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American Kestrel Monitoring Year 6: A Season of Unexpected Results

August 18, 2021 Category: , ,

Each year, the start of American Kestrel Nest Box Monitoring Project season brings great anticipation. How many boxes will be occupied? Will any other species use the boxes? Will we start to see any of our previously banded birds return to nest sites? Many more questions than answers and as the title of this post might foreshadow, there were certainly unexpected results to this year’s monitoring.

Beginning in late March, we noted an early arrival of a number of American Kestrels to the Sax-Zim Bog. Typically, birds nesting in the Sax-Zim Bog start arriving to the area around the first or second week of April. These early arriving birds made us excited as to what this could mean for nesting success because a number of these birds were seen in locations with new nest box placements! Moving into the 6th year of monitoring, we added an additional 14 boxes to the total, reaching beyond 50 boxes placed, with a grand total of 52 across the landscape. On top of this excitement we were able to add a few new volunteers to our roster and expanded research potential by partnering with the Minnesota DNR to put transmitters on adult kestrels nesting in the Bog! With 6(!) new volunteers and additional partnerships with landowners (thank you Simeks!) allow us to continue to answer questions about and support American Kestrel conservation work across the US.

With coordination from Frank Nicoletti, Mark Martell, and Kirstin Hall of the Minnesota DNR we have increased our research potential with the nest box project. With the help of a few other folks, 12 transmitters were placed on adult American Kestrels occupying boxes in the Sax-Zim Bog! These transmitters will utilize the MOTUS network across the US and our tower at the Welcome Center to give an idea of where our kestrels our migrating, when they might leave the area, or when they might return. Adults were banded with a standard aluminum federal band, a blue color band, as well as the transmitter. This will mean we can finally identify adults that are returning to nest box locations. Not only this, but we also have begun color banding nestlings! All nestlings banded this season have an additional blue color band, like the adults. This will not only allow for easier field identification of birds hatched in the Bog during the 2021 season on their migrations, but will also let us know if these birds do come back and nest in the Sax-Zim Bog in the future. We hope to continue with different colors for different years, giving us more potential for research on these wonderful raptors in our area and beyond!

We don’t often get a chance to handle adult American Kestrels, so placing transmitters on adults this season was a special treat. Images from Top to Bottom: Adult female American Kestrel with transmitter, tail of Adult Male American Kestrel, tail of Adult Female American Kestrel, feet of Adult Female American Kestrel showing the aluminum band and new color band!

With the excitement noted above, the most unexpected portion of the season came at the hand of the effects of drought and climate variability in the Bog. Early arrival usually means early nesting for kestrels in the Sax-Zim Bog based on previous years’ experience. That was not the case this season. While we may have had a few early birds, we didn’t detect any nesting activity early on in the monitoring season, which starts May 1 (or mid-April when we clean out boxes!). Usually, most of the American Kestrels nesting in our area have started laying eggs around May 10. This season, the median egg laying date was nearly 10 days later (May 18), with the first eggs being laid around May 11. We also had the latest nesting documented for the project, with a female kestrel in one of the boxes laying her first egg on June 19! To give you an idea of how late that is, the first round of chick banding last season was on June 19.

During this nesting season, in comparison to the previous 5 seasons, weather was extremely variable. We had a few days in early May approaching or surpassing 80 degrees F, followed a few days later by days not reach 30 degrees F! This variability may have impacted the numbers of eggs laid and lateness of first eggs laid. American Kestrels are very well adapted to cold weather conditions, but eggs are not the same as adult birds and are more sensitive to fluctuations in temperature. Drought effects might not be well see over the course of one summer, but will certainly impact food availability in coming seasons. Dry conditions impact the entire food chain and while not immediately noticed in top predators in one year, consecutive years impact food availability at the top of the food chain.

All in all, the monitoring season was not as bad as I might have made it seem above! We had a record numbers of useable boxes, boxes occupied, successful boxes, and a record number of chicks hatched in the 6 years of the project. We also had a record number of nesting failures this year, unfortunately. Warm, dry conditions may have had something to do with the record number of nest failures, however.   

A total of 48 boxes were in useable condition this season. This number was 49 useable boxes, but a couple of trees fell down at a nesting location and no trees remained at the location to replace the box. Luckily, no kestrels were using the box at the time. In a turn of good news, we were very excited that our placement of new boxes was met with rave reviews from kestrels this season: 9 of the 14 newly placed boxes were used! Those 9 boxes were part of the total of 23 boxes used this season in the Sax-Zim Bog, a new record! The unfortunate record from the season was the record number of nest failures, with a total of 6. Most of those boxes were abandoned after eggs were laid, with only one predation event. For the most part, we don’t know why these birds leave boxes. This year, we were able to offer a reason for at least one of these box abandonments… a nesting pair of Red-tailed Hawks using the same habitat! Red-tailed Hawks are not very common nesters in our area, though in the farm fields and open, grassland type habitats in the Sax-Zim Bog we occasionally find a nest. Red-tailed Hawks and American Kestrels are not in direct competition for resources, but adult Red-tailed Hawks are adept bird predators, with American Kestrels serving as prey items from time to time. Perhaps the adult kestrels ended up as a meal for the pair of Red-tailed Hawks or perhaps the kestrels left the area for greener pastures.

Though nesting success may have been lower this season, the boxes above were still fairly typical of this year’s nesting season.

As far as nest success goes, the data is also unexpected! A total of 76 eggs were laid producing 67 chicks. That works out to an average of 4.4 eggs/nest and 3.9 chicks/nest. Both of these numbers are below our project’s average of 4.7 eggs/nest and 4.4 chicks/nest. Less than half of the nest boxes laid a full clutch of 5 eggs, though we only had one clutch of less than 4 eggs laid. Looking back on the season, the numbers are quite a bit better than anticipated from reports from volunteers about late nesting dates, late hatching dates, and little early occupancy. While up at the nest boxes, we sometimes note interesting prey items. This year saw an overwhelming amount of grasshopper parts in boxes! Grasshoppers make up a portion of the diet of kestrels in the Bog, but not at the volume of use this season. There were a couple of boxes that were absolutely full of uneaten grasshopper legs and wings, where in a typical box we may see only a handful of grasshopper parts.

The floor of this box, like many others this season, is covered by the red and yellow legs of grasshoppers!

Considering the above, any nesting success is welcomed, whether or not a record number of eggs were laid or chicks were hatched. Recently published Raptor Population Index data from Hawk Migration Association of North America shows that American Kestrels are only recently stabilizing their long term declines (from a 59% of hawk watches noting declines from 1999-2009 to a 22% of hawk watch sites noting declines from 2009-2019) and kestrels are still declining significantly, especially in the Southeastern United States. In our area, it appears we have created important nesting locations is somewhat suitable habitat. This is important as American Kestrels breeding numbers have been declining according to Breeding Bird Atlas data, with consistent declines documented since 2010. Not all is lost, however! There is increased interest in American Kestrels lately in raptor research circles and our project hopes to contribute to that research to get a good understanding of these wonderful birds.

We could not have the continued success of this project to support these beautiful raptors without the help of our volunteers! This list has grown quite a bit from previous seasons and I would like to thank Sarah, Mary, Jean, Brian, Jeri, Jessica, and Dawn, Gregg and Kellie for their efforts this season! Banding and connections with Minnesota DNR staff couldn’t be done without Frank. And special thanks goes out to the Simek Family for allowing us to place 3 boxes on their property and to our Bog Buddies who have made all of the nest boxes placed in the Bog and continue to support efforts of this projects through donations that support our organization!

Volunteer Brian with his daugther, Wren, checking out a newly banded kestrel nestling!

If you want to learn more about the American Kestrel Nest Box Project, check out our old blog posts here, here, here, and here! In the future, we hope to have an entire page featuring this project on our website, containing webinars, papers, talks, and other important notes about this project. Stay tuned!

— Head Naturalist Clinton

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