Predator exposure trials
Predator exposure trials were carried out from early May to mid-July,
during the rearing time of the nestlings. To trigger mobbing reactions
in the breeding pairs, a human-habituated eagle owl was used as decoy
predator. The eagle owl was set on a perch at forest edges in the
vicinity of the focal red kite nests. To capture mobbing red kites, a
Dho-gaza net (height: 4 m; length: 6 m; 60 mm mesh) was set up next to
the eagle owl, just out of its reach to avoid entanglement
(Bloom
et al. 2007). The Dho-gaza net was mounted perpendicular to the treeline
and to the wind direction in order to guide the course of the swooping.
We selected breeding pairs with nestlings older than 6 days to avoid
disturbance during the very early nestling period. Exposure trials were
carried out between 6 am and 9 pm. The trial generally ended 90 minutes
after the placement of the decoy predator. We managed to conduct a
maximum of five trials per day at different nests. Other bird species
also reacted to the eagle owl and had to be removed from the net.
Because this could have negative effects on the mobbing behaviour of red
kites, predator exposure trials were categorized into trials disturbed
by bycatch and undisturbed trials (resulting in a binary variable
denoted as “disturbance”). Moreover, mobbing behaviour might differ
with vicinity to the forest edge due to limited manoeuvrability. This
was recorded in a binary variable denoted as “proximity to trees”
(decoy predator closer than 5 meters to the forest edge).
In a subset of trials, we recorded the number of swoops and calling
intensity (binary: few callings = 0; frequent calling = 1). Capture
success was highly associated with the number of swoops (Estimate: 0.32;
95 % CI: 0.19 – 0.44), and with calling intensity (Estimate: 1.75; 95
% CI: 1.14 - 2.37). As variation in mobbing intensity is expected to
result in different time-to-capture, we recorded the time-to-capture if
capture occurred. Therefore, we used capture success as a measure of
mobbing intensity. To account for differences between pairs in the
time-to-detection of the decoy predator, we split the trials into two
periods: detection time (the time from predator placement until the
first reaction of one of the members of the breeding pair) and mobbing
time (the time from detection until capture). It was not possible to
record data blindly because our study involved focus animals in the
field and included capturing of focus individuals. No individual
identification of the parents was possible; thus, mobbing behaviour
reflects the behaviour of breeding pairs.