American Kestrel Monitoring Year 9: The more we know, the more we don’t know
The American Kestrel Nest Box Monitoring Project has completed its 9th season, with the last boxes of the year banded today, July 25, 2024. It seems that year after year, we feel like we have a good idea of what to expect: There will be birds, there will be eggs, and we will band some chicks! The number of nests occupied, how many eggs will hatch, and if any banded birds will return, however, is never something we can predict.
For the 2024 season, hopes were high for good results following a relatively mild winter. When March came around, however, we weren’t seeing kestrels arriving any earlier that usual. What’s more, when our kestrels did arrive, they didn’t begin nesting as early as expected (and anecdotally, at least, it seems they laid eggs on average a week later than usual). In fact, the two boxes we banded on July 25 constitute the second latest banding dates ever for the project and the greatest number of boxes banded this late in the year.
With all of the experiences we have had through observing nest boxes and banding American Kestrels in the Sax-Zim Bog, it is hard to imagine what new things we might find during a monitoring season. Weird food items? How about Star-nosed Mole! Extra eggs in a box? We have documented 2 different extra-egg clutches! Males incubating eggs? Surprisingly common in our observations! And the list goes on.
This year, the surprise was the number of birds being eaten by American Kestrels this nesting season. Typically, we will find bird parts in a couple of boxes each season, with most of the prey items observed being grasshoppers, dragonflies, or voles. However, this year not only had smaller numbers of voles, but a relatively large number of birds, with some boxes having multiple species consumed. This year we documented American Kestrels eating American Goldfinch (first observation as prey), Savannah Sparrow (not uncommon to see as prey), Red-winged Blackbird, Bobolink (first observation as prey), Common Yellowthroat, Yellow Warbler, and Chestnut-sided Warblers (warblers are not uncommon to find), a sparrow species (likely Song Sparrow), and Black-capped Chickadee (also not uncommon as prey). That is a lot of species observed and some boxes had multiple prey items present (one box had a Bobolink foot and most of a Savannah Sparrow during our banding visit).
As exciting as the above sounds, much of the rest of the summary of this year’s project is pretty lackluster, if not troublesome. For example, we had our highest number of nesting failures ever for the project, with 11 boxes failing. Some failures are expected each season (especially nest predations), but this year saw the highest number of predations ever (6 boxes). For comparison’s sake, we have recently averaged 4-6 failures in a season, with only 3 predations expected. Four boxes this season were abandoned, with one box abandoned with fairly large (14-16 day old chicks) present. The last failure was due to a failure of the box itself. It came loose from its bolts on a dead tree and will be replaced in a better location next season. All of this adds up to the lowest nest success rate (65%; boxes banded/boxes occupied) over the course of the project and the lowest number of chicks per box (3.25 chicks banded/boxes banded). This all sounds pretty defeating, but the hatch rate (73%) was fairly normal, if not slightly lower than average (chicks banded/eggs laid in successful nests).
As the title alludes, the more we learn every season, the more questions come up. Why did our kestrels predate birds at such a high rate this year? What does that mean in regards to other food sources in our area? Why were there so many nest predations this season, especially with all of the wet conditions present? Who are these nest predators? The current assumption is that increasing numbers of raccoons in our area are leading to higher numbers of nest predations, but we also observed European Starlings as the responsible party for at least one nest failure this year. Was the prolonged period of wet to blame for some of the abandoned boxes or the choice of prey this season? In the previous dry years insects made up a huge volume of the prey observed in boxes during banding, which was not the case this year. Will we see an uptick of birds and productivity with a more normal winter season? Did this mild winter have a greater negative ecological impact than we had expected?
It is good to consider these questions as we look forward to year 10 of the project! There are some exciting things to come with the project, including publishing our data and observations to help others gain a better understanding of American Kestrels in this area. One of the nestling females fitted with a transmitter last summer survived her first migration and spent her summer near Chisolm (after heading as far north as International Falls!). This is excellent news and the best migration path we have been able to observe from a transmitter bearing bird hatched in the Bog! We also, very excitingly, crossed an important threshold for our project…. 500 chicks banded!! There are always rays of light and positivity in a seemingly difficult year for American Kestrels in our area. We look forward to continue to support these amazing raptors in the greater Sax-Zim Bog ecosystem!
It is not lost on us the work that goes into a project like this, especially since volunteers are doing all of the nest checks and data entry! Special thanks to our volunteers Mary, Jean, Nona, Sue, Jessica, Dawn, Brian, Kelly, Steve, Jeri, and Josef this season, without their time and effort, we would not be able to study and support these amazing birds!
–Head Naturalist Clinton