Comments by Bob Dean
Basics: The American Kestrel (Falco sparverius) is the smallest falcon in North America. They are fairly common in open habitats such as agricultural fields, meadows, grasslands, and occasionally marshes with adjoining open areas. They generally overwinter in similar habitats. These habitats provide the food required for survival; primarily insects (grasshoppers, crickets, cicadas, beetles, etc.), small rodents (voles and mice), small reptiles (snakes and lizards), small amphibians (frogs and salamanders), and small birds (sparrow size and smaller). Observations of avian prey as large as mourning dove have been made, and European starling is frequent prey when competing for nesting boxes.
Kestrels are secondary cavity nesters. They do not excavate cavities, but use existing cavities created by large woodpeckers or other natural hollows in trees or buildings. They have also been observed nesting in rock crevices on cliff faces.
The population of American kestrels has been in steady decline for some time across Virginia, except for the coastal plain, where the population is more stable. Since the mid-1960s the kestrel population has declined between 27% and 73% with the highest estimates in the piedmont region.
Many natural nesting cavities have been eliminated by removal or collapse of dead trees. In an effort to offset this loss, the Virginia Kestrel Boxes working group is installing nest boxes in a variety of locations where landowners with suitable habitat are agreeable with the concept of housing kestrels on their land. The project has shown a positive response by kestrels.
It is important to monitor and clean boxes prior to the breeding season to enhance the likelihood of occupancy. Evidence of undesirable competitors needs to be removed at this time and whenever it materializes prior to kestrel usage. Any nesting material introduced by other species that raises the level of kestrel egg laying will likely enhance the opportunity for subsequent predation.
Breeding Season Monitoring: Kestrels arrive in prospective breeding areas in early to mid-March. In northwestern VA they begin laying eggs in late March to early April. Eggs are laid on alternate days until a full clutch of 4-5 (rarely 6) is complete. Incubation begins with the laying of the penultimate egg, which provides for slightly staggered hatching. If food supplies are inadequate the youngest nestlings will most likely be the first to perish. Eggs are incubated by both adults, but the female does far more than the male. The young kestrels will begin hatching in about 28-30 days. They will then be fed by both adults until they fledge in about 30 more days. If a nest is lost to predators or weather early in the breeding season the adults will promptly attempt a second nest. A small percentage of kestrels will nest a second time after a successful first nest attempt.
It is important for accurate monitoring that the initial date of egg laying or incubation is known so that the young can be banded at the appropriate time. On about day 15 after hatching the flight feathers begin to show color. After this time the young can be reliably sexed and banded. After day 25 the likelihood of premature fledging increases, so the banding window is about one week after all young have reached age 15 days.
After fledging the young will remain in the area with the adults for two to several weeks learning how to hunt and where to shelter. After that the adults will either stay in the general area or move to better winter habitat. Some will make long distance flights of 200 km or more. Telemetry studies will undoubtedly yield some interesting and very granular data regarding post fledging movements of both adults and young.
Winter Monitoring: After kestrels have settled into winter ranges it is beneficial to determine the size and usage of each area as much as possible. This is important information in determining conservation and management strategies. Both telemetry and banding data are critical to achieving a better understanding of winter range use and wintering population size. Banding data provide such important metrics as weight, size, age, sex, and general health; useful as research tools.
Kestrel Banding Goals: There are currently 42 kestrel boxes in Clarke and Frederick Counties. The initial aim of the project was to establish 40 boxes in areas of suitable habitat. If this proves to be a manageable number of boxes, more will be added until a new maximum is achieved. The goal is to monitor all boxes adequately to provide the date of initial egg laying and/or incubation so that banding of all young can be accomplished in a timely manner. When checking boxes and adults are found in a box they will be banded. Efforts will also be made to band adults using bal chatri or noose traps baited with mice during both the breeding season and, more importantly, during the fall and winter.
In 2021 there were 60 nestlings and 5 adults banded in Clarke and Frederick Counties. Only one of the adults was caught by hand in a box and 4 were caught using bal-chatri traps. In 2022 there were 57 nestlings and 10 adults banded. Six were caught by hand in boxes and 4 were caught on bal-chatri traps. In 2023 there were 74 nestlings and 11 adults banded. Seven additional adults were caught in boxes that were previously banded.
This is done in collaboration with Virginia Kestrel Boxes, a working group that shares information regarding American Kestrel natural history and results of local banding projects.
References:
Clark, William S., Jill Morrow, Lance Morrow, and Jeffrey J. Kolodzinski. 2021. Aging American Kestrels In-hand. North American Bird Bander 46:59-64.
Editorial Board of the American Kestrel Partnership of The Peregrine Fund. 2011. Declines of American Kestrel Populations.
Heintszelman, D.S. 1964. Spring and summer sparrow hawk food habits. Wilson Bulletin. 76:323- 330.
Smallwood, J. A. and David M. Bird (2020). American Kestrel (Falco sparverius), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA.