British Birds, Volume 80, Number 7, July 1987
Clutch size, nestling aggression and breeding success of the Spanish Imperial Eagle
During 1970-80, clutch size, nestling aggression and resultant nestling mortality, and breeding success of the Spanish Imperial Eagle Aquila (heliaca) adalberti were studied in Extremadura and neighbouring regions of Western Spain. The number of eggs per clutch and the number of young fledged varied between one and four, with remarkable consistency shown by most pairs over several years. Whereas two pairs regularly laid only one or two eggs and frequently failed to rear young, others laid three or four eggs every year and reared two or three (even four) young. There was no evidence for any pair which occupied a territory not attempting to breed. Nestling aggression considerably reduced the number of chicks during the first weeks after hatching; this phenomenon was previously not known for the Spanish Imperial Eagle. In broods with several nestlings, aggression by older chicks almost always led to the death of the last-hatched; in one instance, the way in which the oldest chick killed first the youngest and, two days later, the remaining chick was observed in detail. Whereas, among some other Aquila eagles, nestling aggression (or "Cainism") almost invariably leads to the death of the second chick independently of the food supply, with the Spanish Imperial Eagle food supply does have an influence on nestling survival.