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).