## 2 Materials and Methods
Due to the size and terrain of the paddock in Trial 1, animals were tracked by first identifying their last known geolocation transmitted by the Iridium satellites and accessed using the proprietary software, and then beginning tracking on-site using the handheld Yagi VHF antenna and receiver to locate the cows in the paddock (Sirtrack Ultra, VSR041A, Sirtrack-Lotek, Havelock North, NZ). Animals were tracked on foot or via helicopter (R-22, Robinson Helicopter Company, Torrance, CA, USA). Tracking cows in the paddock was done by viewing the animal GNSS fixes retrieved from the manufacturers' webpage or software, and then using a handheld Yagi VHF antenna and receiver to locate the cows in the paddock. Collars used GPS and Iridium satellite communication and recorded a fix every 30 min. All fixes were stored on board the collar, with every 6 th fix (approximately every 3 h) being sent to the satellite. The frequency of satellite uploads could be selected by the user and changed at any time. These were available to view and download from proprietary software and corresponding company websites. Both collars summarized accelerometer data on board the collar every 5 min for each of the axes with values between 0 (no activity) and 255 (highest activity). In addition, Brand 2 collars recorded raw 2-axis accelerometer data at 4 Hz. The X-axis measures acceleration in forward and backward motions, Y-axis measures sideways and rotary motion, and Z-axis measures up and down movements using gravitational acceleration. Activity was measured four times per second simultaneously on each axis as the difference in acceleration between two consecutive measurements. Measurements were averaged over the user-selected sampling interval (5 min) and presented on a relative scale between 0-255, in units known as 'counts'. These data were then stored with the associated date, time and temperature. Brand 2 VIT recorded temperature and accelerometer data every 5 min, only retrievable by the manufacturer. However, these data were not recorded by the VIT of Brand 1. Once a VIT of Brand 1 was switched on, there was a 1-week delay before it was fully operational to allow the calculation of an average temperature for each animal as a 'benchmark' or 'normal' value. The Brand 1 VIT would not capture births if inserted into cows within 1-week of being switched on. Both VIT's had VHF transmitters to enable recovery in the field. Brand 1 VIT had a wing opening of 57.17 mm, a length of 88.91 mm, a diameter of 16.34 mm and a weight of 26.3 g. Brand 2 VIT had a wing opening of 70 mm, a length of 70 mm, diameter of 20 mm and weighed 30 g (Figure 1). Upon expulsion of the VIT, the drop in temperature, coupled with inactivity, activated the VIT proximity beacon. This signal was received by the cow's collar, triggering a birth alert that was transferred via the Iridium satellite system and sent as an email and GSM message to the user's mobile phone. The thresholds and alert messages varied between the two birth alert systems (Table 1). The major difference was that the location coordinates were provided within Brand 1's birth alert message, but Brand 2 required access to the proprietary software to identify the location coordinates where the birth alert was activated. Brand 1's system triggered the birth alert when there were both 60 min of inactivity and a temperature change of \(\pm 3\)\({}^{\circ}\)C (Table 1). In addition, the Brand 1 birth alert was triggered if the VIT was inactive for 12 h regardless of vaginal temperature. Another major difference was that each Brand 1 collar could receive birth alert pulses from up to any 8 VIT beacons. This meant that once a VIT had been expelled (triggering birth alert pulses to be picked up by collars), any Brand 1 collar within UHF range (terrain dependent) could identify the birth alert and send a satellite notification of the birth. Therefore, birth alerts may be received multiple times for one VIT if several collars were nearby.