The Importance of Juvenile Cannibalism in the Breeding Biology of Certain Birds of Prey

Abstract

For the majority of birds it is manifestly of considerable importance to synchronize, so far as is possible, the hatching date of their eggs. Indeed, this is essential if the entire brood is to leave the nest at approximately the same time, which, in most species, is necessary if they are all to survive. The approximate synchronization of the hatching date is, of course, normally effected by postponing full-time incubation until the clutch has been nearly, or wholly, completed; but it may be further assisted if the more recently laid eggs require a slightly shorter time to hatch than those deposited earlier (See Jourdain, in Witherby et al., 1: xxi; 1938). As a uniform hatching date is indubitably a desirable feature in the breeding biology of most birds, why is it that with the majority of raptors there is no synchronization in the hatching of the clutch? On the contrary with many of them, intervals of one, two, or more days commonly occur between the birth of each nestling. In large-brooded species, since incubation usually starts with the first egg laid, this may even result in differences of a fortnight or more between the ages of the first and last hatched nestling. What are the advantages gained by graduating the ages of their young in this way? I suggest, at any rate so far as the larger-brooded raptors are concerned, that the habit is beneficial for the following reasons. First, by "staggering" their ages, the task of sustaining a large and growing family of carnivorous young, instead of becoming progressively more difficult and perhaps ultimately impossible, is simplified, and to a great extent eased, by being spread over a prolonged period. And, secondly, as it is eugenically preferable to rear, let us say, one or two healthy well-nourished progeny rather than six or seven weaklings, this "staggering" is of the highest importance since it offers perhaps the only satisfactory way-namely by controlled cannibalism -of numerically reducing the family to a feedable size when this has become vitally necessary owing to a food shortage. As the food supply of some of the owls-and this is especially true of Asio flammeus-is often largely dependent on fluctuations in the population of certain rodents, a ready and rapid means of adjusting the size of the brood to the amount of food available is clearly desirable. Without a marked disparity in the age and size of the fledglings, fratricide would be virtually impossible and consequently no such