Camera set up
For the camera setup, we used a Raspberry Pi 4 microcomputer circuit
board with a 5-megapixel 1080P camera (25 mm x 24 mm x 17 mm) with a
130⁰ adjustable night vision fisheye lens (focus 2.5 cm) and a single 5
mm infrared LED light (Fig. 1). We removed the round part of the
infrared LED light using a belt sander and added a small
semi-transparent piece of sticky tape to diffuse the light throughout
the filming chamber (Fig. 1). A Rpi-RTC DS1307 clock module keeps
accurate time while not connected to the internet. The microcomputer was
placed in an IP65 waterproof electrical junction box (158 x 90 x 60 mm)
with four 1 g silica gel desiccant packets. Three holes were drilled in
the junction box: two on the long side for waterproof cable glands (fits
cables between 3–6.5 mm) and one on the bottom for the PVC male adaptor
(SKU 436-010; 3 cm). The power source cable (USB type C) and the USB
extension cable were inserted individually through one of the two cable
glands and together into a 1.5 m long stainless-steel shower hose to
prevent damage by rodents (which chew exposed rubber or plastics). On
the side of the stainless-steel hose, the cables were inserted
individually through cable glands in an IP67-rated waterproof PVC box
(38.8 x 28.9 x 24.3 cm). In this box, the USB power cable is connected
to a power converter (12 V to 5 V) connected to a 130 Ah LiFePO4 lithium
battery (10 kg). A 1 TB USB drive is connected to the USB extension
cable (Fig. 1).
Trials by (Mouy et al. , 2020) have found that USB storage used
more energy than SD card storage, therefore lowering battery life, and
USB storage was less reliable because of its more fragile connection.
However, O’Brien et al. (2023a, b) and Kallmyer et al.(2017) successfully used 64 GB and 32 GB USB storage in their study,
respectively. In this study, we chose a 1 TB USB storage because the USB
drive and battery are separated from the microcomputer; which has the
benefit that the USB drive and battery are replaced in the field after
21 days (often during rain and 90%> humidity), we do not
have to open the box with the microcomputer and camera to potential
damage by moisture. The total cost for the 21-day camera setup is
AUS$790.75 (Table 1).
The camera length can be easily adjusted for different habitats. For
example, when filming animals under leaves or artificial shelters (Fig.
4), we used the camera setup with the camera directly placed in the PVC
male adaptor (4 cm width) and the lens focusing at 2.5 cm. To ensure
that water could not enter the PVC male adaptor, a PVC coupling was
placed with a glass lens glued inside and a rubber O-ring was placed
between the connections. This way, if necessary, we could still change
the focus of the camera by removing the coupling. For filming under
natural or artificial wood shelters (‘logs’; see paragraph “artificial
shelters” below), we use a 20 cm long and 4.2 cm wide PVC pipe in the
converter, which goes over the male adaptor. The camera and the infrared
LED light were held in place at the end of the PVC pipe with a
3D-printed holder against the glass lens. Both the camera and battery
box were covered with square mesh (mesh size 1 x 1 cm) to protect the
plastics from chewing by rodents. To record the temperature and humidity
a datalogger (Hygrochron iButton) was placed in the chamber when the
camera was recording (Fig. 5).
The Raspberry Pi was coded to start recording when it powered on, and
for continuous video recordings of 15 minutes each. These recordings run
back-to-back but a 15-minute duration was chosen to keep the file size
of each recording manageable (O’Brien et al. , 2023a). The
‘continuous’ recording stretched for 21 days. Videos were saved to a 1
TB USB drive, creating folders of the month followed by folders of the
day. The videos themselves were labelled with time (hour.minute.second)
(Appendix 1; for detailed instructions on how to program the Raspberry
Pi see e.g. Hereward et al. , 2021; Youngblood, 2019).