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Glasby (2013) hedgehog GPS performance accuracy evaluation

Title: Evaluation of the performance and accuracy of Global Positioning system bug transmitters deployed on a small mammal, European Journal of Wildlife Research DOI 10.1007/s10344-013-0770-3. 2013

Authors: L. Glasby & R. W. Yarnell

Country: UK

Background to study

GPS bug transmitters can provide accurate, unbiased details of movement and resource use of focal animals with increased spatial accuracy, reduced survey effort and the ability to obtain data from remote or inaccessible habitats.  This study is an investigation into their effectiveness for monitoring movement patterns on hedgehogs.

Method

  • Static tests were conducted on the GPS bug transmitters (13g) to investigate fix success rate (% successful fixes against total number of attempted fixes) and accuracy in five different habitat types frequented by hedgehogs (adjacent to buildings, hedgerow, open pasture, reedmace beds and woodland). Two bug devices were placed per habitat type between October and November 2012 and set to record location fixes at 2-min intervals for a minimum of 4 hours.
  • Field tests were carried out to compare the performance of GPS bug transmitters with VHF radio transmitters at recording hedgehog movement:
    • 11 hedgehogs were fitted with VHF radio transmitters and tracked for three nights per week between April and October 2012. Fixes were obtained at least twice per night and locations recorded with a handheld GPS. 
    • GPS bugs were attached to the radio tagged individuals from June and set to record fixes at 20 min intervals between 2200 and 0400 hrs (June to mid-August) and between 2000 and 0600 hrs (mid August onwards). Bugs were attached to individuals for a total of five full nights after which they were removed and the location data downloaded.
    • Home range estimates were calculated from both VHF and GPS bug location data using minimum convex polygons and compared alongside a cost analysis in terms of generating home range estimates.

Key results

  • 3538 locations fixes were obtained from 4484 attempts during the static tests and both fix success rate and accuracy varied significantly between habitat types.
  • The accuracy and performance of GPS bug transmitters were best in open pasture and worst in woodland.
  • GPS bug transmitters fix success rate was 84.6% during the field tests, with an average of 96 successful location fixes recorded per attachment.
  • The mean number of failed fixes per hedgehogs was significantly higher during the hour after sunset and the hour before sunrise compared to the time period in between.
  • There was no significant difference in the home range estimates obtained from location data retrieved using the GPS bug transmitters collected over 5 days and the VHF radio transmitters which were attached for 6 months.
  • Total home range project costs for one hedgehog tagged with GPS bugs was £3872 compared to £2520 for VHF, however just 2 hours of labour were required for the GPS bug (£22) compared to 50 h for VHF (£550) based on obtaining >50 fixes from one animal. 

Key messages to landowners and managers derived from these results

  • GPS bug transmitters are an effective method for obtaining unbiased and representative data on home range and habitat use for hedgehogs with substantially reduced effort required compared with VHF radio transmitters.
  • GPS bug transmitters will be most effective for studies outside of closed habitat types, thus may not be useful for identifying winter hibernacula.

 

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