Photo: Rose Lincoln, Harvard Gazette Staff Photographer
In November 2018, arborist Ben Kirby and I mounted a half dozen artificial nests throughout the Arboretum landscape. Made from old tree planting baskets and landscape fabric and filled with twigs and wood shavings, the nests were created with a goal to increase nest availability for great horned owls. Incapable of building their own nests, this species typically utilizes nests constructed by other large birds or relies on natural cavities in large trees.
After a season of vacancies, we were lucky when a mating pair of owls moved into one of our artificial nests in late January 2020. Due to the location, we were able to observe and collect data on the entire nesting process while remaining on the ground, a rare opportunity. Since the Arboretum is a Chapter of the Cornell Lab of Ornithology’s NestWatch program, our submitted data will help ornithologists better understand great horned owl breeding behavior and population trends.
The photos below chronicle this season’s nesting process, from egg laying in early February to fledging in late April. Since posting photos of active owl nests on social media typically results in increased human disturbance (which can endanger the female and her young), these photos were purposefully withheld until the young had already fledged. The photos were taken from over 150 feet away, with care to limit the time and frequency of each visit in order to minimize disruption.
Photo: John DelRosso
Special thanks to Lucy Merrill-Hills, Stephen Baird, John DelRosso, Fran Perler, and Rose Lincoln for generously allowing use of these images. For permission to copy or share these photos, please email the author for contact information. Even greater thanks to the vast majority of visitors who respected and stayed beyond the roping at the nest site, who limited their observation time, and who reported their observations over the last several months. Easily observing this nesting attempt was unique and very lucky; those who respect our wildlife protection policies make projects like this possible in the landscape.
If you happen to see any species of owl on our grounds, please remember to follow ethical owling practices. Do not approach the owl, keep your voice low, and do not publicize the active known location of any owl on social media. Getting too close, or encouraging others to search for it, could spook the owl from its hiding place and expose it to attacks from crows and hawks. Help us make the Arboretum a safe habitat for all wildlife and with any luck we may be able to observe this charismatic species once again.
A male great horned owl. While rarely seen, this non-migratory species is widely distributed and can be found in a diversity of habitats, from Alaska to South America. Male great horned owls are extremely territorial; the individual in this photo is likely the sole male in the Arboretum landscape. His loud, low pitched calls in early winter (hoo-hoohoo-hoo) advertise his presence to rivals and potential mates alike. Females are a third larger than the males and have similar but higher pitched calls, which they use in a response known as “dueting”. Great horned owls mate for life and it is likely that the pair shown in these photos has been nesting (and will hopefully continue nesting) in the Arboretum for years. Photo: Lucy Merrill-Hills
These owls are fierce apex predators and have no natural enemies. However, they are vulnerable during nesting season. Crows and hawks harass the exposed female and the roosting male whenever they can find them during the day. Owls also regard humans very suspiciously at this time and do not rest when they know people are nearby. Cumulative disturbance prevents them from sleeping and the presence of humans can prevent them from feeding their young. The signage and roping around the nest site prevented visitors from getting too close and other signage encouraged people to spend no more than 5 minutes per infrequent visit. Thanks to the majority of visitors who respected the wildlife by following these recommendations.
A female great horned owl in our artificial nest in early February. Great horned owls are one of the earliest nesting species in North America, beginning their nest attempts between January and March, depending on the region. Incapable of building their own nests, great horned owls in Boston typically take over red tailed hawk nests. Elsewhere, options include raven nests, heron rookeries, osprey platforms, squirrel dreys, tree cavities, and cliff edges. They will even fight for (and often win) nests built by bald eagles. Photo: Lucy Merrill-Hills
Arboretum Head Arborist John DelRosso took this photo while tying emergency support to the nest structure. Female owls typically lay two to four eggs per season, each two days apart. Note the soft feathers surrounding the eggs, likely plucked by the female from her own chest feathers.
The female incubates the eggs by pressing against them with her brood patch (an unfeathered section of skin on her chest). The warmth from her skin keeps the eggs at 98 degrees. She remains on the nest day and night for an average 33 days, leaving only if absolutely necessary. Meanwhile, the male roosts in a nearby tree and watches for threats. He hunts at night and brings her food while she incubates. The owl’s clearly visible “horns” in this photo are actually just feathers. They likely function as daytime camouflage by breaking up the owl’s outline against trees and branches. Though they look like ears, they play no role in the owl’s hearing. Photo: Lucy Merrill-Hills
The owlets in this nest hatched around March 3-4. They emerge blind and with only a light covering of down feathers. They cannot yet regulate their temperatures so the female must continue brooding them. Note that she is sitting more upright to avoid squishing the nestlings; her chest feathers are fluffed out to cover the young birds. She remains with them constantly for another two weeks to keep them warm. The male brings food to the nest, which she tears into small pieces to feed the nestlings. Photo: Lucy Merrill-Hills
At two full weeks old, the owlets can see and are anxious to move about. They have enough insulating feathers to leave the protection of their mom. They begin to explore the nest but rely on their mother to feed them and keep them warm during cold and rainy weather. Photo: Lucy Merrill-Hills
This owlet is still covered in fluffy down, but is growing rapidly, gaining over an ounce a day. At the Arboretum, its diet is likely heavy on mice, voles, chipmunks, squirrels, and rabbits. However, this species is renowned for its diversity of prey. A more expansive list includes skunks, possums, small raccoons, cats, snakes, frogs, crows, coots, ducks, geese, herons, peregrine falcons, hawks, and even osprey and bald eagle nestlings. Although the average great horned owl weighs less than 4.5 pounds, they can lift off with heavier prey. Photo: Lucy Merrill-Hills
The eldest owlet is visible; the mother is still brooding its two siblings. Owlets hatch two days apart in the order the eggs are laid. As a result, the eldest owlet is visibly larger and more developed than the youngest. Photo: Lucy Merrill-Hills
At three weeks, the owlets begin to show their contour feathers. These feathers cover their insulating down and provide shape and camouflage. Note the brown wing bars now showing on the wings. Photo: Lucy Merrill-Hills
The “horns” (actually just feathers) also begin to grow in as well as their white “bib”, just barely visible now beneath its bill. Photo: Lucy Merrill-Hills
Mother and four week old owlet. Great horned owl eyes are proportionally among the largest of terrestrial vertebrates. With binocular like vision and the capability of swiveling their heads 270 degrees, very little escapes their attention. Although fencing prevented visitors from approaching the nest, the female attentively tracked anyone who walked in the area. At this stage, the nestlings are large enough to observe and track potential threats in the landscape as well. Photo: Rose Lincoln, Harvard Gazette Staff Photographer
The owlets are now four weeks old and the female begins to spend less and less time with them. She often leaves and roosts in a nearby tree, all the time watching her young. She may still be regaining her strength after spending over 6 weeks straight in the nest, incubating and brooding her young. Photo: Stephen Baird
The young begin exercising their wing muscles. The dark black undersides of the flight feathers are visible on this nestling. These are pin feathers and blueish sheaths still encapsulate the new growth. The waxy sheaths will crack and fall off naturally as the feathers develop or will be preened off by the owl. The owlets are still incapable of flight. Photo: Stephen Baird
Mother and owlet again, this time after two full days and nights of continuous rain. Grown too large to easily brood, the female did her best to protect her young by pressing against them and covering them with her wings. Great horned owls’ feathers are less water resistant than other species and they cannot fly if completely saturated. Normally, the female would have sheltered close to the trunk of a densely branched tree to avoid getting soaked. Fortunately, the owlets now have sufficient contour and down feathers to survive in the rain. Photo: John DelRosso
The insulating layer of down feathers keeps adult and nestling warm through rain and cold. The owlet is puffed up for warmth, while the female may be purposefully deflated so that the heavy rain runs off her feathers. Photo: John DelRosso
Great horned owl nestlings stare at their mother who has arrived with food. The adult owls typically deliver food to the young in early evening before setting off to hunt. Extra food is often cached in trees around the nest for easy delivery. Photo by Fran Perler
At five weeks, the female now only rarely spends time in the nest. Male and female hunt constantly at night and into the morning, storing extra food in the surrounding trees. Each owlet now requires between four to eight mice every night to continue growing (or the equivalent vole, chipmunk, squirrel, rabbit, skunk, duck, goose, heron, songbird, etc.). Great horned owls are excellent rodent control. Lucy Merrill-Hills
Almost strong enough to leave the nest if necessary (note the more developed flight feathers), it is not uncommon for owlets to attempt flight, fall or glide from the nest, and spend several more days of their development on the ground. The parents will continue to bring them food but the young birds are very vulnerable to animals such as coyotes and dogs as well as well intentioned humans. Never approach an owlet on the ground unless it is injured or in a highly trafficked location. It is illegal for anyone except a certified wildlife rehabilitator to provide care for an injured owl. Photo: Lucy Merrill-Hills
At this age, the owlets spend much time eating leftover food in the nest, picking at one another, stretching their legs, flapping their wings, and preening their feathers. Preening is an essential skill for birds and contributes to keeping their feathers in good condition (perhaps similar to rubbing oil on leather work boots). Preening also works loose the waxy sheaths remaining on their pin feathers. Photo: John DelRosso
Increasingly inquisitive, they actively shuffle and hop around the nest in order to peer at objects in the landscape. The young owls bob their heads up and down and side to side when looking at something. This helps them judge distance with depth perception, an essential skill for this species. Great horned owls are ambush predators, preferring to sit still on a branch while patiently using their sharp hearing to locate prey and eyes to narrow in on its movement. Young birds must learn how to gauge depth perception in order to feed themselves when they leave the nest; many juvenile great horned owls sadly fail to catch enough food during their first year. Photo: John DelRosso
This owlet has nubby horns, white bib, and barred feathers characteristic of the species. By the following year, it will have lost all of its juvenile plumage and gained the darker brown plumage of its parents. Photo: John DelRosso
An owl’s posture can reflect the threat it perceives in an environment. Sitting tall, feathers sucked in, with horns upright is meant to help them blend in with the branches. The horns (when fully developed) are thought to help break up the outline of their bodies. Sometime the nestlings lay flat in the nest. This helps them to avoid attention and attacks by red-tailed hawks and crows, which occasionally harass the nestlings. When the adults see a threat, they begin hooting loudly, signaling the young to take cover. The female will fly down to the nest to remain with the nestlings if the threat persists. Photo: Stephen Baird
The facial disks (which funnel noise to the owls’ ears) are clearly defined on these young. Their “ears” are actually small openings located under their feathers, just to the outside of either eye. Having the ears closer to the front of the face allows them to listen for prey and then narrow in on its location with their strong vision. They cannot move the pupils in their eyes so instead they turn their entire heads, which can rotate 270 degrees. Photo: Stephen Baird
Using the definition of their facial disks as a proxy for development, it appears that the eldest nestling may be in the front, the second oldest in the back, and the youngest to the left. Photo: Stephen Baird
Another look at a potential age line up. The owl on the right seems to be the youngest of the siblings. Note the less developed facial disk and horns compared to the nestling on the left.
Although space is now limited, they find a way to exercise their growing muscles. Note the smaller amount of feather sheaths still visible on the wing feathers, at the point where the feathers attach to the wing.Photo: Stephen Baird
Note the small amount of feather sheaths still visible on the wing feathers, at the point where the feathers attach to the wing. Photo: Stephen Baird
Stationary practice flight is a sign that these large nestlings are almost ready to leave the nest. To practice, however, the owlets must hold tight to the nest or adjoining branch while flapping. This puts pressure on the nest and is part of the reason that great horned owls rarely reuse nests for more than a few consecutive years. The combination of adults landing and taking off, young owlets practice flapping, and degradation by weather eventually destroys most nests they occupy. Photo: Stephen Baird
Early naturalists referred to this species as “Tiger of the Skies” or “Tiger Owl”. This is partly due to the gold and brown baring visible on this nestling’s outstretched wings and mostly due to their ferocity as apex predators. These owls have no natural predators in New England and, as previously noted, attack animals larger and heavier than they are. This nestling’s wing feathers have almost finished developing but its tail feathers are still growing in. Note the pin feathers, with feather sheaths still intact. Photo: Stephen Baird
Three owlets, over six weeks old. Their infant fuzzy down feathers are now being replaced or covered by contour and flight feathers. Photo: John DelRosso
Although physically large, male nestlings typically weigh little more than two pounds when they fledge while females (which are always larger) may weigh closer to three. Photo: John DelRosso
The nestlings looking towards their parents in the tree tops. Soon, they will leave the nest and either walk out along the adjacent branches (a life stage fittingly called “branching”) or fly to the ground and attempt to climb into nearby trees or shrubs (they are surprisingly good climbers). The parents increasingly call to them as if encouraging them to leave. When they are capable of flight, they will fly towards their parents whenever they catch sight of them, often while emitting begging cries for food. Photo: Fran Perler
The three nestlings sharing the nest, two visible and one laying down behind. A big yawn from the nestling on the left. Photo: Fran Perler
Two of the three owlets fledged (left the nest) on Sunday, April 19. The last nestling remained in the nest for several days after its siblings departed. It may be the case that this remaining nestling was the youngest of the three, up to four days younger and less developed than the eldest. Note the darker feathers near the base of the nestling’s tail feathers, an indication of its future coloration. The young owl spent much of its time preening and pulling out its downy feathers. Photo: John DelRossoPhoto: John DelRosso
The adults continued to feed the solitary nestling, while also attending to a fledged sibling in a nearby tree. The owlet had more space in the nest to move about and stretch and seemed to enjoy the room. Photo: John DelRosso
This nestling successfully fledged at some point during the evening of April 22. Depending on its hatch sequence, it would have been somewhere between 44-48 days old. At this age, the owlet was capable of making short flights. It likely walked along the branches then flew to a nearby tree (or flew to the ground and climbed up). After 83 consecutive days of providing a safe place for the adult owls to raise their young, the nest is now empty. Photo: John DelRosso
In the weeks following, at least one of the young was seen nearby. It is important not to frequently look for or disturb these juvenile owls as they face longs odds of survival and need all the rest they can get. Even with parental help, fewer than 30% percent survive their first year. With skill and luck, though, the survivors may reach over 25 years old in the wild. Give this owl a helping hand by remembering to owl ethically. If you see an owl in the landscape, do not approach it, keep your voice low, and do not post its location on social media.
While the nest may now be empty, the fencing around the nest tree will remain in place for several more weeks. If they survive, the young will gradually become more adept at flying. Eventually, they will follow their parents to other parts of the Arboretum. They will stay under their parent’s watchful eyes throughout the summer and into the fall, begging for food while becoming proficient hunters themselves.
By next September, the survivors will increasingly resemble the adults. At this point, the adults will stop feeding them and may actively try to lose them in the landscape. If the juveniles do not take the hint, the male will finally chase them from the grounds when breeding season begins anew next winter. They will have to find their own territories, mates, and nesting sites, none of which are abundant in urban areas.
The Arnold Arboretum is committed to enhancing the ecological value of our unique urban landscape in order to better support wildlife such as great horned owls. Thanks to all who respected the well being of these fascinating birds by following the Arboretum’s policies, signage, and roping put in place to protect them. Visitors and the many members who support the Arboretum help make these types of projects safe for wildlife and possible in the first place—thank you.