THE FUNCTION OF POSTFLEDGING EXPLORATION - A PILOT-STUDY OF 3 SPECIES OF PASSERINES RINGED IN BRITAIN

The post-fledging movements of young birds are poorly understood and difficult to study. Relatively little attention has been paid to the exploration-like elements of post-fledging movements. Thus, young birds visit and perhaps revisit a number of sites and collect sufficient navigational information to allow them to return, often months later, to at least some of these sites. My aim in this paper is twofold: (1) to consider some of the potential functions of post-fledging exploration and the characteristics that each may impart to the behaviour; and (2) to suggest analytical techniques by which researchers with access to suitable ringing returns may use their data to investigate the behaviour. In part, post-fledging exploration appears to be aimed at the location of areas suitable for breeding (the following summer) and, where appropriate, for overwintering. However, a pilot study of three species of passerines ringed in Britain suggests that longer post-fledging journeys do not necessarily translate into the adoption of more-distant breeding sites. Instead, the distance and direction of post-fledging exploration is a function of the furthest distance a bird is ever likely to travel from its birth site. Taken together, these data support a ''navigational target'' hypothesis. The size and orientation of this target may be optimal for the risk of navigational error to which each species is exposed. The larger targets may be elongated along a N-S axis and birds may bias the direction of their first spring return migration to one side (E-W) of the target.