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Breeding of Non-Adults and Effects of Age on Productivity in the Spanish Imperial Eagle Aquila adalberti

We studied annual and geographical variation in the proportion of breeding non-adult (<4.5 years old) Spanish Imperial Eagles Aquila adalberti in relation to the species' recent demographic recovery. The proportion of non-adult breeders in the populations studied increased significantly during 1983–2004 in parallel with the population increase. Non-adult pairs were more often associated with ‘young’ territories (occupied recently) than with territories that had already been occupied for longer time (mostly used by adult pairs). Adult pairs produced significantly more chicks than mixed adult— non-adult pairs, which in turn had a higher productivity than pairs in which both male and female were non-adult. Adult pairs attempted breeding more often than non-adult pairs, and clutch and brood size were larger. The poorest reproductive output was found in pairs with non-adult males. These differences were not associated with differences in fledging rates. Once the influence of territories was controlled for (comparing the productivity between adult pairs and non-adult pairs present in the same territory) the age effect on productivity was significant. Our results emphasise the important role which the recruitment of non-adult birds into the breeding population may have played in the recent population recovery of the Spanish Imperial Eagle.