Male American kestrel. Photo by J. St.John, Wikimedia Commons
(Click on the following link to listen to an audio version of this blog … American kestrel
The American kestrel (Falco sparverius) is the most abundant and also the smallest falcon in North America. It ranges in size between a blue jay and a mourning dove (8.7 to 12.2 inches long, 20 to 24 inch wingspan, and 2.8 to 5.8 ounces in weight). As is true for most raptors, the female is larger than the male. Kestrels are found from Alaska all the way down to the tip of South America. There are 17 distinct subspecies of the American kestrel across their very broad range of habitats.
Kestrels have distinct gender plumages. Males have blue-gray wings with black spots and white undersides, rufus-colored backs, white bellies and sides with black spots and rufus tails with white or rufus tips. Females have rufus-colored backs and wings, with dark hatchings, cream-colored bellies with brown streaks, and rufus-colored tails with narrow black bars. Both males and females have white heads with rufus caps with very distinctive double, vertical, black facial lines on each side. Other falcons also have black facial lines, but their lines are singular.
Photo by A. Lours, Wikimedia Commons
Kestrels are quite tolerant of people and can be found hunting and nesting in farm fields, parks, cities and suburbs. They can also be found in open forests, deserts, meadows and grasslands. A key habitat requirement is the presence of high perches where they can rest and watch for prey and from which they can swoop down on their potential food items. Kestrels are quite omnivorous and will take small mammals, small birds, snakes, lizards and frogs as food. Their primary food source, though, are large insects like grasshoppers.
Kestrels are cavity nesters, but they are unable to carve out their own nesting spaces. So, nests are frequently built in the hollows of dead trees, in old woodpecker or squirrel holes, in rock crevices, or in protected spaces in and on buildings. They also readily nest in human-constructed nest boxes. Kestrel nests are bare and unadorned with no soft nesting materials for the eggs or nestlings.
Kestrels are monogamous and breed for life. The female takes on the primary role of egg and hatchling incubation while the male assumes the job of providing food for the nestlings and for the female. The quality of the male as a hunter, then, is critical for the success of a clutch. Clutches have between 4 and 5 eggs, and a mating pair can have either 1 or 2 broods per year.
Female kestrel feeding young at nest box. Photo by K. Greene, Wikimedia Commons
Successful rearing of fledglings is greatly affected by the age and experience of the mating pair. Young kestrel couples (first time parents) seldom have any of their brood survive. More experienced mating pairs, though, almost always have at least one of the offspring survive into adulthood.
Females choose the male with whom they will mate, and they choose carefully. Older females almost never choose younger males. Younger females who choose older males increase their chances of having surviving offspring by about 50%.
The average life span of a kestrel in the wild is 5 years. In captivity, kestrels have reached 14 years of age. Often pair bonding and mating doesn’t occur until the birds are 2 years of age , so an average mating pair will have 3 to 6 breeding opportunities.
Kestrels that live in high latitude locations typically migrate in the winter to warmer climates (those breeding in Alaska, Canada and the northern regions of the United States, for example, migrate to southern states or to Central America). Kestrels living in more moderate climates tend to be year-round residents.
American kestrels are not as heavily muscled as other falcons probably because they tend to be ambush predators that swoop down from their high perches to pounce on their prey rather than engaging in the more energetically demanding practice of high speed pursuit and mid-air strikes that are more commonly seen in other types of falcons. The ambush-hunting style typically causes kestrels to stake out hunting spots and elevated stalking perches where they will be regularly observed. When we lived in Pennsylvania, we named a road near our house, “kestrel road” because of the extreme likelihood of sighting a kestrel or two up on the utility wire overlooking the nearby corn fields. Here in Colorado, in the Spring of 2021 we saw a pair of kestrels just outside of town hunting along the side of the highway (from Signs of Spring 9, April 29, 2021):
“Leaving the shelter of town, we also felt the full force of the northwest wind. It was blowing at a steady twenty-miles-per-hour and was skimming loose dust off of the fields and sending it flying across the road in fine clouds. There were two kestrels in the field buffer strip just to the east of the road that were using the wind to hover above the still brown weeds while hunting their prey. We saw them each suddenly fold their wings and plunge down into the vegetation. We had heard and seen grasshoppers on our morning, neighborhood walks. My guess is that the kestrels (great insect-eating little falcons that they are (which is why they are called “the farmer’s friend!”)) were hunting for grasshoppers. We didn’t see if they snagged any, though.”
American kestrel with captured American pipet. Photo by G. Smith, Wikimedia Commons
Most raptor species in North America have had a resurgence of numbers and distribution since the 1970’s. The banning of DDT and the recognition that many other pesticides can negatively affect predatory birds has allowed populations of bald and golden eagles, peregrine falcons, ospreys, red-tail hawks and many other hawk species to at least begin to return to their former densities and habitat distributions. A notable exception to these overall trends, though, is the American kestrel. Although this small falcon is still the most abundant falcon species in North America, its numbers have declined almost 50% since 1970.
Why?
An intensive study sponsored by the U. S. Geological Survey and U. S. Fish and Wildlife Service involving over 50 researchers is investigating possible causes of the kestrel decline. They are also evaluating some of the on-going programs that were designed to help kestrels regain their pre-1970 population densities.
An initial assumption about the declining kestrel numbers was that their nesting habitats had been too disrupted to allow efficient reproduction. Farmlands and parklands were being cleared of dead trees which were absolutely vital for kestrel nesting cavities. Extensive programs were initiated to establish kestrel nesting boxes in suitable habitat areas. In fact, one of the last things I did before we moved away from Pennsylvania in 2020 was help to put up a kestrel house in the “Bat House Meadow” at Harrison Hills Park.
These nesting boxes, initially, seemed to have the desired impact on kestrel numbers. Often, the first years of nesting box placement resulted in increased nest building and increased numbers of hatchlings and fledglings. However, most nest box programs found that not only did the number of boxes being used for nests quickly plateau, but they even began to decrease over a few years. Many boxes were ignored by the kestrels and went unoccupied through the breeding season.
Suitable nesting sites, then, did not seem to be the key factor in the observed decline of the kestrels!
Male American kestrel. Photo by BLM, Public Domain
Other hypotheses that have been explored include the conditions in the kestrels’ overwintering habitats, the possible impacts of rodenticides on the mouse and rat eating kestrels, the impacts of the still widely used neonicotinoid pesticides, and the possible influence of modern agricultural practices on the post-breeding quality of kestrel-inhabited farm habitats. Climate Change and Global Warming have also been suggested to be factors in the kestrels’ decline. It is agreed that most likely the falling numbers of kestrels is being caused by multiple factors.
One of the researchers participating in the U. S. Geological Survey study, though, thought that possibly one word could explain why kestrel numbers were decreasing in North America; grasshoppers! Young kestrels depend upon an abundance of large insects for most of their food in the critical first months of their independent lives. Large insects (like grasshoppers) are abundant and easy to catch. The young kestrels can then feed themselves well as they hone their hunting skills for more difficult prey.
Insect numbers in general have greatly decreased over the past few decades (see Signs of Summer 5 (July 5, 2018) and Signs of Winter 13 (February 21, 2019)). Large insects, like grasshoppers, have become increasingly scarce. The consequential impact on young kestrels, then, is starvation in their adolescence. They don’t live long enough, then, to breed.