Diurnal duck migration over the Maltese Islands

Abstract

Since the postulation of the 'combination' approach to bird migration, ornithological literature has witnessed an influx of evidence for leading lines. The new approach combined the two conflicting lines of thought that upheld broad and narrow fronts. It was suggested that there was indeed a broad directional trend to migration, the birds flying in a 'standard direction' typical of their particular population while over uniform terrain or the sea; but in addition there were 'leading lines', delineated by the boundaries between favourable and unfavourable terrain or particular topographical features. When migrants encounter such a factor they tend to fly along it, forming a narrow and concentrated stream. Once the obstacle or feature is surpassed the stream widens out into the broad front again (Matthews 1968). It is now ascertained that (especially) diurnal migrants respond to topographical features and often follow water courses, coastlines and ridges, more so when these are oriented in the direction of their movement (Gauthreaux 1980, & Gill1990). These features provide a line that is easy both to perceive and follow, making it easier for a bird to compensate for any tendency of a crosswind to displace it sideways from its track. The disadvantage in using a leading line is that many birds following the same route increase the competition for potentia·! staging areas (Baker 1984 ).