a remnant from an oilskin rainslicker; and bleached-out bones from domestic animals like cattle and sheep) and the t w entieth (rubber boots; bicycle tires; Hula Hoops; sections of television antennas; and styrofoam cups and plastic hamburger containers).
Ospreys in North America nest from northern Alaska eastward to Labrador; along the Atlantic coast as far south as the Florida Keys and along the Pacific coast as far south as Mexico. Once the nesting season is over almost all migrate, some wintering as far south as Central and South America. First-year birds stay put when spring and the adult birds return to the north. The immatures won't leave the wintering grounds until their second year, and they won't breed until they're three. The older birds return to the site of last year's nest, the male arriving first. He may greet a female with an undulating courtship flight to entice her to the nest.
Ospreys, unlike bald eagles, which space their nests at least a mile apart, often nest colonially. A colony may contain 50 or more nests. Such gatherings are found only where fish are unusually plentiful. In earlier times Gardiner's Island in Long Island Sound had a colony of approximately 300 nesting pairs. Some of the Gardiner's Island ospreys nested directly on the ground, something that has been observed in the Florida Keys as well. Apparently this occurs because suitable trees aren't always available. A typical osprey nest isn't on the ground. It's up high, and unlike the bald eagle this raptor doesn't seem to mind being out in the open. Ospreys seem to deliberately chose highly visible nesting sites, or perhaps it's more accurate to say that such a site makes the surroundings highly visible to the osprey. Besides tall trees, preferably dead, the birds favor utility poles, billboards, communication towers and even channel marks. One Florida pair slapped a nest together on the boom of a sidelined construction crane.
The species' preference for out-in-the-open nests may explain why nestling ospreys wear camouflaging coats of mottled down. Aerial predators such as bald eagles, gulls, crows, and ravens would have difficulty in spotting the young from above. The nest is constructed of sticks and branches, and is added to annually, although not to the extent that bald eagles nests are. The average weight of 400 pounds is therefore more modest than that of a bald eagle's nest but is still sizable.
Like the osprey (but unlike the bald eagle), the peregrine falcon is found throughout the Old World as well as the New World. In North America the peregrine is usually a cliff-dweller, although sometimes the "cliffs" are man-made substitutes - high-rise buildings or large bridges.
Peregrines are highly specialized birds, exquisitely adapted to finding and catching other birds from the air. While accipiters such as goshawks or Cooper's hawks frequently hunt other birds, they don't typically use an aerial search to find their prey. Accipiters employ still-hunting-hunting from a perch-or they glide swiftly from tree to tree, moving like ghosts through the branches. The peregrine, by contrast, is a bird of the open spaces, and its chosen habitat includes water. Its old name in America was "duck hawk" because it often takes waterfowl. Some peregrines have learned that the movement of small boats along waterways will scare up prey, and the falcons will deliberately follow the boats, waiting for prey to take to the air and thus expose itself to the peregrine's slashing aerial attack.
Sleek, streamlined predators, elegantly designed for their predatory role, peregrines are terrible home builders. As a matter of fact, they don't even try. What is crucial to a peregrine looking for an eyrie site is not a tall nest tree, but a high vantage point and a situation that enables the falcon to sneak up on its prey. Cliffs and canyons fill the bill (literally), and thus the peregrine is not typically a woodland nester, but a haunter of high places, with open vistas laid out below. Lakes, rivers, and seacoasts are almost always in the picture because of the prey they attract. As is the case with osprey and bald eagle nesting sites, favorable peregrine nesting locations are not easy to come by, and the birds tend to return to the same spot year after year. Or rather a peregrine pair will use the same nest site year after year, but not necessarily the original pair. Observers in days gone by used to tell of eyries where the same birds had nested for sixty years or more. Recent banding experiments have shown that theaseme ryrie may well be occupied by different peregrines after a few years, with another turnover in another few years. So strong is this site fidelity that some eyries in England have had an almost constant peregrine tenancy for centuries.
If in fidelity to nesting sites peregrines are comparable to bald eagles and ospreys, the falcons are totally different as architects. Peregrines do not deign to construct nests (Although very rarely they will utilize the nest of another bird. Before DDT wiped out the eastern peregrine, peregrines of the central Mississippi Valley used the abandoned nests of other species. This highly unusual, strictly local adaptation disappeared when the falcons died out.). I've often wondered why. Peregrine beaks are notched and "toothed," specially designed to break the neck of their prey; peregrine toes are long and more delicate than the powerful feet of the osprey and the eagle. Are falcons not properly constructed for nest construction? That seems unlikely, since other birds with equally specialized beaks and feet manage to build themselves perfectly serviceable nests. A more likely explanation is that, typically, peregrine terrain doesn't lend itself to building a stick or grass nest. Peregrines favor heights, rocks and cliffs that give their young protection from land predators like foxes and weasels as well as from aerial predators.
So the "nest" is no nest at all really, for the peregrine it is a cliff face, giving protection from below and above . A good cliff area, such as the one in the Grand Canyon overlooking the Colorado River, offers a variety of ledges to the adult falcons for roosting and for plucking and eating prey. The male, who does the hunting for the female as her eggs develop within her body, as she incubates, and later as she broods the developing young, will need a nearby ledge from which to transfer food to the female. She in turn will need to get away from her growing young from time to time. So several ledges are desirable. The nesting ledge often faces south, and may be positioned under an outcropping of rock that serves as protection from the elements, as well as hiding the vulnerable young from the eyes of predatory ravens and great horned owls.
As the breeding season approaches, the male peregrine will fly from ledge to ledge on the selected cliff, scraping suggestively at the surface of each with his feet and calling to the female. She follows, watching his behavior. Her selection will determine the nesting ledge. She may try out several, literally trying each on for size. She'll foot-scrape the ledge. scratching aside the thin soil and rock debris. She may start with a natural depression in the rock. Foot-scratching (sometimes the beak is used as well) is followed by body-molding, as the female uses her breast to push the hollowed-out area into her own body's shape. If the result doesn't feel right she'll try another spot on the same ledge. Or she'll abandon the ledge altogether, selecting another rock shelf somewhere else on the cliff. The early scratch-shapes are often rejected. Perhaps trying out several potential nests is part of peregrine courtship ritual. Once the actual nest site is selected and scratched out, that's it - no greenery is added, no plucked feathers soften the rocky surface. The eggs, usually three or four, are laid in this shallow depression that for peregrines functions perfectly effectively as a nest. Now, of course, there's a modern variant. Peregrines are nesting in cities (almost invariably cities like Boston, New York, Chicago and Springfield that have a nearby body of water), choosing ledges on high rises or bridges. These man-made ledges apparently work as well as cliff ledges for the falcons, since the height of the artificial cliffs gives the birds the vantage points and the protection they need, the city pigeons and starlings provide them with plentiful food uncontaminated with agricultural pesticides, and two dangerous predators of the young - ravens and great horned owls - are not present. One theory as to why Massachusetts peregrines have not recolonized their former mountain eyries is the threatening presence of ravens, which have greatly expanded their range in this state in the past few decades.
Great Horned Owl
The great horned owl preys not only on young peregrines, it preys on virtually everything that crosses its path. Because it is a nocturnal hunter, it can and does capture diurnal birds at night when they are helpless. It frequently raids crow roosts, which may be why crows will mob this species unmercifully when they encounter it abroad by day. Great horneds have also been known to tackle such large and potentially dangerous birds as great blue herons, ospreys, and even bald eagles. Strongly territorial. this big and aggressive owl, largest of the Northeast's breeding owls (the snowy owl outweighs it but does not nest in the Northeast), does not allow other owl species in its nesting territory. It's not welcoming of other birds of prey in general. An oft-repeated story is that great horned owls and red-tailed hawks will share a nesting area. The birds don't get in each other' s way, goes the story, since the hawks hunt by day and the owls take over at night. Like so many charming stories this one doesn't hold up under close scrutiny. True enough, the two species are at home in the same type of habitat, typically woodland broken by fields or meadows that offer a variety of hunting opportunities. That isn't to say, however, that the hawk and the owl dwell in different-shift bonhomie. With rare exceptions, neither species will allow the other a close approach to its nest, and again with rare exceptions when a dispute arises it's the owl that emerges victorious. One researcher, investigating eleven such territorial disagreements, discovered that the owls won nine of them.
Won what, exactly? Won the actual, physical nest, because the great horned owl is the "thief' of this article's title, a bird that does not construct its own nest. Owls as a group rarely build nests of their own and when they do attempt it, by all accounts the result is nothing that will be featured in Architectural Digest. Great horned owls do lay their eggs in natural locations such as broken-off tree snags. They infrequently utilize cliff ledges, caves, even cacti . In our area, however, the nest of choice is a pre-built one in a live tree. The rightful owner may be a hawk. a heron, a crow, or a squirrel. Since red-tailed hawks occur in the same habitat as great horneds, it is often this hawk that loses its nest to the owl. Male great horneds move back into their breeding territories in November, and courtship starts up in December. The female may be sitting on eggs in February, which makes this species our earliest nester. Therefore. while red-tailed hawks are still occupied solely with hunting, the owls will have quietly selected a home and moved in. By the time the hawks are serious about looking over last year's nest, the owls are caring for young, and are in no mood to vacate the premises, stolen or not. The red-tails, who rarely risk an outright battle with great horneds, will in all likelihood quickly realize that discretion is the better part of survival, and move on. The hawks may build an entirely new nest, or they may refurbish an old one. Red-tails often construct several nests in their territory, and may use a different nest from the preceding year's even if unmolested by owls. The nest the hawks occupy, whether new or used, may be fairly near the owl's nest. But with rare exceptions it will not be within the owl's nesting territory, which occupies anywhere from one-third to two square miles.
So great horneds are nest-appropriators, taking over the dwellings of others (in some cases, such as when they claim a squirrel's drey, the rightful resident may be eaten first). Once the owls take possession, do they alter the nest in any way? Not by much, if at all. A few feathers from the female may be added, but greenery isn't. The eggs may be laid directly onto a hard surface, therefore, but their distinctly ovoid shape will help prevent them from rolling. In the Northeast, typically two are laid; in the Midwest and West, where more food is available, the average clutch size rises to three or even four eggs.
A great horned owl nest may not be too far from your house, even if you live in the suburbs. This change is recent: formerly great horned owls tended to nest far from human habitation. Leon Augustus Hausman, writing in Birds of Prey of Northeastern North America, published in 1948, says that: "The more unsettled parts of our country where impenetrable swamps, deep lowland forests, and wild wooded mountain and hillsides abound are the common domains of the Great Horned Owls. Seldom are they found close to human dwellings. Unlike many of our native birds, they shun civilization and retreat before man's progressive occupation of the land." Well, that's no longer exactly the case. Great horned owls have adapted fairly well to humans and their manipulation and alteration of the landscape, and now the birds do nest at times fairly close to people. One great horned owl in California is reported to have nested behind one of the letters of a supermarket sign. It would be interesting to know if this particular owl fledged any young. Great horneds nesting near people may find themselves so disturbed by well-meaning birders that they finally desert the nest, leaving the young to fend for themselves. That happened to a great horned owl nest on the North Shore of Massachusetts, and the young ended up being cared for by rehabilitators until they were old enough to take care of themselves.