The American kestrel (Falco sparverius) exhibits distinct dimorphism between males and females. This difference in size and plumage between the sexes is quite noticeable in this smallest North American falcon.
Size Dimorphism
There is a clear size difference between male and female American kestrels. Males tend to be smaller with an average weight of about 3.5-5.5 oz (100-150 g). Females are noticeably larger, weighing around 6-8 oz (170-225 g). This size disparity gives the female an advantage during incubation when she needs to cover the eggs, as well as provide warmth and protection to the hatchlings.
Sex | Average Weight |
---|---|
Male | 3.5-5.5 oz (100-150 g) |
Female | 6-8 oz (170-225 g) |
The difference in size between the sexes is also apparent in their measurements. Males have an average length of 9-12 inches (23-30 cm) and wingspan of 21-25 inches (54-63 cm). Females are bigger with an average length of 11-14 inches (28-36 cm) and wingspan of 25-29 inches (63-74 cm).
Plumage Dimorphism
In addition to the size difference, the plumage or feather coloration and patterns also differ noticeably between male and female American kestrels. The most striking dimorphism can be seen on the wings, tails, and heads.
Wing Color Patterns
Male American kestrels have slate-gray wings with black spotting. The back of the wings have reddish barring. In contrast, females have more brownish, rufous-colored wings with narrow black barring throughout.
Tail Color
Males have a rufous or reddish tail with a single wide subterminal black band and white tip. Females have a brownish tail with numerous narrow black bands and white tip.
Head Pattern
The head exhibits very distinct sexual dimorphism in the American kestrel. Males have a bright gray crown and sides of head, with black spotting especially above the ear coverts. Females have a brownish crown and stripes through the eye region.
Purpose of Dimorphism
What is the purpose behind this pronounced size and color dimorphism in the American kestrel? There are a few leading hypotheses proposed by ornithologists:
Different Roles in Reproduction
The larger body size of the female kestrel allows her to more effectively incubate eggs and brood and defend the hatchlings. The smaller male is more agile and suited for hunting to provide food for his mate and the chicks.
Sexual Selection
The distinct plumage coloration allows kestrels to easily distinguish between potential mates. The bright colors and patterns may serve as sexual ornaments, allowing males and females to assess each other’s fitness for breeding.
Niche Partitioning
The size differences may allow males and females to target different sized prey without having to compete directly for resources. This partitioning of the feeding niche contributes to more efficient hunting.
Geographic Variation
While the sexual dimorphism described above applies to most American kestrel populations, there are some regional differences across the large breeding range of this falcon. Here are some of the key geographic variations:
Larger Northern Subspecies
The subspecies F. s. sparverius that breeds across Canada and Alaska tends to be larger in size. Males in the north average 4-6 oz (110-180 g) while females weigh 7-10 oz (200-280 g). They need the extra bulk to survive cold winters.
Increased Size in the Southeast
The Southeastern American kestrel F. s. paulus is 25% larger than the nominate sparverius subspecies. These are the heaviest kestrels in North America, thought to be an adaptation to year-round residence rather than migration.
Brightly Colored Southwestern Birds
The subspecies F. s. cinnamominus breeding in southwestern states has intensified colors, with bright rufous on the wings and tail. This may provide better visual signals in open desert environments.
All-gray Cuban Subspecies
The Cuban kestrel F. s. sparveroides lacks the typical reddish plumage seen in mainland birds. Both sexes are entirely gray except for black wing tips, an example of evolution in an isolated island population.
Juvenile Appearance
While adult male and female American kestrels can be easily distinguished by size, plumage, and color patterns, young juveniles look similar in both sexes. Juvenile kestrels resemble adult females but have buffy streaking on the underside and a duller facial pattern.
Juvenile male kestrels start to acquire adult plumage after their first spring molt at around one year old. Juvenile females molt into adult plumage more slowly over the first two years of life.
Conclusion
The American kestrel displays some of the most striking sexual dimorphism seen in falcons. The differences in size, colors, and patterns between males and females are genetically programmed. These variations likely serve important purposes from courtship and breeding to niche partitioning. While highly dimorphic across most of the range, geographic subspecies show local adaptations in size and color.