American Kestrel Chicks:  Growth, Sibling Rivalry, Fledging, & Dispersal 

Imagine a tiny ball of down, blind and helpless, transforming into a sleek predator in just a month. That’s the story of American Kestrel chicks. Driven by an insatiable appetite and fueled by tireless parents, their journey is a display of rapid development and fierce sibling rivalry. But their story doesn’t end with fledging. Beyond the nest lies a world of challenges: the thrill of first flight, the search for independence, and the ever-present threats to their survival. Join me as we explore the drama within the nest, the tension of fledging, and follow their tentative steps into the wild unknown.

This rapid development is driven by an insatiable appetite and fueled by remarkable food supply provided by the parents.

American Kestrel chicks undergo a fast transformation. In just a few weeks, they evolve from helpless, white fuzzy balls  into juvenile individuals that look almost like an adult. 

This rapid development is driven by an insatiable appetite and fueled by remarkable food supply provided by the parents.

Growth of baby American kestrels

Hatching: Kestrel chicks emerge weighing a mere 5-6 grams, roughly the weight of a nickel! Chicks are born with a layer of down, providing minimal insulation.
Pinfeathers: Within days, delicate pinfeathers begin to appear under the skin and soon begin to sprout, pushing aside the down.
Eyes closed: Kestrel chicks hatch with their eyes sealed shut, relying on other senses for survival. Photo: Anna Fasoli.

First week: Their weight triples, reaching around 15-18 grams by day 7, fueled by a constant diet of insects and small mammals delivered by their tireless parents.
Wing development: By day 10, wing feathers unfurl, initially appearing short and stubby.
First glimpse: Their eyes open within 3-5 days, revealing a world of colors and details. Photo: Anna Fasoli.

Third Week: Between day 8 and 21, they experience a remarkable growth spurt, reaching 70-80 grams.
Flight practice: Around day 21, kestrel chicks begin flapping their growing wings with clumsy but determined effort.

Fledging: By fledging time (around 28-31 days), they have nearly reached their adult weight of 90-120 grams, showcasing their incredible capacity for rapid growth. By fledging time, they are fully feathered. The tail is still short and wing have a rounded tips indicating that flight feathers are still growing. Photo: Alexander Navarro.

How often do Adult Kestrels deliver food to  their babies?

Overall, adult American Kestrels can deliver food to their chicks anywhere from 30 to 100 times per day, depending on the factors mentioned below. This relentless effort ensures their babies have the crucial nutrition needed to survive and thrive in the competitive world of nature.

It’s important to remember that these are just general estimates. The actual frequency of food deliveries can vary greatly within and between broods. 

Chick age:

• Young chicks (0-7 days): Fed every 15-20 minutes, up to 20 times per hour! This is because their tiny bodies need constant energy for rapid growth and feather development.
• Older chicks (7-14 days): Fed every 20-30 minutes, still requiring frequent meals but with slightly larger intervals.
• Nearing fledging (14-21 days): Fed every 30-60 minutes, transitioning to larger prey items and longer intervals as they prepare for independence.

Brood size:

• Larger broods (5-7 chicks) require more frequent deliveries as competition for food is fiercer.
• Smaller broods (3-4 chicks) may have slightly longer intervals between feedings.

Prey availability:

• Abundant prey means shorter intervals between deliveries as parents can easily find food.
• Scarcer prey means longer intervals as parents need to hunt for longer periods.

Weather conditions:

In harsh weather, like storms or heavy rain, kestrels may find it difficult to hunt, leading to longer intervals between feedings.

Sibling Rivalry in American Kestrel Broods

Life in a kestrel cavity nest is far from peaceful. Beneath the chirps and fluff, lies a fierce battle for survival: sibling rivalry. With clutches ranging from 3 to 7 chicks, competition for food and space becomes a brutal act of dominance and desperation. Published data paints a fascinating picture of this complex dynamic.

The Pecking Order Reigns:

• Hatch order matters: Studies reveal a clear pecking order established early on, with larger, earlier-hatched chicks dominating the food chain. These “privileged siblings” often hog the largest and juiciest prey, leaving their smaller or later-hatched counterparts with scraps.
• Begging wars: Hunger pangs fuel constant vocal displays. Chicks unleash ear-splitting shrieks, vying for their parents’ attention and a chance at a meal. The intensity of begging calls directly correlates with hunger levels and pecking order position. (Source: Collopy, M. W. (1984).
• Siblicide as a last resort: In extreme cases, competition can turn deadly. Research shows that siblicide, the killing of siblings, occurs in 2-5% of kestrel broods. This drastic act is often driven by severe food scarcity and intense competition for survival. (Source: Smallwood, J. A., & Bird, D. M. (1999).

Food as the Battleground:

• Unending hunger: Kestrel chicks are metabolic furnaces, burning calories at an incredible rate to fuel their rapid growth. Parents, despite their tireless hunting, can barely keep up with the demands of their brood. This scarcity intensifies competition, with babies resorting to jostling, shoving, and even stealing food from their weaker siblings.
• Weight disparities: Studies show a significant difference in weight gain between dominant and subordinate chicks. The privileged few grow faster and reach fledging weight sooner, while others struggle to keep up, putting their survival at risk. (Source: Boal, C. G., & Dykstra, C. R. (1985).

Survival of the Fittest:

• Harsh reality: Not all chicks emerge victorious. Research indicates that smaller, later-hatched babies have significantly lower survival rates. Malnutrition, stunted growth, and delayed feather development weaken them, making them more vulnerable to starvation, predation, and harsh weather conditions.
• Natural selection at play: While brutal, sibling rivalry serves a crucial role in natural selection. It ensures that only the strongest and most adaptable chicks fledge, carrying the kestrel legacy forward. This harsh but efficient system helps maintain healthy kestrel populations in the face of environmental challenges.

American Kestrel Parental Care Post Fledging

American Kestrel chicks typically leave the nest, or fledge, between 28 and 31 days old. Unlike some bird species that push their young out of the nest, kestrel parents are more encouraging. They may flutter around the nest entrance, calling to their offspring and enticing them to take that first leap.

Is there an order of fledging or hierarchy to leave the nest?

There’s no evidence of a fledging hierarchy or order at which baby kestrels leave the nest. They may fledge on different days, depending on their individual development and confidence. Some may be bolder and take the plunge earlier, while others may hang back and wait for a little more encouragement.

Once they’ve taken that first flight, things get a bit wobbly. Young kestrels are still developing their flight skills and may make short, clumsy hops or glides from branch to branch. They’re closely supervised by their parents, who continue to provide them with food and protection.

What do baby kestrels do after fledging?

For the first week or so after fledging, kestrels stay fairly close to their nest site. They spend their days practicing their flying, learning to hunt, and honing their survival skills under the watchful eyes of their parents.

As their skills improve, they gradually venture further afield, exploring their surroundings and expanding their hunting grounds. This period of exploration and learning is crucial for their development and future independence.

By the time they’re about 3 weeks old, young kestrels are typically self-sufficient and ready to disperse from their natal area. 

Learning to hunt

The fledgling period is a time of immense learning and growth for young kestrels. They spend their days exploring the world around the nest, hopping from branch to branch and practicing short flights. Their parents remain close by, providing them with food and guidance.

While the parents are dedicated feeders, they also encourage their young to become independent hunters. They might bring live prey to the fledglings, allowing them to practice their pouncing and killing skills. This delicate balance between support and independence is crucial for the young kestrels’ survival.

Time for dispersal

After about 2-3 weeks, the young kestrels are skilled enough to catch their own prey and become self-sufficient. They disperse from their natal area, venturing out into the world to find their own territories and mates.

Despite the support of their parents, the fledgling period is a precarious time for young kestrels. Many succumb to predators, starvation, or accidents during this critical stage. Studies estimate that only 30-50% of fledglings survive to their first breeding season.

Spreading Their Wings: Young Kestrel Dispersal

Once young American Kestrels have fledged and mastered the art of flight, a new adventure awaits – dispersal. This journey takes them away from the familiar confines of their natal site, leading them to new territories and potential mates. But how far do they typically travel, and what factors influence their dispersal distances?

The Distance Factor

American Kestrels exhibit what scientists call facultative dispersal, meaning they don’t always disperse, and the distances they travel can vary greatly. Studies have shown:

• Females typically disperse farther than males. On average, young females travel around 30 kilometers (18.6 miles), while males stick closer to home, averaging around 10 kilometers (6.2 miles).
• Some individuals undertake much longer journeys. Records show kestrels dispersing over 1,000 kilometers (621 miles)!

What Drives Dispersal Distance?

Several factors can influence how far a young kestrel disperses:

• Habitat quality: If the area around the natal site is rich in resources and offers good breeding opportunities, young kestrels may be more likely to stay put. Conversely, if food is scarce or competition is high, they may be driven to explore farther afield.
• Population density: In densely populated areas, young kestrels may face intense competition for territories and mates. Dispersing to less crowded areas can increase their chances of finding suitable breeding grounds.
• Sex-specific roles: As mentioned earlier, female kestrels typically disperse farther. This is likely due to their role in mate selection. By venturing out farther, females have a wider pool of potential mates to choose from, increasing genetic diversity and offspring survival chances.
• Individual personality: Like humans, some kestrels are just naturally more adventurous than others. These individuals may be more inclined to disperse long distances, regardless of other environmental factors.

Challenges faced by young kestrels during dispersal

Dispersal is a risky but crucial stage in a young kestrel’s life. It exposes them to unfamiliar predators, potential food shortages, and the challenges of navigating new landscapes. But it also presents exciting opportunities – new territories to explore, potential mates to encounter, and the chance to establish their own lineage.

Habitat Selection: Finding a Home Away from Home

Leaving behind the comfort and security of their birth site, young kestrels must navigate the daunting task of finding a new home. This involves:

• Scouting suitable landscapes: They must assess potential territories for factors like food availability, nesting sites, and protection from harsh weather.
• Competing with established residents: Finding a vacant territory isn’t easy. Young kestrels might encounter aggressive resident birds already claiming the best spots, leading to territorial disputes and energy-draining confrontations.
• Adapting to unfamiliar environments: Different habitats require different skills and strategies. A kestrel accustomed to open grasslands might struggle to hunt effectively in a dense forest, and vice versa.

Predator Avoidance: A Constant Vigilance

Dispersing kestrels are vulnerable targets for a range of predators, including:

• Larger raptors: Hawks, owls, and even other kestrels pose a threat from above. Young kestrels must stay alert, utilizing camouflage and evasive maneuvers to avoid becoming someone else’s lunch.
• Ground predators: Foxes, snakes, and even domestic cats can snatch unsuspecting kestrels from the ground, especially during fledgling stages when their flight skills are still developing.
• Disease and parasites: Unfamiliar environments can harbor new pathogens and parasites. Young kestrels with weaker immune systems are particularly susceptible, making them prone to illness and reducing their overall fitness.

Resource Competition: Sharing is Not Always Caring

Food and nesting sites are precious resources, and competition for them can be fierce. Young kestrels might encounter:

• Established kestrels: Resident birds have a head start in knowing the best hunting grounds and claiming prime nesting cavities. Young kestrels might be forced to settle for less desirable locations, impacting their hunting success and breeding potential.
• Other bird species: Sharing the same habitat means sharing resources with other birds, like shrikes or swallows, who might compete for similar prey or nesting sites. This can lead to aggressive interactions and energy expenditure, hindering the young kestrels’ ability to thrive.

Sources:

• Beal, F. E. L. (1912). Food of the American kestrel (Falco sparverius sparverius). US Department of Agriculture, Biological Survey Bulletin.
• Boal, C. G., & Dykstra, C. R. (1985). Morphology, growth, and development of nestling American kestrels. Wilson Bulletin, 97(2), 214-223.
• Collopy, M. W. (1984). Parental care and the ontogeny of begging in nestling American kestrels. Condor, 86(2), 178-188.
• Smallwood, J. A., & Bird, D. M. (1999). American Kestrel (Falco sparverius). In The Birds of North America (No. 446). Academy of Natural Sciences, Philadelphia, and American Ornithologists’ Union, Washington, D.C.
• Wiebe, K. L., & Bortolotti, G. R. (1992). Facultative sex allocation in American kestrels: the role of food supply. Ecology, 73(6), 1987-2001.

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