The American kestrel is a pint-sized yet ferocious aerial predator notorious for taking out tree swallows or bluebirds in mid-air. About the size of a mourning dove, this raptor is the smallest and most colorful falcon in North America.

But the species is also in danger. Kestrel populations have been falling, with the Mid-Atlantic region experiencing the greatest decline. According to the U.S. Geological Survey, the species’ ubiquity has decreased about 93% over the past half-century.

In 2013, it was listed as endangered in Delaware, and two years later, the state added the bird as a Tier 1 Species of Greatest Conservation Need in its Wildlife Action Plan. That status persists today.

The suspected culprits causing the kestrel decline are loss of nesting habitat, invasive species, pesticide accumulation, climate change and disease, although ultimately questions about what’s driving the population decrease remain.

“Until we determine if it’s one cause of decline or multiple or maybe different causes of decline in different areas, unfortunately, we’re still a little bit far from that finish line,” said Jordan Brown, the raptor, grassland and forest bird biologist with the Delaware Department of Natural Resources and Environmental Control’s Division of Fish and Wildlife.

To learn more about the change, the Brandywine Zoo began setting up kestrel nesting boxes in suitable habitats in 2014. Though the initiative primarily focused on New Castle County initially, the DNREC Division of Fish and Wildlife has expanded the program to cover the entire state, and today, there are about 90 such boxes.

The program has been very informative so far, Brown said. While there are no current estimates of the number of kestrels in the state, DNREC and its partners observed kestrels at 20 nesting boxes in 2024, including breeding pairs at 14. A total of 40 eggs were laid, though only 21 successfully hatched. Four new adults and 11 chicks were banded.

As it happens, eggs were laid late in the 2024 nesting season. Therefore, chicks hatched later than usual, while increased temperatures in the nesting boxes resulted in chicks leaving the nest earlier than anticipated. Thus, most were gone when DNREC staff visited to band and tag the baby birds.

Kestrels like nesting in or by open fields, so many boxes have been set up on agricultural land. They’re stationed on a mix of public and private land, including properties owned by Winterthur Museum, Mt. Cuba Center and the Delaware Nature Society as well as individual homeowners who have agreed to take part.

“Anywhere that you’re going to find voles, mice, grasshoppers, dragonflies, all of those types of things, is where the kestrels are going to be hunting,” Brown said.

Many locations were chosen either because someone reported seeing a kestrel nearby to DNREC or on eBird, an online database of bird observations.

After the Brandywine Zoo began the project, other entities got involved, including the Division of Fish and Wildlife, and the Delaware Kestrel Partnership was formally established.

Oversight was later transferred to the division.

Monitoring and Banding

DNREC’s bird experts focus their monitoring efforts on breeding season, which typically covers April and May. Before that, though, the nesting boxes must be cleaned. Debris is removed and wood shavings are added.

DNREC is assisted by a team of more than 30 dedicated volunteers with a passion for our natural world.

Beginning in March, experts and trained volunteers visit each nesting box about twice a month. Monitors use a GoPro-style camera on a telescoping pole to check inside the nest box for evidence of a “scrape,” or depression in the floor, which indicates a kestrel is preparing the box for nesting, since the wells they form in the shavings create a perfect bowl to keep their eggs from rolling around.

When a kestrel is sighted near a nest box, the monitoring changes to once per week so DNREC can determine the date of the first egg laid. After that is observed, it becomes a waiting game.

DNREC schedules 30 days from clutch completion to return to check for chicks hatching and, from there, schedules chick-banding windows of between 20 to 25 days.

Incubation begins once kestrels complete their clutch of about five eggs. A week or two later, someone will return to the box to trap any adult kestrels present.

Trapping is done by trained professionals using a net on a long pole, with half the net covered by a plastic board zip-tied in place. The board side of the net covers the nesting box entrance hole to secure the bird inside while someone sets up a ladder, climbs it and then removes the bird.

Chicks are easier to retrieve since they are banded before they can fly. They are placed into individual bags for safety and holding and handled with the utmost care.

The same banding process is used for chicks and adults. Handling and banding are done by the same trained experts who have the necessary state and federal permits.

First, someone weighs the bird in its holding bag, being sure to subtract the weight of the bag itself from the total.

DNREC measures the bird’s tarsus, or “shin” length, as well as the talon on its back toe called the hallux. Then personnel measure the beak length from the flesh of the kestrel’s nose to the tip of its beak, from the “wrist” to the tip of the longest primary feather and, finally, tail feather length. DNREC also checks for “crop fullness,” which indicates how recently the bird has eaten, its body condition (such as whether it’s over- or underweight) and if it has any parasites.

When it’s time for the banding itself, the width of the bird’s “shin” is first measured with a special tool to determine what size band should be utilized. The band is then opened with a pair of special pliers and crimped around the bird’s leg. The unique band number is recorded and voila — this part of the process is complete!

If it’s an adult, it is then allowed to simply fly away, while chicks are placed back into the nest. The young’uns will fledge from the nest at around one month of age and continue to linger in the nest box area for several weeks after that.

In 2024, DNREC began using radio tags, which basically ping radio towers on the same frequency when a tagged bird passes by. That enables the Department to keep a closer eye on kestrels by understanding their movements.

Through late mid-March, towers have signaled the arrival of six of the 16 radio-tagged birds, showing they are returning from migration and hopefully preparing to set up nests in the area.

As the cause of the kestrel’s decline is still unknown, banding and feather sampling are ways of understanding where kestrels are going, whether they return to the same areas for nesting annually and how the species’ dispersal may impact its long-term survival.

Because the American kestrel is near the top of the food chain, it is an indicator species, Brown said.

“When anything is going wrong with our ecosystem and our environment, those are typically where we see the effects,” she said.

That, in turn, can be used to influence habitat management as officials consider what Delaware may be doing — or not doing — that is causing the population decline.

For more information or to volunteer to assist DNREC with its kestrel monitoring efforts, visit https://dnrec.delaware.gov/fish-wildlife/conservation/american-kestrel/volunteer/.

Related Topics:  American kestrels, animals, conservation, education, fun, health, kestrel, nature, outdoor delaware, science