ORNITHOLOGICAL LITERATURE SEXUAL SIZE DIMORPIIISM IN HAWKS AND OWLS OF NORTH AMERICA. By Noel F. R. Snyder and James W. Wiley. Ornithological Monographs No. 20, 1976:vi + 95 pp. Amer- ican Ornithologists ’ Union. $7.50.-The chief original contributions of this paper are (1) a table summarizing food habits of all species of hawks and owls in North America for which significant data are available in Fish and Wildlife Service files or published litera- ture; (2) a table of dimorphism indices and the mean measurements of wing chord, culmen, and weight on which these indices are based; (3) extensive data on food, feeding rates and timing of reproductive losses at nests of the Sharp-shinned Hawk (both Puerto Rican and mainland subspecies), Coopers’ Hawk, Goshawk, and Red-shouldered Hawk; (4) excellent new data demonstrating substantial sexual differences in diet in Sharp-shinned and Coopers’ hawks ; and (5) 4 bivariate scatter diagrams that in- dividually relate average dimorphism in both hawks and owls to % birds (rn = .79), % mammals (rs = .06), % lower vertebrates (rs = .16), and % vertebrates (rr = .43) in the diet. The correlation of dimorphism with % birds is highly significant; that with % vertebrates is weakly significant, resulting mainly from the inclusion of the component of avian prey. From this the authors conclude (p. 9) that, “The correlation between size dimorphism and taking of avian prey is sufficiently strong that we consider it to be the most important fact to be handled by any hypothesis concerning the function of size dimorphism in raptorial birds.” On the basis of these findings, Snyder and Wiley present a novel refinement of existing theory that relates sexual size dimorphism to adaptively-broadened intraspecific niches. Specifically, they propose that substantial dimorphism in bird-feeding raptors has been selected for because it reduces intra-pair competition and allows a wider range of food sizes to be taken during the crucial period late in the breeding cycle when both sexes are foraging and when such predators are food stressed because of reduced bird popula- tions. Young of these birds of prey, requiring extended practice to develop the expertise necessary to capture birds, may be especially vulnerable to food shortage during their pro- longed post-fledging dependency period. Although the authors achieve only moderate success in supporting their argument with field data, especially those obtained from nesting accipiters, even when their efforts lead to rather equivocal results one gains the impression that they are seeking answers in the correct places. It is unfortunate that the general paucity of published information on feeding ecology of hawks and owls late in the breeding season renders their hypothe- sis difficult to test with information from other species. Nonetheless, Snyder and Wileys’ stimulating thoughts on dimorphism deserve continuing consideration as new information accumulates. In addition to a thorough review of ideas proposed in the past for the sexual size dif- ference in raptors, they also discuss the related question of why the female is usually the larger sex. After briefly entertaining the notion that reversed dimorphism “is pos- sibly a chance effect,” they conclude, in at least partial agreement with Amadon (Raptor Research 9:1-11, 1975), that the reversed nature of the dimorphism is likely to be “re- lated to advantages in copulation, incubation, brooding, and nest defense for large females.” The most impressive aspect of this monograph is the thoroughness and balance of the discussion, which interweaves both old and new explanations for dimorphism in a most satisfying way. The many alternative views already in the literature are weighed fairly, with no apparent urge to seduce the reader into following poorly-illuminated paths. In 145 146 THE WILSON BULLETIN * Vol. 90, No. 1, March 1978 TABLE I RELATIONSHIP BETWEEN SIZE DIMORPHISM AND ‘j& BIRDS IN DIET IN NORTII AMERICAN HAWKS AND OWL? T&-G3 Type of Number of Statistical included dimorphismindex species r* significance Hawks and owls Average ’ 44 0.79 P < 0.001 Owls Average 17 0.71 P = 0.001 Hawks Average 27 0.86 P < 0.001 Hawks below D. I. of 7.5 Average 13 0.13 P > 0.100 Hawks above D. I. of 7.5 Average 14 0.71 P = 0.010 Hawks and Owls Body weight3 35 0.93 P < 0.001 Owls Body weight 16 0.54 0.01 < P < 0.025 Hawks Body weight 19 0.81 P < 0.001 Hawks below D. I. of 8.1 Body weight 9 0.09 P > 0.100 Hawks above D. I. of 8.1 Body weight 10 0.36 P > 0.100 l Based on analysis, with the Spearman Rank Correlation Test, of data from Table 1 of Snyder and Wiley. z “Average Dimorphism Index” of Snyder and Wiley is computed ils the mean of separate in- dices calculated for wing chord, cuhnen, and body weight. 3Based on cube roots of mean weights of males and females. addition to discussion of the possible dimorphism-food relationship, the paper is replete with ad hoc hypotheses involving other ecologic features and their possible relevance to the size difference between the sexes. Thus, correlations are attempted between de- gree of size dimorphism and incidence of double-brooding, degree of coloniality, polyg- yny, and sex ratio in various raptorial species, including those that do not fit neatly into the apparent relationship of degree of dimorphism and percent avian prey. To be sure, many of these comparisons do not yield convincing correlations, suggesting that the explanation underlying the evolution of dimorphism could be more complicated than researchers in this area have admitted. If the selective pressures that influence dimor- phism vary among species, through time, and by geographic region, as could well be the case, then the data necessary to expose the real biologic correlates of sexual size dif- ference will need to be correspondingly refined and extensive. Those who interpret this conclusion as an unnecessary retreat into complexity are reminded that it also could represent an advance into reality. Simple answers are unlikely to questions that have puzzled naturalists for over a century. Because the authors do not separately analyze the various groups of raptors included in their biologically important and statistically significant plotting of $‘ birds in diet against average dimorphism (Fig. 1, p. 71, I have taken the liberty to examine their data in this light. The results of my comparisons are presented in Table 1. Of immediate in- terest is that owls follow the same trend as hawks even though they do not reach the striking indices of dimorphism shown by the latter group. Inspection of Snyder and Wileys’ plotting of the hawks alone exposes yet another area of concern. Below an aver- age dimorphism index of approximately 7.5, only species that feed to a minor extent on birds are represented and no correlation of sexual size difference with the taking of avian prey is seen (rs = 0.13). Above an average dimorphism index of 7.5, however, strong correlation is demonstrated. Thus, a gradual increase of dimorphism with increasing avian March 1978 * ORNITHOLOGICAL LITERATURE 147 feeding is not seen in approximately one-half the species of hawks. But when dimor- phism reaches a certain (threshold?) level, the sexual size difference is dramatically correlated with percent birds in the diet. Most impressive of all is that the highest levels of dimorphism in owls occur in the same region of the scale where the hawks shift from non-correlation to correlation with avian diet. Comparison of these trends in dimorphism with those of birds of prey of other geographic regions should be most rewarding. Space does not permit speculation here on the reasons for these patterns in North American species. In the opinion of many, body weight is far superior to other measurements as an as- sessment of overall size. Thus, I was disappointed to note that Snyder and Wiley base all their size comparisons on an average dimorphism index calculated as the mean of 3 separate indices, those of wing chord, culmen, and weight. For many species the indices are rather similar and nothing would appear to be gained by using the body weight in- dex. But for others the difference is dramatic and an index obtained by averaging simply conceals the true magnitude of the size difference between the sexes. For example, in the Great Gray Owl the index based on weight is 10.9, versus 6.3 for the average index. Comparable figures for the Short-eared Owl are 6.1 versus 2.5. Therefore, comparisons of dimorphism indices based on body weight with % avian food are of interest, and I present these data in the bottom half of Table 1. In general, the findings compare favor- ably with those derived from the average index. The combined groups of birds of prey show a substantially higher correlation than when the average index was used. In- dependent comparisons of owls and of hawks again provide statistically significant Spearman correlation coefficients. Once more a break is evident in a plotting of the hawks, in the vicinity of dimorphism index 8 and between 10 and 20% birds on the scale. Surprisingly, when the 2 groups on each side of the shift are analyzed separately, neither correlation is statistically significant. Although the reasons for this are not clear, I suggest that in hawks, the dimorphism index based on weight correlates well with % of birds taken as food, mainly because the means of 2 rather independent groups are correlated. Within either group, however, the scatter is substantial and no significant cor- relations are evident. Thus, there are still many puzzling aspects of the relationship be- tween degree of dimorphism and diet in addition to those illuminated by the authors.