American Kestrel (Falco sparverius) Science Article 3
The degree of dimorphism between males and females has been examined in a number of raptors (Storer 1966, Reynolds 1972, Balgooyen 1976, Snyder and Wiley 1976), but few studies have reported the amount of dimorphism between mated pairs of birds. If the maintenance of a size difference within pairs is adaptive, then one would predict that selective mating would occur to minimize instances of overlap. American Kestrels are only slightly dimorphic, with some overlap between the sexes (Brown and Amadon 1968).
REED BOWMAN, Wilson Bull., 99(3), 1987, pp. 465-467