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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