Size Dimorphism and Food Habits of North American Owls

Although in most species of birds the male isFor a thorough discussion of the foregoing characteristically larger than the female, the arguments and of the evolution and ecologic direction of size dimorphism among many significance of dimorphism in birds in general, birds of prey is reversed; the female is larger see Amadon ( 19159), Cade ( 1960), Selander than the male.This reversal is present in the ( 1966), and Storer ( 1966).orders Falconiformes and Strigiformes and WhiIe these workers agree that reversed size in the families Stercorariidae and Fregatidae dimorphism is associated with a predatory ( Amadon 1959:534).mode of life.their studies have concentrated Several authors have discussed the possible on the Falconiformes.To our knowledge there origin and significance of sexual dimorphism in has been no detailed examination of the relasize in raptorial birds.Briill ( 1937), Hagen tive degree of sexual dimorphism among the (1942), Dementiev (1951), Storer (1952, 1966), other birds of prey.Because current generaland Selander ( 1966) believe that the increased k izations on reversed dimorphism in predatory size of the female o r the male permits differ-birds are based in large part on data from ential niche utilization by the sexes; that is, a hawks, it is of considerable interest to deterdifference in sizes of the sexes would allow an mine to what extent data on other raptors increase in the optimal size range of prey spe-corroborate these findings.cies and would reduce intersexual competition.The primary purpose of this paper is to de-Cade (1960) feels that the larger size of the scribe the extent and nossible function of L female in hawks is related to the difficulty of interspecific and intraspecific size dimorphism pair bonding in such birds.Hagen (1942) and in species of North American owls.Secondly, Amadon (1959) postulate that this dimor-we summarize principal food habits of these phism may have evolved in order to protect the species from data in the literature and from young from possible predation by *the male unpublished noaations on museum specimen parent.The male birds of prey may have labels.We have little to sav about the reversed weaker parental instincts but stronger preda-direction the dimorphism has usually taken; of tory instincts than do the females and thus greater ecological relevance is the degree of represent a threat to the young; a larger, domi-dimorphism and its possible relationship to nant female could better ,g rotect her young competition.Finally, this compilation has from possible cannibalis .demonstrated the lack of basic data on bodv , The paper by Hill (1944) seems to have weights and food habits for certain forms, and been overlooked by most recent authors deal-we wish to call attention to the need for this ing with dimorphism, although he presents information.data (unsupported by statistics) on sexual differences in size for 49 forms of Falconiformes METHODS AND MATERIALS based on measurements of over 2200 museum Our analysis of dimorphism is based upon body specimens.Although he attempts correlations weights (n = 1144) and wing lengths (n = l' eO2) of amount of dimorphism with various aspects of museum specimens of 32 forms of owls.Stomach of behavior and of breeding biology, no con-contents listed on labels of specimens of known sex, body weight.and wine lenath are of narticular intervincing relationships are shown.He concludes est because they dem&straie the food habits of indi- that the more primitive members of the Fal-viduals of known size and sex.coniformes show less dimorphism than do the All wing measurements (the chord of the arc of the more highly derived forms and that the evolu-unflattened wing) were made by Johnson.Wing measurements were not used if the owl was molting or if tion of the size difference involved a decrease the wing feathers were incompletely grown. in the size of the male rather than an increase Because weight is the best indicator of overall body in the size of the female.size, it is unfortunate that body weights are not commonly recorded for museum specimens.Thus, sam-