Healthy Homes

June 01, 2020

Carolyn Bernhardt, Illustrations by Nathan Pasch

“At The Raptor Center (TRC), we look at how well a nest is doing to learn about the health of the raptors within,” says Lori Arent, MS, assistant director at TRC. “However, they also tell us about the health of the environment in which the birds live.” For this reason, Arent says that people who find a raptor nest should leave it alone. “They can, of course, always call us if they sense a problem.”

A nest’s contents reveal a lot of valuable information. “For example, if the female is healthy, she may be more likely to lay the maximum clutch size for her species,” says Arent, “but if she is more nutritionally stressed, she may not lay eggs.”

Additionally, the things raptors use to line their nests can indicate what is readily available in their environment. “Raptors have certain things they prefer to line their nests with, and if unusual things are used—sometimes these can pose hazards to the young—it could reflect habitat loss or that they have easy access to things humans leave behind in the environment,” Arent says.

TRC has long taught its volunteers to identify nests and to bolster them. A few times a year, the rehabilitation coordinator at TRC leads a group of volunteers through the process of building and reinforcing original nests in order to help raptors in the wild be more successful. Plus, this effort helps TRC anticipate which nests will be successful for fostering the nestlings that come into the clinic, upon their release.

“At TRC, we focus our energy on either returning youngsters to their original nests or fostering them into different nests of the same species, rather than raising them ourselves. No matter how hard we try, we cannot do so nearly as well as parents can,” says Arent. “And we are always looking for additional volunteer climbers to help us place nests we have constructed. Training would of course be provided.”

TRC’s investment in nests, as well as in planting the trees for future nesting habitats, is all in an effort to create a sustainable future for raptors near and far.

Bald eagles
stick nesters

Average size: Among the largest nests made by birds, bald eagle nests are 5–6 feet in diameter and 2–4 feet tall. They can be cylindrical, cone-shaped, or round and flat, depending on the tree in which they are built.

Location: Bald eagles tend to use tall, sturdy pine trees that emerge above the forest canopy for good flight access and visibility. They usually arrange their nests high up in the tree and near the trunk.

Materials: Sticks provide the nest with structure and stability; dead grass, cornstalks, moss, and downy feathers insulate and soften the interior.

Strategy: Both the male and female gather materials for building their nest. The female arranges the nest, the male rearranges it, and the female arranges it again. The pair weave sticks together, filling gaps in with grass, moss, or cornstalks. It can take the breeding pair up to three months to build their nest, and many tend to reuse (and add to) their nests year after year.

Clutch size range: 1–3 eggs

Timing: 34–36 days of egg incubation, followed by nestlings in the nest for 56–98 days

From the expert: “Bald eagles are faithful to a breeding site, not necessarily a breeding partner,” Arent says. “If they find a place they like that provides them with ample food options and a safe place to raise their young, they will breed there year after year.”

Northern harriers
ground nesters

Average size: The exterior is roughly 16–24 inches wide by around 1.5–8 inches high, while the interior is about 8–10 inches wide by roughly 2–4 inches deep.

Location: Tall grasses in fields

Materials: Thick-stalked plants like cattails, alders, and willows are used for the outer part of the nest. The inner lining is made out of grasses, sedges, and rushes.

Strategy: Nest building takes the breeding pair 1–2 weeks. Either the male or the female may choose the nesting site. The male may start building the nest platform, but the female finishes it. Later, both bring in materials, but the female arranges them.

Clutch size range: 4–5 eggs

Timing: 28–36 days of egg incubation, followed by nestlings in the nest for 14 days

From the expert: “At about two weeks of age, northern harrier chicks start taking short walking excursions from their nest site,” says Arent. “This frees the female to leave and help the male meet the increasing food demands of the growing chicks. Before this time, the male is rather busy as the sole food provider.”

American kestrels
cavity nesters

Average size: Since kestrels often choose natural cavities, it is hard to pinpoint an average size. However, when building a nest box, TRC recommends 16 inches high by 9–10 inches wide. But the most important thing is that the access hole is about 3 inches in diameter.

Location: Kestrels usually nest in trees along wood edges or in the middle of open ground, but they will readily take to human-made nest boxes.

Materials: Kestrels pluck down feathers from their abdomen to create a spot where their skin can have direct contact with the eggs—known as a brood patch. Typically, these plucked feathers are the only materials used to line the nest.

Strategy: The male kestrel searches for possible nest cavities. When he finds a few options, he shows them to the female, who ultimately decides where the pair will nest. She hollows out a small dip in the organic materials at the base of the nest.

Clutch size range: 4–5 eggs

Timing: 26–32 days of egg incubation, followed by nestlings in the nest for 28–31 days

From the expert: “Nest boxes for kestrels have to be made a certain way and placed in the proper environment,” Arent says. “An area close to an open field bordered with trees is ideal because the bird doesn’t have to fly too far to hunt. TRC has a nest box plan for anyone interested in building a box.”

Great horned owls
opportunistic nesters

Average size: This varies based on what animal built the nest.

Location: Great horned owls usually take over nests built by other animals, so their location varies. But great horned owls can also nest in cavities in live trees. They may also nest in some unexpected places, such as dead snags, deserted buildings, cliff ledges, and human-made platforms.

Materials: Adopted nests are usually made by red-tailed hawks, crows, or squirrels and made out of sticks and vegetation.

Strategy: Great horned owls do not work to maintain the nest they adopt, so by the time their clutch of eggs hatch, it may be a little worse for wear due to harsh winter weather. They rarely reuse a nest in subsequent mating seasons. In some areas, they may line the nest with shreds of bark, leaves, downy feathers plucked from their own breast, fur or feathers from prey, or trampled pellets. In others, they may not line the nest at all.

Clutch size range: 1–4 eggs

Timing: 30–37 days of egg incubation, followed by nestlings in the nest for up to 42 days

From the expert: “At TRC, we often see young great horned owls in the clinic, both because great horned owls are the most common large owl found in the Twin Cities area, and because their young frequently fall from their haphazard nests after hatching in the spring,” Arent says. “There is estimated to be roughly one great horned owl nest every two miles in the Twin Cities.”

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