Age and territory-quality effects on fecundity in the Spanish Imperial Eagle (Aquila adalberti)

Over a period of 19 years, we studied 237 breeding attempts of Spanish Imperial Eagles (Aquila adalberti) in Doñana National Park (southwestern Spain), including 29 pairs with at least one immature member, to investigate age-related effects on population fecundity. Without considering effect of territory quality, adult pairs were significantly more productive than immature pairs. Highly significant differences in breeding performance among territories were independent of age-classes of birds occupying them. Low-quality territories were more frequently occupied by immature pairs, whereas high-quality territories were occupied mostly or exclusively by adult birds. Therefore, age and territory quality appear to be interrelated. We found no effect on average population fecundity due to breeding by immature pairs.We are indebted to L. García, R. Cadenas, and M. L. Chacón for helping in the fi eld, and to I. Bustamante for helping with the English text. This article has greatly benefi ted from conversations and comments by K. Bildstein, G. Janss, M. N. Kochert, M. Marquiss, M. T. Murphy, I. Newton, J. L. Tella, and two anonymous referees. Support for this project was provided by FUNGESMA (Glaxo-Wellcome) and Consejo Superior de Investigaciones Cientificas.Peer Reviewe