Some Kestrel Notes

I’ve previously stated that, like raptors in general, American Kestrels have sexual dimorphism: the female is larger. But the difference is slight for these small falcons, and the literature says that there can be some overlap. The male of a pair can sometimes be larger.

Over the last three months, the #BrooklynKestrels have sometimes appeared evenly matched. Sometimes she seems larger. Sometimes he does. I’ve been attributing his appearance to fluffed feathers, as above. On Monday, I happened to see — they are so noisy! — them mating yet again, and, viewed from the vent, the female is definitely larger.Monday.

From a study of nesting in a Kentucky Wildlife Management Area in the 1980s:

“Average clutch size for the three years (N = 10 clutches) was 4.2 eggs (mode = 4, SD 0.91). Fifteen of 43 (34%) eggs failed to hatch. One of these containeda fully developed embryo, and 14 were lost to predators. Nine eggs were lost from 4 nests between consecutive observations. At a fifth nest, 5 eggs were lost over a period of 7 days. Of 28 kestrel eggs that hatched, 12 young (43%) fledged from 3 nests. Of 16 kestrels that hatched but did not fledge, one died (probably from an infection), while 15 were killed by predators. Thus, overall fledging success for three years was 12 fledglings from 43 eggs laid (28%) or 1.1. fledglings/active nest. In 1983, 2 of 5 nests were successful, yielding 9 fledglings (1.8 fledglings/active nest), in 1984, none of the 4 active nests produced fledglings, and in 1987, 1 of 2 nests was successful and produced 3 young (1.5 fledglings/active nest). Additionally, two adult females were found dead; one below a nest from which 4 eggs had disappeared, and the other within a nest from which I of 4 young was missing. Matted feathers on each female’s head and neck may be an indication that snakes had tried to swallow them.

Incubation by individual males accounted for from 0% to 60% of total incubation (x = 32%). Males were not recorded incubating at two nests (N = 5 and 4 observations, respectively) while at three other nests males were recorded incubating in 3 of 5, 2 of 5, and 4 of 9 observations, respectively. Males performed 47% of the incubation (9/19 observations)for pairs in which males were observed incubating. Although incubation by male kestrels was common in 1983, they were never observed brooding (N = 15 observations of brooding adults).”

~ “Nesting Success and Incubation Behavior of American Kestrels in Central Kentucky” by Christopher Kellner and Gary Aitchison, The Wilson Bulletin Vol. 100, No. 2 (Jun., 1988), pp. 317-319. Authors say nesting success was lower than in the literature: snapes! Rat snakes were a big problem. Tuesday.

Another study, in the boreal forests of northern Saskatchewan in the 1990s, found that weather played a bigger role in size and survival of nestlings than did food abundance (Red-backed voles almost exclusively up there). Cold and wet are just awful for mortality.





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