Abstract

Global Positioning System (GPS) tags are nowadays widely used in wildlife tracking. This geolocation technique can suffer from fix loss biases due to poor satellite GPS geometry, that result in tracking data gaps leading to wrong research conclusions. In addition, new solar-powered GPS tags deployed on birds can suffer from a new “battery drain bias” currently ignored in movement ecology analyses. We use a GPS tracking dataset of bearded vultures (Gypaetus barbatus), tracked for several years with solar GPS tags, to evaluate the causes and triggers of fix and data retrieval loss biases. We compare two models of solar GPS tags using different data retrieval systems (Argos vs GSM-GPRS), and programmed with different duty cycles. Neither of the models was able to accomplish the duty cycle programed initially. Fix and data retrieval loss rates were always greater than expected, and showed non-random gaps in GPS locations. Number of fixes per month of tracking was a bad criterion to identify tags with smaller biases. Fix-loss rates were four times higher due to battery drain than due to poor GPS satellite geometry. Both tag models were biased due to the uneven solar energy available for the recharge of the tag throughout the annual cycle, resulting in greater fix-loss rates in winter compared to summer. In addition, we suggest that the bias found along the diurnal cycle is linked to a complex three-factor interaction of bird flight behavior, topography and fix interval. More fixes were lost when vultures were perching compared to flying, in rugged versus flat topography. But long fix-intervals caused greater loss of fixes in dynamic (flying) versus static situations (perching). To conclude, we emphasize the importance of evaluating fix-loss bias in current tracking projects, and deploying GPS tags that allow remote duty cycle updates so that the most appropriate fix and data retrieval intervals can be selected.

Highlights

  • Many wildlife movement studies employ Global Positioning System (GPS) location data [1]

  • The 13 bearded vultures tracked in a 7 year period from 2007 to 2014 provided a total of 83,231 GPS fixes (48.1% were recorded by CTTs and 51.9% by Platforms Transmitters Terminals (PTTs) tags)

  • Our results demonstrate that none of the tag models was able to accomplish the duty cycle programed initially

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Summary

Introduction

Many wildlife movement studies employ Global Positioning System (GPS) location data [1]. Both basic ecology and conservation studies use location fixes obtained from animal with tags carrying GPS receivers. There are studies on habitat-selection [2,3], animal behavior [4,5] or human-wildlife conflict [6,7]. Obtaining a GPS fix, and how reliable. Biases in solar GPS-tags contract from the Ministerio de Educacion, Cultura y Deporte (FPU13-03484). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

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