WINDEN J VAN DER, POOT MJM & HORSSEN PW VAN (2014) Satellite transmitters provide information on location, size and fidelity to foraging areas of Purple Herons in the Netherlands. LIMOSA 87 (2): 82-90.

Purple Herons have a restricted distribution in the lower parts of the Netherlands (Fig. 1). They breed in marshes and although feeding densities are highest in marsh habitat, the largest numbers feed in agricultural grasslands with high densities of ditches. So far foraging ranges and location of foraging sites have been described on the basis of sightings of flying and feeding herons. However, since 2007, 15 adult Purple Herons were deployed with a satellite transmitter of which five also had a gps module. This made it possible to quantify individual flight distances to foraging sites and determine the size of these sites. Although conventional satellite transmitters provide relatively inaccurate positions, these can well be used to pinpoint feeding sites if enough are available (Fig. 2). This information showed that the Purple Herons show a high fidelity to feeding sites within and between seasons (Fig. 3). Few birds used alternative sites at more than 1 km from their preferred location. The average foraging flight length of the herons was 5.3 km with a maximum of 13.4 km (Tab. 1). This is well within the range of 20 km based on visual sightings. The few individuals that bred in larger colonies showed larger flight distances. This is consistent with the hypothesis that a lack of suitable breeding sites in the Netherlands forces Purple herons to perform long flights. The average measured instantaneous flight speed to and from feeding sites was 41.5 km/h (range 31-72 km/h; Fig. 5). This implies that some individuals might spend more than 20% of their time flying when feeding chicks. All Purple Herons with transmitters indeed foraged in grasslands and three of them also in marshes. The herons use relatively small areas to feed usually with two or more core areas. A Kernel prediction model was used to estimate feeding ‘home-ranges’, neglecting unused areas between the cores. With this method we estimated the mean size of individual feeding areas at 60-80  ha (range 25-230). The data confirm that Purple Herons have overlapping feeding sites with conspecifics and that the size of the used area is negatively correlated with habitat quality (food density).

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