Odour cues influence predation risk at artificial bat roosts in urban bushland:
Electronic Supplementary Material
Caragh Threlfall, Bradley Law and Peter B. Banks
Supporting Methods
Study Area
This study was conducted in Sydney Harbour National Park, Sydney, Australia. The study area
supports populations of potential predators, including the introduced black rat Rattus rattus and native
common brushtail possum Trichosurus vulpecular, which may use bat guano as an olfactory cue for
prey, and have been observed to use nesting cues in previous studies [1]. The local bat assemblage
consists of some species that roost in tree hollows and some that roost in caves, including Gould’s
wattled bat Chalinolobus gouldii (predominantly hollow roosting), and eastern bentwing bat
Miniopterus schreibersii oceanensis (cave roosting) [National Parks and Wildlife Service Wildlife Atlas
data, accessed 2010; 2].
Roost Cue
Scats were collected from individual bat species that have been found within Sydney Harbour
National Park. Scats were collected from harp traps directly, or from holding individuals in cloth bags
usually overnight. Clean gloves were used when handling bat guano. In each situation, bat numbers
were estimated to calculate an approximate amount of scat produced/individual. All scats were frozen
(-20°C) in airtight containers until use. Scats were subsequently mixed and sub-sampled for use in the
experiment. Species from which guano was collected included Gould’s wattled bat Chalinolobus
gouldii, large-footed myotis Myotis macropus, eastern bentwing bat Miniopterus schreibersii
oceanensis, Gould’s long-eared bat Nyctophilus gouldi, eastern horseshoe bat Rhinolophus
megaphyllus, little forest bat Vespadelus vulturnus, and large forest bat V. darlingtoni. Multi-species
roosts occur naturally, so mixing guano in this way mimicked natural conditions, although it likely that
different predator responses may be found for cavity and cave roosting species.
Guano collected was weighed and divided by the number of individuals from which it was collected.
On average (n = 8), ~0.1 g of scats were produced over one day by an individual bat across all
species, and as such 0.1 g was used to represent a solitary bat. In order to simulate a group roost of
10 roosting bats, 1 g of cue was used. Control roosts were identical to other treatments, where water
was used instead of odour. Artificial roosts were made out of enclosed plastic containers, where an
entry hole of approximately 6 x 6 cm was cut into the side. The containers were painted dark brown to
reduce their visual conspicuousness. Roosts were placed > 50 m apart and attached to a tree (>10
cm DBH) via cable ties approximately 1.5 - 2 m above the ground. Australian bats roost at varying
heights from the ground [3, 4], and this height was chosen to reasonably mimic natural conditions and
allow for ease of monitoring. Roosts were placed in areas considered likely to support roosting bats,
and were distributed within an area of coastal woodland where all roosts were in areas considered to
be equivalent habitat.
Two methods were used concurrently to detect roost visitors. Non-sulfurous plasticine (Rainbow
modelling clay, Newbound Pty Ltd) was placed inside the entrance to the roost to record indirect signs
of visitation, such as footprints, claw, beak and teeth marks. Roosts were also monitored directly
using motion censored infrared video cameras (Scoutguard 550v set to maximum sensitivity), to
record predator identity, behaviour and time of visit. Each roost was inspected each day, and the
plasticine smoothed over if it had been disturbed. A visit was defined by any investigative behaviour to
the artificial roost, recorded via either monitoring method. A visit to a roost was classified as
independent from the camera data, if occurring greater than three minutes from a previous visit by the
same species. Visitation may also be influenced by local hollow availability, and roost height, which
was not measured in this study, however may provide additional insights into the intensity of roost
predation.
We used 90 artificial roosts were used to simulate natural roosts (n=30 solitary, 30 group and 30
control roosts). Odours were deployed to simulate roosts of differing group size (solitary or group),
and longevity (single use, or re-used on consecutive days). The odour cue (0.1 g, 1 g or water =
control) was placed inside the roost entrance. The level of visitation by other species to these roost
types was compared to control roosts. One replicate from the ‘group’ treatment was removed from the
design due to site inaccessibility. In addition to the odour cue placed inside the roost, an additional
dilute cue was sprayed on the surface of the roost entrance to facilitate olfactory detection by potential
predators. Odour used on the entrance to the roost consisted of 0.05 g.L-1 for solitary roosts, and 0.5
g.L-1 for group roosts. Half of all treatments (15 replicates) were re-applied daily for five days, to
represent the re-use of the roost (‘re-used’ treatment). In the other half, the faeces were only applied
on the first day to mimic switching (‘switched’ treatment). Renewed odour included both the scats
inside the roost, and the diluted odour on the entrance of the roost. Water was resprayed as a control.
The roosts were placed in each area seven nights prior to the start of the experiment to habituate
animals to the experimental equipment. Odour (solitary, group or control) was placed in and on the
roost on day eight, and roost visitation was monitored the following five days.
Analyses were carried out using JMP (SAS Institute, version 7.0), following [5] and [6]. All data have
been deposited in Dryad (doi:10.5061/dryad.r01j0).
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