Barn Swallow (Hirundo rustica) Science Article 3
To compare fitness data not only between adults and juveniles but also between recently-fledged and older juvenile Swallows we looked for an externally-visible character to distinguish these two cohorts in the field. So far, no such criteria seem to have been described (Svensson 1992, Suter 1985, Cramp et al 1988). Here we present data to demonstrate that scoring the stage of development of the bill edges is a useful method for that purpose.
Johann von Hirschheydt, Beat N f-D nzer and Lionel Maumary, Ringing & Migration (2005) 22, 209-211