The Reversed Size Dimorphism in Birds of Prey: A Reply

Abstract

Yom-Tov and Ar criticize, in a loose manner, our hypothesis (von Schantz and Nilsson 1981) on the causes of the reversed dimorphism in birds of prey. They conclude that the larger sizes of females cannot be explained by either considering the relative cost of egg production or their ability to carry large prey alone (our italics). We agree with this. But the major point is, as we suggested, that there are in fact several selective pressures working together on both sexes. The resultants of these give rise to the observed species' patterns, with a varying degree of dimorphism in different species (see also Andersson and Norberg 1981). Some further comments on Yom-Tov and Ar's arguments. We concluded, that for central place foraging birds, which are specialized on small prey, the benefits of being large (e.g. at egg laying) are counterbalanced by greater energy requirement due to a greater number of provisioning trips. For birds of prey, this negative effect is lessened as they take proportionately larger prey and thereby make fewer provisioning trips. This made Yom-Tov and Ar to expect that in nidifugous species size dimorphism favouring large females will be common (our italics). However, it is not so. As we wrote (p. 130), for bioenergetic reasons, nidifugous species or species which make few provisioning trips to their young should have a greater potential for evolving large body size' either through sexual selection favouring large males or through selective pressures favouring large females, or both. We stressed this prediction by pointing out that nidifugous birds generally are larger than nidicolous species. So, since none of the parent birds make provisioning trips to the chicks, both sexes can evolve large body size. In the true precocial orders (Anseriformes, Galliformes, and Ratites) the males seldom provide food for the chicks. Accordingly, in these species there are no selective pressures on the males to be small as is the case in birds of prey. On the contrary, many precocial species are polygamous which hence favours large males through sexual selection. We don't see how the arguments on the energetic part weaken our idea. The energy content of one egg of birds of prey and passerines is 103 and 45% of BMR respectively (Ricklefs 1974). Passerines lay one egg each day whereas birds of prey usually lay one egg each second or third day. Thus, according to Ricklefs (1974) the daily