BERND-ULRICH MEYBURG

On Cainism in the Lesser Spotted Eagle Aquila pomarina

Summary

Detailed observations were made of Cainism in a Lesser Spotted Eagle's (LSE) nest, whereby this procedure generally leads to the death of the second chick a few days after hatching. The time-lapse between hatchings amounted to two and a half days (60 hours). At the time of hatching the chicks weighed 64 und 58 g respectively. During the first and second days after the second chick (Abel) hatched, no signs of Aggression could be observed and herb chicks received a liberal supply of food. On the third day, however, herb chicks were strenuously fighting, with Abel being the first to attack. On the fourth day the female was displaying an unmistakable tendency to favour the older chick (Cain), which clearly held a stronger attraction for her. On one occasion the feeding by the female abruptly ended even though Abel was still actively begging for food. On this day Abel was indeed still given food, but its begging and its own attacks were, however, very haphazard. At four days old Abel weighed 12 g less than it did on hatching. This chick probably died the next day, even though there was an abundance of food in the nest.

Observations at another nest (see Table 1), in which Abel had reached an age of about three weeks, confirmed previous findings by experimental translocation that the weaker chick's acceptance of intimidation is ultimately responsible for its death when other factors (smaller weight at hatching, later hatching date resulting in less adroitness and tenacity at feeding times, neglect by the female) do riet lead to it.

The three cases so far considered, in which the second chick could be closely observed from hatching to death, show that Cainism in the LSE can take a variable course. This is amply confirmed by the case under study in which Abel survived for up to three weeks.

The existence of Cainism could be explained by the LSE´s being today at an intermediate stage in its evolution, of transition from two-egg to one-egg clutches.

Abstract: The following factors are responsible for Cainism (fatal Cain-and-Abel struggle) in the Lesser Spotted Eagle:
1. The time lapse of several days between the hatching of the two chicks, giving the first-born (Cain or C1) a more or less considerable developmental advantage.
2. Cain’s greater weight at the time of hatching. It usually also hatches trom the larger egg.
3. The aggressiveness of the chicks towards each other.
4. The acceptance of intimidation by the inferior chick, even when there is virtually no difference in weight (e. g. in translocation experiments), to the extent that this chick hardly ventures to take part in feedings.
5. The scant attention paid by the female to the second chick. given its smaller size and, above all, its greater clumsiness in taking food due to its retarded development, shows that it holds a waning power of attraction. The food supply plays no part in this. At this time lhe nest contains an ample supply of prey.
Up to now there has been much disagreement over the meaning of second-egg laying, leading as a rule to the fledging of only one young, and over the significance of Cainism within the framework of evolution in this and other eagle species. None of the attempts to explain it so far published is really convincing. One possible explanation for the phenomenon of Cainism may well be that the species is at an evolutionarily intermediate stage of transition from two-egg to one-egg clutches. For unknown reasons, and in contrast to earlier times, the raising of one young per breeding season is today sufficient to maintain the species. At a later stage in the course of its evolution this species, which at present lays a second egg clearly smaller than the first – one could regard this as vestigial – may well confine itself to laying no more than one egg per clutch.

Key words: Aquila pomarina, Lesser Spotted Eagle, cainism, Cain-and-Abel struggle, evolution.