4.2.3 Assessing the differences in quantity and quality of data collected through tracking technology
To effectively describe the animal movement and behaviour critical to seed dispersal predictions, we need detailed tracking data. Large time gaps and short study durations can limit inference and may lead to over or underestimating dispersal distances. Both tag duration (length of time attached to an individual ) and total location points (number of locations collected per individual tag ) significantly related to the tracking method. Studies using GPS tags captured almost 18 times the number of locations compared to studies using radio tags and were deployed for almost 5 times longer. Increased battery capacity and solar powered tags enable data collection over many months and possibly years (Silva et al, 2017).
GPS tags are increasingly used to track frugivores; since 2015, 28 studies have used GPS tags compared to just 11 studies during the previous 36 years. This has also allowed for a larger diversity of species to be tracked. Before 2003, the largest frugivore equipped with a tracking device was 3kg. Since then, a further 21 species of frugivore have been tagged, with body mass ranging from 3 to 4750kg. This trend could be linked to the remote download capabilities of GPS tags. Larger animals tend to have larger home ranges (Harestad & Bunnel, 1979) and GPS tags can now be downloaded from many kilometres away or via remote upload to satellites or Wi-Fi (Kays et al, 2015). This is particularly important for migratory animals, where it is often impossible to stay close enough to use radio transmitters (Guilford et al, 2011). With the introduction of remote downloading, GPS tags are preferred because data download is guaranteed after the initial device attachment, without the need to recapture individuals or search for radio transmissions. Additionally, many GPS tags also include the option of onboard accelerometers (Shepard et al, 2008; Brown et al, 2013), which can be used for defining specific behaviours, such as foraging events, that can be incredibly useful when determining seed shadows.
On the other hand, many bird species are still tracked with radio transmitters. Radio transmitters tend to be smaller and can be used on smaller species, but this result may also relate to habitat. Most tropical birds reside within thick vegetation (MacArthur & MacArthur, 1961). GPS tags require low vegetation cover for successful fixes. In dense vegetation GPS fixes can fail or overestimate movement tracks for up to an additional 28% (DeCesare et al, 2005).