American kestrel eggs are variable in color and markings. This variation appears to be linked to the order in which the eggs are laid by the female. In this article, we delve into the kestrel eggs physical characteristics, egg-laying period, clutch size, and incubation process. Join us as we uncover details about America’s smallest bird of prey’s eggs.
American Kestrel
Breeding
Nest
Eggs
Chicks
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Physical characteristics of the American Kestrel eggs
Shape
American kestrel eggs generally have an oval shape, a little more rounded than a chicken egg. But shapes vary with some females laying consistently more oval or slightly elliptical eggs. This variation can happen within the same clutch.
Color
The American kestrel eggs are variable in color and marking patterns. They range from white to cream to yellowish or light reddish-brown with gray, black and reddish dots and blotches of varying sizes.
The dots, blotches, and mottling are often concentrated around the wider end of the egg. Eggs that have been incubated for some time may show stains resulting from soiling by the adults’ feet and body fluids.
Generally, the first egg laid has more saturated colors and markings. The last-laid egg is usually paler than the others in the clutch.
Size
As with other birds of prey, the size of the American Kestrel eggs varies with latitude. In general eggs in the south are smaller than eggs in the north. Kestrel eggs in Florida and other southern states are the smallest in North America.
Kestrel Egg Measurements
| Egg Length | Egg Breadth | |
| South | 1.3 inches | 1.1 inches |
| North | 1.1 inches | 1.1 inches |
Weight
As it would be expected, American kestrel eggs increase in weight from the south to the north. Interestingly, studies on breeding kestrels found that eggs decrease in weight as the breeding season progresses, meaning that eggs laid late in the breeding season are smaller than their earlier counterparts.
American Kestrel eggs’ weight range is roughly from 9.81 to 18.6 grams.
Eggshell Thickness
The eggshell thickness of American kestrel eggs ranges from 0.17 to 0.20 mm (Smallwood and Bird 2020). Measures of egg thickness taken before and after the DDT crisis in North America show little variation suggesting the American kestrels were not heavily affected by the widespread use of this pesticide.
Kestrels primarily feed on small herbivore mammals and large insects, making them less affected by the bioaccumulation of organochlorine pesticides compared to top carnivores like the Bald Eagle (Haliaeetus leucocephalus) and Peregrine Falcon (Falco peregrinus).
Egg-laying, latitude and migratory behavior
Latitude significantly influences the timing of egg-laying of the American Kestrel. Kestrels breeding in the lower latitudes such as in the North American south lay eggs earlier than those breeding in northern regions such as Canada and Alaska.
Egg-laying has to do with Kestrels’ migratory status. Birds breeding in Canada and Alaska are considered obligate migrants, meaning they must migrate annually to escape harsh winter conditions. As latitude decreases, kestrels’ migratory behavior transitions from obligate to facultative, and eventually to resident.
This flexibility is primarily driven by local environmental factors, such as food availability and winter temperatures.
Comparative studies conducted in Saskatchewan (Canada), the State of Iowa, and Floria (USA) illustrate the variation in the initiation of the breeding season on North American Kestrels. (G. R. Bortolotti, D. E. Varland and JAS; cited in Smallwood and Bird 2020). The study found that:
- Egg-laying in the State of Florida begins the first week of March.
- Egg-laying in the State of Iowa begins the third week of March.
- Egg-laying in Province of Saskatchewan (Canada) begins the last week of April.
How Many Eggs Do American Kestrels Lay?
The clutch size (as ornithologists call the number of eggs per nest) varies according to multiple factors. The American Kestrel typically lays 4 or 5 eggs per clutch. However, this may vary depending on the availability of food and nest sites. Some kestrels may lay as few as 2 eggs or as many as 7 eggs in a single clutch.
How many clutches do kestrels lay every year?
A pair of kestrels normally lay a single clutch of eggs every nesting season. However, if the clutch is lost, particularly early in the season, the pair may lay another one.
In northern and colder parts of the American kestrel range, adult kestrels may not lay a new clutch if they lose it. This is because the breeding season in northern latitudes is shorter than in the southern ones.
Food and nesting sites may be more scarce in northern latitudes (such as Canada and Alaska), which can make it difficult for kestrels to raise two broods in a single breeding season.
In a study conducted in Saskatchewan, Canada, researchers found that 12% of American kestrel pairs laid a second clutch in a breeding season. This was compared to 25% of pairs in a study conducted in Texas. The researchers concluded that the shorter breeding season and the scarcity of food and nesting sites in Canada were the main reasons why kestrels were less likely to raise two broods in that region.
Incubation of the eggs
Like most female birds, the female American Kestrel waits until she lays the penultimate egg before starting to consistently incubate her eggs. She may sit on her eggs intermittently prior to completing the clutch, but incubation starts before she lays the last egg.
Division of Labor
The female American kestrel does most of the egg incubation. She spends the nights and most of the day sitting on the eggs while the male brings food to her. The male takes over incubation duties for only short periods of time when she takes breaks.
Even though the male brings food to the nest, the female may also go foraging for food during her breaks.
Video Cameras of nesting American kestrels show how careful the adults are with their eggs. Both parents are very gentle and take their time to position themselves to incubate the eggs properly.
Both the male and female kestrels develop brood patches during the incubation period.
What is a brood Patch? A brood patch is a bare or featherless area on a bird’s belly. This bare patch is well supplied with blood vessels that help transfer body heat onto the eggs during incubation. Most birds develop brood patches during the incubation period.
How long does it take for an American kestrel egg to hatch?
American kestrel eggs typically hatch after an incubation period of 26-32 days, with an average duration of 30 days. Incubation periods tend to be slightly shorter in warmer regions like the southern states compared to cooler northern states.
Do American kestrel Eggs Hatch All at Once?
A clutch of American Kestrel eggs takes 2-3 days to fully hatch. Each egg requires approximately 48-52 hours from the moment the hatchling creates a hole in the eggshell until it is fully hatched.
The sequence in which eggs hatch is influenced by the incubation time each egg receives. Eggs laid first receive intermittent incubation, and nearly a full day’s worth of incubation occurs before the last egg is laid. At this point, the female initiates full incubation of the entire clutch.
What animals eat American kestrel eggs?
Common known predators of American kestrel eggs include:
- Snakes: Snakes, such as rat snakes, corn snakes, and gopher snakes, are common predators of American kestrel eggs. They can climb into kestrel nests and eat the eggs.
- Raccoons: Raccoons are opportunistic predators that will eat just about anything, including American kestrel eggs. They are also very good climbers and can easily access kestrel nests.
- Jays: Jays, such as blue jays and Steller’s jays, are known to prey on American kestrel eggs. They are intelligent birds that can learn to locate and access the eggs in kestrel nests.
References
- Johnsgard, P. 1990. Hawks, Eagles, and Falcons of North America. United States: Smithsonian Institution.
- Smallwood, J. A. and D. 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.
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