BOSCH M VAN DEN & VLEK R (2006) Autumn migration of Purple Herons Ardea purpurea over Ridderkerk, 2002-2005. LIMOSA 79 (4): 129-138.

At Ridderkerk (southeast of Rotterdam, Zuid-Holland, The Netherlands, 51°52'N, 4°35'E) an extensive evening migration of Purple Herons was noted on 26 August 2002. After this discovery, evening counts (18h00-21h00) were carried out until the end of September when migration ceased. In 2003-2005 these counts have been repeated during the complete migration season (August and September). On average 560 migrating Purple Herons were counted per autumn, ranging from 520 (2003) to 689 (2004) (Tab. 1). Variation in the seasonal totals reflects the number of breeding pairs in Holland only to a minor extent, presumably due to differences in breeding success between years. Migration was concentrated on seven to 14 evenings in a period of about 3.5 weeks from late August to mid September (Fig. 1,2). On many evenings no migrating herons were observed, despite apparently favourable weather conditions. Migration generally peaked in the first or second week of September, or in late August (2003). 2005 showed a late passage, with herons migrating until early October. Within a season several peaks could be recognized, possibly related to the separate passage of adult (late August) and juvenile birds (early to mid September).
Purple Herons migrated in flocks of on average 8.0 birds. The largest flock recorded consisted of 43 birds (11 September 2002). Purple Herons seldom migrated alone. It was regularly observed that herons from the nearby breeding colony in Kinderdijk joined migrating groups. Migration strongly peaked in the 1.5 hours before sunset. Apparently the Purple Herons use this period to form groups and orientate for their nocturnal flights. Migrating groups headed SSW, avoiding a nocturnal passage over the wide estuaries in the province of Zeeland. Grey Herons A. cinerea were regularly seen migrating together with Purple Herons, particularly later in the season.
The extensive migration of Purple Herons near Ridderkerk is presumably the result of its strategic position in relation to five large breeding colonies (Fig. 3). The social departure from the breeding grounds is a phenomenon that is unique for Western Europe. Other migration hotspots for this species are only to be found in the Eastern Mediterranean.

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