Monitoring of the age of mates, a method proposed to detect early warning signals for demographic trends in long-lived bird populations, showed different patterns when the study was performed on a single Bonellis eagle subpopulation of Andalusia (South of Spain) than when the whole region is taken into account. In this respect, we discuss the role of the spatial scale, the origin of reduction in non-natural mortality and the definition of the used age classes. For a correct monitoring of the age of mates in the Bonellis eagle, we propose the two age classes previously suggested, adult and non-adult, but paying particular attention to the late subadult individuals, specially when the monitoring is performed in a wide region by different field work groups. Likewise, it is nec- essary for monitoring age of breeders in long-lived species with deferred maturity to collect data from a sample well distributed over space, taking regularly into account pairs from the edge and from the centre of population. Besides, it is important to monitor those subpopulations with different known threats, providing them their relative importance among the whole population. Finally, con- ventional monitoring of the age of mates seems to generally be a reliable way to predict viability changes of bird populations, except when adult by adult replacements take place by reduction in juvenile mortality. 2005 Elsevier Ltd. All rights reserved.