Ageing nestling Barn Swallows Hirundo rustica: an illustrated guide and cautionary comments

Abstract

Ageing is a crucial requisite for any analysis of growth and development. However, ageing by regular nest visits around hatching is time-consuming. Thus, species-specific developmental patterns are used to estimate nestling age at infrequent nest visits during the nestling period. Unfortunately, comprehensive species-specific data on the timing of other developmental traits than body mass and feather growth are rarely published. Here, we present an illustrated guide to ageing nestling Barn Swallows, Hirundo rustica, based on daily observations of 27 broods from hatching to 15 days of age. The occurrence of feather tracts at different parts of the body was the most reliable key to ageing nestlings in the first days of life. Afterwards, the most useful trait was the emergence of feathers from their sheaths. In periods of cold weather, nestling body mass and feather length was significantly lower than that attained in normal weather conditions, especially from day 9 onwards, leading to an ageing bias of one to four days. To ensure accurate estimates of nestling age, ageing should be accomplished as early as possible in the nestling period. This is of special importance if differences in growth and body condition between individuals are key aspects of the study.