Southeastern Naturalist
R.M. Elsey and P.L. Trosclair III
2016 Vol. 15, No. 1
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2016 SOUTHEASTERN NATURALIST 15(1):76–82
The Use of an Unmanned Aerial Vehicle to Locate Alligator
Nests
Ruth M. Elsey1,* and Phillip L. Trosclair III1
Abstract - Coastal marshes of Louisiana provide nesting habitat for Alligator mississippiensis
(American Alligator). Helicopters are typically used to locate Alligator nests in remote
interior marshes. We tested the use of an unmanned aerial vehicle (UAV) to detect Alligator
nests on Rockefeller Wildlife Refuge in Grand Chenier, LA. Three brief flights with a combined
search time of approximately 25 minutes and 9 seconds were conducted in a single
afternoon, covering 28.2 ha. While in the field, we observed 6 Alligator nests with the UAV,
and later review of video imagery recorded allowed us to detect an additional 6 Alligator
nests. The use of UAVs may be a useful tool for detecting Alligator nests.
Introduction
The use of unmanned aerial vehicles (hereafter UAVs, or drones) has been examined
as a practical tool for wildlife management and research in recent years. As early
as 2002, preliminary work to examine potential applications of an UAV to assess
Alligator mississippiensis Daudin (American Alligator, hereafter Alligator) populations
was undertaken (H.F. Percival, Florida Cooperative Fish and Wildlife Research
Unit, USGS-BRD, and University of Florida, Gainesville, FL, pers. comm.). A more
detailed assessment of small UAVs for wildlife research captured imagery of numerous
wildlife species, including the Alligator, as well as an Alligator nest (Jones et
al. 2006). Other studies using drone technology documented the size of an Alligator
without requiring actual capture and handling (Martin et al. 2012, Watts et al. 2010).
A recent publication reviewed the evolution and development of the technology of
this wildlife tool, and the benefits and drawbacks of the use of drones in wildlife work
(Martin 2014); others have summarized some 15 years of applications of drones in
wildlife research and management (Burgess et al. 2015, Carthy et al. 2014). Drones
have been used as a tool for behavioral research, including a case study that successfully
identified 2 Crocodylus porosus Schneider (Estuarine Crocodile) nests in 5
flight hours in the Malaysian state of Sabah (Evans et al. 2015).
The state of Louisiana has a large commercial Alligator-farming program,
which includes legal collection of wild Alligator eggs (egg “ranching”) under
permit from nests on privately owned wetlands (Elsey and Kinler 2012). Alligator
nests are typically located by helicopter, and less often by ultra-light aircraft.
Each summer, biologists in Louisiana’s Alligator-management program receive
requests from licensed Alligator farmers for the estimated peak of Alligator nesting,
to aid in scheduling helicopter flights. It is advantageous for commercial
1Louisiana Department of Wildlife and Fisheries, Rockefeller Wildlife Refuge, 5476 Grand
Chenier Highway, Grand Chenier, LA 70643. *Corresponding author - relsey@wlf.la.gov.
Manuscript Editor: Michael Cove
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egg collectors to work economically and efficiently, by initiating flights to locate
nests only after the majority of each season’s Alligator nests have been constructed
and egg deposition has occurred. This avoids conducting nest flights too early,
and thus requiring a second flight over the same wetlands to find later-constructed
nests. Helicopter rental can be expensive, at approximately $300–$500/hour or
more in Louisiana, and possibly higher in other regions. The inherent danger
of using airplanes and helicopters for wildlife work has been well documented
(Sasse 2003). We initiated this study to determine if drones could be used to detect
Alligator nests in coastal marshes.
Methods
We performed the study on the State of Louisiana’s Rockefeller Wildlife Refuge
in Grand Chenier, LA, on the afternoon of 23 June 2015. The refuge boundaries
and predominant vegetation have been described (Selman and Baccigalopi 2012),
and this study was conducted in the south Superior System of the refuge, which is
known to support dense Alligator nesting (Joanen and McNease 1989). We tested
a drone obtained from DronesMadeEasy (DJI Phantom 2 Deluxe Mapping Bundle,
GoPro Hero4 Black +1.2W Vtx) and used a LCD screen (FlySight 32 Ch Black
Pearl 5.8 Ghz 7” Diversity Monitor) for immediate viewing.
We selected a site on the refuge that had not been surveyed by helicopter for
Alligator nests but that we knew from past experience was excellent Alligatornesting
habitat. We launched the drone from an airboat; after a brief period of
hovering and the drone was stabilized, we began searching (using manual controls)
for Alligator nests using low-level flights within approximate grids, but
without set established transect lines nor pre-planned flight paths. Winds were
light (estimated at 5–10 knots) from the southeast. This project was not an attempt
to get a total nest count/Alligator nest density; it was simply a pilot study to determine
if we could visualize Alligator nests using the drone. We quickly ascertained
a height of about 8–10 m allowed us a good overview of the marsh; viewing
approximately 50 m to each side. Some glare on the LCD screen made field observations
imperfect, but we could make some immediate observations.
Results
On the initial flight, we observed 1 Alligator nest and, after landing the drone,
we visited the nest site via airboat and confirmed a nest containing Alligator eggs
approximately 10–12 days old. As we approached the nest, we observed the female
Alligator, which proceeded to actively defend the nest while we opened it to confirm
the presence of eggs.
We then travelled ~1045 m southeast for the second test flight. On the second
trial flight, we observed some areas of dead vegetation making detection of Alligator
nests more challenging, but we did observe what we thought might be an
Alligator nest. After visiting the site, we discovered that the Alligator nest was
partially constructed but was now inactive and presumed abandoned. While
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travelling to check the inactive nest, we observed another Alligator nest we had not
seen with the drone in the field (but later noted it was visible on video recorded);
this nest contained eggs that were 7–8 days old. The female Alligator was present,
but not aggressive. We then travelled via airboat some 230 m west for the next test
flight. The third flight led to field observations of 4 Alligator nests. Upon verification
in the field, we noted that 2 of these contained poorly calcified oviductal
contents, 1 nest contained eggs that were estimated to be 8–10 days old, and the
final nest contained similarly aged eggs. Since Alligator nests are built gradually
over a 2-week period (Joanen 1969), fully completed Alligator nests are larger (and
thus would likely be more readily visualized) than nests still under construction.
Although females defend their nests infrequently (9.2%; Joanen 1969), it is possible
that observing movement or presence of an attending female might also cue
observers to a nest that otherwise might be overlooked.
Upon return to the office, we reviewed video footage obtained with the drone
and detected several additional Alligator nests (Table 1). Alligator nests were
Table 1. Results of drone testing to detect American Alligator nests. Times noted are in minutes and
seconds.
Total Nest Battery Nests Additional nests Total Approximate
video search remaining seen observed after nests hectares
Flight duration time after flight in field office review detected searched
1 10:38 8:11 28% 1 2 3 11.5
2 12:14 10:41 26% 1 3 4 11.5
3 8:43 6:17 33% 4 1 5 5.2
Total 25:09 6 6 12 28.2
Figure 1. Aerial view of an Alligator nest from video imagery captured during a drone flight
within Rockefeller Wildlife Refuge in Grand Chenier, LA, 23 June 2015.
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easily visible at a distance (Fig. 1), and became clearer as the drone was overhead
(Fig. 2). In calculating the “search times” for a flight, we defined it as ending when
we clearly began a return path towards the airboat in preparation for landing. The
first flight began at 13:36 h and we completed the final flight at 14:34 h; active
nest-search time was only 25 minutes and 9 seconds, with the remaining time being
to “ground-truth” the field observations, changing batteries between flights, photo
documentation of nests, and airboat travel between nest sites. We might have been
able to conduct longer flights, but were conservative and returned the drone to the
airboat with ample battery life remaining (Table 1). Results of quantity of nests
observed in the field, additional nests later noted upon video review, and approximate
hectares covered in each flight are summarized in Table 1. Incidentally, while
using the drone to survey a heron rookery on the refuge in early June, an additional
Alligator nest was observed. Also, 2 more Alligator nests were seen during a brief
flight conducted near the refuge headquarters a few days after the pilot study flights
described above.
Discussion
Our results suggest drones may be useful tools for detecting Alligator nests in
coastal marshes. Nests were relatively easy to see while in the field despite glare on
the screen and our limited experience with drones. Additional nests were detected
after review in an office setting of the recorded video footage. Screen anti-glare
devices would likely improve field observations. One of us (P.L. Trosclair) had
moderate experience using the drone in the field (15–20 prior flights of short duration),
whereas other personnel (R.M. Elsey and a co-worker) could see some of the
nests in the field despite no prior experience with the drone. We acknowledge we
Figure 2. Closer view of the Alligator nest seen in Figure 1; note shadow of drone beside
the nest.
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did select an area where we anticipated high nest-density from past experience.
Excellent habitat conditions and water levels in summer 2015 contributed to high
levels of nesting efforts in the region. Also, our extensive prior field experience
with Alligator-nesting research enabled us to diligently search habitat areas with
landscape features (e.g., remote marsh with small ponds for dens) where we anticipated
Alligator nests might occur. Our small sample size in this preliminary study
precluded performing estimates of detection probability, which might be an area for
future research.
Short flights such as those described herein may provide information to biologists
and commercial Alligator egg ranchers as to when they should schedule
subsequent helicopter flights to most efficiently fly once the majority of that year’s
nests appear to be under construction. They can also utilize this information to
coordinate timing of egg collections once all eggs have been deposited. Alligators
can take up to 20 days to complete their nests (Deitz and Hines 1980). Drones
could allow investigators or commercial egg ranchers to estimate the status of nest
construction via close low-level observation of the nest size and structure, so as to
not schedule collections too early. Drones might allow observers to visualize the
high cone (Deitz and Hines 1980) of a completed nest, or even a tail drag left in
the mounded vegetation after the female leaves the nest. We do not suggest drones
would be used for total nest counts, but rather as a tool to guide timing of scheduling
helicopter flights that cover hundreds or thousands of hecta res.
Drones may help readily identify nests where a female was seen in nest attendance
or still on the nest while the nest was under construction; such information
could aid researchers attempting to capture nesting female Alligators for specific
reproductive ecology studies (such as Davis et al. 2001). Additionally, tropical
storm surges and heavy rainfall can lead to nest losses due to flooding, and drones
might help landowners rapidly identify areas where rising water levels might be
a concern in order to guide egg collections sooner in areas at risk of flooding.
Drones might also be useful in some non-coastal habitats where canopy cover reduces
the efficacy of using helicopter flights to detect Alligator nests. The smaller
size and agility of drones might make it possible to more effectively search wooded
areas. However, challenges might occur for the controller as the drone travels
a distance away in heavily forested areas due to interference from trees. Drones
might also allow detection of additional nests missed on some properties, due
to the advantage of reviewing video footage obtained. Indeed, even experienced
observers detected only ~77% of Alligator nests on helicopter surveys in Florida
(Rice 1992). However, the use of drones would likely be limited to days with
clear weather and calm winds.
There is the potential for nest abandonment if female Alligators are disturbed
by checking the nest prior to oviposition (Wilkinson 1983). Our limited data
suggested the drone caused little disturbance to the female, as we observed 2
females at nest sites when we “ground-truthed” the nests soon after the drone
over-flight. Helicopters can be loud with a decibel level of 100 dB (Purdue University
2015), whereas small, non-military drones are much quieter; however,
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in either case the actual time spent hovering at any nest site when searching is
only a few seconds.
Drone technology is rapidly advancing, and more sophisticated models with
geo-referencing and extended range of flights are available. The model we tested
was relatively inexpensive ($2799 when obtained in November 2014), and numerous
field applications would soon make this cost efficient, considering the costs for
helicopter rental to locate Alligator nests as noted above. At an estimated cost of
$500/hour for helicopter rental, the cost of less than 6 hours of flight time would
pay for a drone similar to the one tested in this study. Furthermore, aviation accidents
are also a concern when conducting Alligator nest work, and the potential
costs (both in dollars and human lives) and impacts of such mishaps can be greatly
reduced with use of drones for some surveys.
Future work in this area might help determine if there is an optimum time of day
for using drones to detect Alligator nests or perhaps basking Alligators as a rough
population index. In addition to Alligator work, we have found drones to be useful
to select potential sites for construction and delineation of a Grus americana
L. (Whooping Crane) holding pen, detect nesting Ardea alba L. (Great Egrets)
and Egretta caerulea L. (Little Blue Herons), evaluate status of possible flooded
Whooping Crane nest/eggs, and review progress of wetlands-mitig ation projects.
Acknowledgments
We thank Dr. Franklin Percival for assistance with helpful references prov ided.
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