Background Accurate estimation of nestlings' age is essential in avian demography studies as well as in population ecology and conservation. For example, it can be useful for synchronizing nest visits with events of particular interest, such as the age at which young can be safely ringed, or in choosing the best period to attain the most accurate calculation of laying or hatching dates.

Methods We constructed a photographic guide for aging European Bee-eaters (Merops apiaster) nestlings to 3-day age classes and evaluated the aging method by performing a validation exercise with several observers with no previous experience in aging bee-eater nestlings.

Results The aging guide for bee-eater nestlings allowed estimating age to within 3 days with an average accuracy of 0.85. We found the optimal period for aging nestlings was between days 13-18 (with accuracy between 0.94 and 0.99), during which the status of feather development was more easily distinguishable from the preceding and subsequent age classes. During the first 3 days after hatching, nestlings could also be aged with high accuracy (0.93). The small size of the nestling in relation to the eggs and the nestling's inability to raise its head during these first days allowed for good discrimination from the subsequent age class. Between days 25 and 28, nestlings were correctly aged in only half of assignments (0.55 sensitivity) and nestlings belonging to class 7 (days 7-9) were the least correctly identified (0.38 sensitivity). However, by visiting the nests at 12 days intervals it is possible to achieve the highest accuracy in age estimation with the smallest disturbance and logistic investment.

Conclusions This study highlighted how indirect methods and a simple protocol can be established and employed to quickly estimate nestling age in cases where handling nestlings is challenging or impossible, while minimizing disturbance in and around the nest.