The American Kestrel & Cavity-Nesting Owls
Results
As a result of three years of field observations, we now know that breeding kestrels arrive to Genesee around April 18th and start laying eggs about one month later. In total, over three years we observed 50 kestrels during the Roadside Raptor Surveys or incidental observations. During 2021-2023, we also trapped and banded adults and nestlings before leaving the nest boxes, for a total of 17 adults and 38 young kestrels.
Between 2021-23 we monitored 65 nest boxes suitable for American kestrels - 20 boxes in 2021, 19 boxes in 2022, and 26 boxes in 2023. Each year, six or seven are used by kestrels, despite an increase in the number of available boxes. It is quite possible that this number represents the carrying capacity of our study area, but it is also possible that other kestrels are nesting in natural cavities available within the Genesee Bird Project boundary. While the number of kestrels breeding in nest boxes has remained fairly constant, the total number of eggs laid has decreased, and the hatching success has been only 50 – 54% (Figure1). Fledging success varied between 76 and 100 %, but this is expected as food availability or inclement weather during chick rearing could have an important impact on the actual number of young raised by each pair.
A part of the concerns raised by the low hatching success (which in our case it is not due to lack of parental care as we observed kestrels incubating eggs for nearly two months) is a worrying trend represented by the overall number of fledglings, that seems to be decreasing each year (Figure2). This means that each year, less and less kestrels are entering the existing population of kestrels, and the trend follows a scenario presented by several other research papers, when an initial high occupancy rate and good breeding output is followed by a steady decline. At this point, it is not apparent what is causing this situation, although in 2023 we documented an increased presence of European starlings (Sturnus vulgaris) that are direct competitors for cavities, and when present in large numbers, they might displace kestrels or cause nest failures of birds trying to nest in the same area. We also noticed that our kestrels in 2023, while arriving to Genesee around the same time each year (18th of April), began laying eggs a week earlier to previous years. This can also lead to clutch failures, especially if hatching and chick rearing is decoupled from prey availability. While we are hoping that this decrease it is just a part of a natural cycle and will reverse in the following years, we will continue to gather data to try and answer factors contributing to the kestrels’ nesting success.
Figure1. The number of eggs laid and hatched by American kestrel (Falco sparverius) compared to the number of available nest boxes within the Genesee Bird Project boundary between 2021-2023. The numbers in the brackets represent the number of nest boxes where at least one kestrel egg has been laid each year.
Figure2. The number of nest boxes occupied by European starlings (Sturnus vulgaris) compared to the number of American kestrel (Falco sparverius) fledglings produced in nest boxes provided within the Genesee Bird project boundary between 2021-2023. The R-squared value displayed on the linear trendline fitted to the fledgling data supports the decreasing trend over time, although many more years of data is needed.
During 2022-2023 while conducting ground-based surveys, we also identified 10 Northern Saw-whet owl breeding territories, although none nested in the 20 nest boxes that we placed in November of 2022. As of 2023, we started to search for a suitable location where we could start a fall migration monitoring station for these owls. During eight hours of nighttime mist netting, we managed to capture eight birds that we fitted with bands issued by the Banding Office.