WINDEN VAN DER J, ELZEMAN S, HORSSEN VAN P, STIP A, CLEMENTS R, VERBEEK R & TERLOUW S (2013) Onset of Purple Heron Ardea purpurea migration from the Netherlands. LIMOSA 86 (2): 68-79.

The Dutch breeding population of the Purple Heron migrates to West Africa during August-September. To date only one migration watchpoint (Ridderkerk, Zuid-Holland) has been known for substantial numbers passing during late afternoon and evening, and this site attracts many birdwatchers in this period. As Purple Herons take off for migration from their post-breeding foraging areas, it was hypothesized that the migration must occur on a broader front. For this reason, in August-September 2009-2012 we organised simultaneous evening counts at several migration watchpoints along the southern edge of the Purple Heron feeding areas (Fig. 1), to study migration patterns, timing and numbers. The herons took off from feeding areas and near night-roosts and colony sites in the late afternoon and evening (Fig. 2). The migration indeed occurs on a broad front from the feeding areas, though weather and social behaviour modify the birds' routes and direction (Fig. 4). The herons form flocks ranging from 2 to 45 birds at departure or when passing night roosts and colony sites, while actively calling and circling while gaining height. Mixed flocks with Grey Herons A. cinerea were recorded in 14% of the cases. Departures only occur on days with a back wind component (WNW to E), and on such occasions, peak numbers of 300-500 Purple Herons are counted. The migration direction is predominantly SW to WSW but is highly influenced by wind direction and speed. During western winds, the herons headed SE while with other wind directions they flew SW to WSW (Tables 1,2, Fig. 5). From the simultaneous counts we estimate that 1200-1800 Purple Herons departed from the western part of The Netherlands in the study years (Tab 3). This coincides very well with the present size of the regional breeding population (600-650 pairs), yielding additional evidence that adult Purple Herons do not disperse before leaving to Africa.

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