Why don't all siblicidal eagles lay insurance eggs? The egg quality hypothesis

Abstract

Several species of birds lay second eggs that are eliminated by the siblicidal behavior of the first-hatched chick. A widely accepted explanation for the occurrence of these second eggs is insurance against complete nest failure. However, if insurance is seen as an important breeding strategy for two-egg (c/2) layers, the question arises why single-egg species do not lay insurance eggs. The insurance-egg hypothesis predicts that extra eggs should occur where hatch failure is not trivial, which may be particularly prevalent in dense populations. Neither prediction was supported for siblicidal Wahlberg's eagles Aquila wahlbergi. Neither could food constraints or allometric relationships explain the small one-egg clutch (c/1) of this species. Instead, clutch size was experimentally shown to be related to optimal brood size: parents given two young were unable to rear them, and subsequent breeding opportunities were significantly curtailed. Since clutch and brood size are similarly related in c/2 eagles, insurance may be an exaptation of the second egg. One-egg species, however, appear to trade second (insurance) eggs for large, high-quality eggs, which enhance hatchability and chick viability. This was borne out by comparison of the world's c/1 eagles, which lay significantly (p