2019’s Truncated American Kestrel Nesting Season

American Kestrel - Photo Mick Thompson

Nesting seasons involve many things such as the arrival of adult birds, courtship, selecting and winning nest sites, laying eggs, feeding nestlings, fledging events, etc., but for most conservationists that offer nestboxes to birds, they describe a nesting season as the period when their nestboxes are active with eggs and young.

The Delaware County Health Department launched the American Kestrel Nestbox Project prior to 1993. Elementary and Middle School students were encouraged to donate money from recycled aluminum cans and other material to support the construction of ten kestrel boxes by the Delaware County Bird Club. The Ohio Department of Transportation granted permission for the boxes to be attached to traffic signs along major highways that ran east and north from the city of Delaware, Ohio. In 2000, boxes from the expanded program were moved from traffic signs to seventeen utility poles owned by Consolidated Electric Company and one free-standing pole at Gallant Woods Park.

Since the project’s first kestrel nest in 1995, detailed data on every nest has been recorded, and by the end of the 2011 nesting season, data from 164 kestrel nests was analyzed to better describe Central Ohio’s nesting seasons. March 14 became the earliest first-egg-date for kestrels, and August 26 was the latest fledging date, to make a possible season of 166 days.

Two other dates emerged from the 2011 calculations that are  very important when planning management procedures: the latest first-egg-date was June 25, and if kestrels start their first clutch of eggs by April 12, then there is enough time for their nestbox to raise two families during the same season.

So, how did the 2019 season compare to the 2011 calculated standard? Dick Phillips and I always start checking the project’s boxes in mid-March to make sure all nest cavities are ready to accept our small falcons. On March 17, we found “kestrel cups” in 14 of 18 boxes. A kestrel cup is an oval shaped indentation in the white pine bedding that fits a kestrel’s body. We added bedding to four boxes that had been excavated by European Starlings, and we added bedding to two boxes where kestrel cups had exposed wooden floors.

Our second box check took place on April 7 and we found five kestrel nests with eggs. Three boxes had starling eggs. We remove starling eggs but not the nest. If kestrels want a box with a starling nest, they will reshape the starling’s round nest cup to fit their own body. Starlings don’t lay first eggs after the first week of June.

Once it is determined that a kestrel’s clutch is complete, we no longer check the nest until the young will be between 14 and 24 days old, the time to apply U.S. Fish and Wildlife Service leg bands while being able to sex the young. Then after we revisit the nest for banding, we no longer check the nest until at least one week after the projected fledging date.

We banded our first nest of five males on May 20, followed by three families on May 26, six families on June 2, three families on June 15, and five Preservation Parks staff members and one of their mothers helped with banding the last family on June 26.

When we monitored Delaware County’s eighteen kestrel nestboxes for the ninth time on July 13, we expected to find several new clutches of kestrel eggs. We found no new clutches, and yes, we were disappointed, but we were also impressed. An analysis of 2019’s data revealed a memorable season.

Of 14 nest attempts, all nests were successful, and all but one nest started with five eggs. Of the 13 nests that started with five eggs, 12 nests hatched and fledged all five. Only one nest of five eggs, hatched four and fledged three. A lone nest of four eggs managed to fledge three of its four eggs. The overall statistics for the 67 eggs laid is that 97.1% hatched, 95.7% of the eggs developed to fledge, and after eggs hatched, 98.5% fledged. These are the highest percentages during the 25-year history of the project.

Sixty-six kestrels were banded and fledged during the 2019 season making the grand total of 1,230 falcons fledged since the project’s first successful nest in 1995.

Kestrel with Five Nestlings - 2019

On May 20, 2019, this female kestrel had been brooding or feeding her five nestlings.

The first-egg-date for 2019’s kestrels was March 31, which was worrisome at the time. Also, finding no second clutches to mark the end of the season on July 13 was also a concern. The 2019 kestrel season was only 100 days long once the last fledging date of July 8 was calculated. The recent season was more than two months shorter than the historic length of 166 days calculated in 2011. What could be the reason for such a short, but successful, season? Could it be rainy weather?

I had to wear knee boots to keep my feet dry during most of my bluebird trail monitoring trips since I had to walk through standing water. Weather forecasters were reporting that 2019 was the wettest year on record in Central Ohio and the previous fall was so wet that cover crops could not be planted and the spring was so wet that many farmers could not plant their corn or soybeans.

Bare fields and standing water forced many of the kestrel’s prey items to seek cover and concentrate along roadsides and other places where cover still existed. Also, as I checked my bluebird boxes, I saw floating earthworms and bare infant rodents in the fields that had experienced the fatal bad luck of saturated soil and standing water. So, the kestrels found food after a late start to their season, then they had sufficient food to support healthy egg clutches and successful broods, but as the season progressed, the concentrated food supplies were harvested down to a level that could not support second clutches.

Bluebird Box Hole Restrictor

A 1-1/2-inch hole restrictor covers the three inch entrance to protect a bluebird family from hungry kestrels.

On our last monitoring trip during the nesting season on July 13, we saw eight kestrels on the utility wires near one of our boxes. The eight most likely formed from two families that had united. Parents will watch over their fledglings for several weeks then the siblings tend to stay together after the parents set them free. Young kestrels will join other families to form flocks. So, when you see a kestrel in late summer or fall, look around and you might see more.

Also, on our last trip we found two kestrel boxes that were occupied by bluebirds, the species that led us into the world of nestboxes. One kestrel box had three bluebird eggs and a second box had three bluebird nestlings about eight days old. I screwed hole restrictors made from old bluebird nestbox fronts to both kestrel boxes that reduced the three-inch openings to 1-1/2-inches so the bluebird families will not become lunches for curious kestrels. While helping kestrels, we cannot neglect our bluebirds. Raptor on!