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2012 SOUTHEASTERN NATURALIST 11(3):447–454
Responsiveness of Mangrove Cuckoo (Coccyzus minor)
During Call-playback Surveys in Southern Florida
Rachel D. Frieze1,*, Stephen M. Mullin2, and John D. Lloyd3
Abstract - Point-count surveys are useful in collecting information on breeding birds;
however, species that are elusive, occupy dense forests, or call infrequently may be undersampled.
In this study, we examined the responsiveness of Coccyzus minor (Mangrove
Cuckoo) to call playbacks in southern Florida from May to June in both 2010 and 2011.
Our objective was to determine if playback surveys would increase the detectability of
Mangrove Cuckoos. At each of the 111 experimental points, either recorded Mangrove
Cuckoo vocalizations (treatment) or no vocalizations (control) were broadcasted. We
detected Mangrove Cuckoos at 14 of the 67 treatment points (20.9%) and at 1 of the 44
control points (2.3%), suggesting that using call-playbacks significantly increased the
likelihood of detecting a Mangrove Cuckoo (P = 0.01) as compared to passive pointcount
methods. In recent years, sharp declines in Mangrove Cuckoo populations have
been noted, and little is known about their overall ecology. Before conservation for this
species can take place, it is necessary to fill basic information gaps such as distribution
and abundance. Once distribution is better understood, critical habitat can be protected
and monitoring the effects of climate and habitat change can occur. Increasing the detectability
of these birds is an important step toward achieving these goals.
Researchers frequently use point-count surveys to collect population information
on breeding birds (Farnsworth et al. 2002, Kubel and Yahner 2007). While
useful for many applications, methods that rely on observation and/or passive
listening as means to detect individuals may perform poorly for birds that are
elusive, occupy dense forests, or call infrequently (Allen et al. 2004, Gibbs and
Melvin 1993). Broadcasting vocalizations of a species to increase its probability
of detection by eliciting a response is a survey method referred to as “call playback”
(Johnson et al. 1981). Previous studies have used call playbacks to monitor
secretive waterbirds such as Podilymbus podiceps L. (Pied-billed Grebe), Botaurus
lentiginosus Rackett (American Bittern), Rallus limicola Vieillot (Virginia
Rail), and Porzana carolina L. (Sora) (Allen et al. 2004, Bogner and Baldassarre
2002, Gibbs and Melvin 1993). Additionally, playback surveys were used to study
western populations of the Coccyzus americanus L. (Yellow-billed Cuckoo) in
Nevada, Arizona and California, as they are secretive and occupy dense forest
habitats (Halterman 2009). Variation in responsiveness may depend on several
factors such as time of day, weather, season, breeding stage, sex, or type of call
used (Legare et al. 1998), and the effectiveness of call playback should be tested
for each target species.
1211 East Lowell Avenue, Tracy, CA 95376. 2Ecostudies Institute, 167041 SW 298th Terrace
Street, Homestead, fl33030. 3Ecostudies Institute, PO Box 106 South Strafford, VT
05070. Corresponding author - firstname.lastname@example.org.
448 Southeastern Naturalist Vol. 11, No. 3
Coccyzus minor Gmelin (Mangrove Cuckoo) is one of the most poorly understood
birds in North America (Hughes 1997). Rare to uncommon in Florida,
their secretive and quiet nature, and the inaccessibility of the coastal mangrove
forests that they inhabit, make them extremely difficult to study. As such, they
are virtually unstudied in their North American range, and little is known of
their ecology or their population status. However, sharp declines in population
size have been noted in some parts of Florida (Lloyd and Doyle 2011), lending
immediacy to calls for additional research on this species. One of the most
pressing needs is for the development of survey methods that will allow for unbiased
estimates of distribution and abundance. Given the apparent difficulty
of detecting this species, call playbacks may prove useful, but their efficacy
with this species is untested. In this study, we examined the responsiveness of
Mangrove Cuckoos to call playbacks in southern Florida. Our objective was to
determine if playback surveys would increase the detectability of Mangrove
We conducted surveys on public land within the coastal mangrove forests
of southern Florida including Biscayne National Park, Everglades National
Park, The Florida Keys, and 10,000 Islands National Wildlife Refuge (Fig. 1).
The mangroves in which we conducted surveys ranged in structure from shrublands
to open woodlands to tall, closed-canopy forests, and were dominated
by varying mixtures of four different tree species: Rhizophora mangle L. (Red
Mangrove), Avicennia germinans (L.) L. (Black Mangrove), Laguncularia
racemosa (L.) C.F. Gaertn. (White Mangrove), and Conocarpus erectus L.
(Buttonwood). Small pockets of tropical hardwood hammock were present at
The points that we surveyed in this study were established in 2008 or 2009
as part of an ongoing separate study collecting abundance and distributional
data of mangrove landbirds throughout Florida’s mangroves using point-count
methodology (J.D. Lloyd, unpubl. data). Point counts for this study were completed
prior to the playback experiment. The location of points was determined
using a general randomized tessellation stratified (GRTS) survey design (Stevens
and Olson 2004). Survey locations were selected at random from all areas
supporting mangrove vegetation in Florida, with the constraint that points
chosen for surveys were on public land and were accessible by boat or foot. In
2010, we randomly selected 63 of these points to include in this study. Points
were randomly assigned to either the treatment group (n = 42) or the control
group (n = 21). At each point, whether control or treatment, we began by conducting
a passive, 10-minute point count that was part of a separate study. For
2012 R.D. Frieze, S.M. Mullin, and J.D. Lloyd 449
Figure 1. Overview of the study area, including specific locations of experimental points
(black circles), where surveys for Mangrove Cuckoos were conducted in conjunction
with broadcast recordings of Mangrove Cuckoo vocalizations, and control points (white
circles), where only passive surveys for Mangrove Cuckoos were conducted.
450 Southeastern Naturalist Vol. 11, No. 3
points assigned to the treatment group, we broadcasted using a small handheld
speaker and an Mp3 player a recording of the typical vocalization (the guttural
series of repeated “cah” notes) of this species that is heard during the breeding
season. The equipment was tested before experiments took place to insure
that the broadcast vocalizations could be heard from a distance of 100 m. The
function of this vocalization is unknown, but that it is limited to the breeding
season argues for a role in territorial defense and advertisement. The recording
lasted for 8 seconds, at which point we then listened quietly for 52 seconds.
This process was repeated 4 more times, for a total survey period of 5 minutes.
During the survey period, we sat quietly in one place and noted the presence of
any Mangrove Cuckoo heard or seen. Distance was estimated for each visual
or vocal response as well as the time to response after the recording was broadcasted.
At control points, the initial 10-minute point count was followed by 5
minutes of quietly listening and looking for Mangrove Cuckoos. No playbacks
were used at control points.
In 2011, we drew points from the pre-existing set where ≥1 Mangrove Cuckoo
had been detected during the 2008–2011 point-count surveys of mangrove landbirds
(J.D. Lloyd, unpubl. data). The change in methodology from 2010 to 2011
was implemented because a preliminary power analysis indicated that a large
sample size (n > 500) would be required to compare control and treatment sites
given the number of points at which no cuckoos were detected. We suspected that
a low response rate was partly due to the fact that many of the points we visited
were not inhabited by Mangrove Cuckoos, either because habitat was unsuitable
or because the regional population is small enough that not all suitable locations
are inhabited. Increasing the power of the experiment required eliminating, to
the extent possible, points at which cuckoos have never been detected. We do not
understand Mangrove Cuckoo habitat requirements sufficiently to predict where
they would occur based on habitat, so as an alternative we used the presence of
cuckoos in previous years as a surrogate that allowed us to predict where they
might occur in 2011. Of the points at which ≥1 Mangrove Cuckoo had been detected,
we randomly selected 47 points to include in this study. As in 2010, we
randomly allocated points to the treatment group (n = 25) or the control group
(n = 22). Points surveyed in 2010 were not surveyed again in 2011. Thus, our total
sample size was n = 110.
In both years, we conducted all surveys between 0600 and 1000 during May
and June, putatively the peak breeding season for Mangrove Cuckoos in Florida
(Hughes 1997). All survey locations were accessed by foot or boat.
We compared the frequency of responses between treatment and control
points using a generalized linear model. We assumed a binomial distribution
of errors and used the logit link function. In addition to the main predictor
of interest—a dummy variable indicating whether a playback was used
at the point—we also included several covariates that we thought might
2012 R.D. Frieze, S.M. Mullin, and J.D. Lloyd 451
independently influence the probability of detecting a Mangrove Cuckoo.
First, we included the date on which each point was visited, because we suspected
that the propensity of Mangrove Cuckoos to vocalize spontaneously or
to respond to broadcasts of recorded vocalizations might vary seasonally as a
function of what, if any, breeding activities were underway. Second, because
the sample of points included in this experiment was drawn without knowledge
of whether Mangrove Cuckoos were present, we also included a dummy variable
that indicated whether any Mangrove Cuckoos had been detected that year
at the point during surveys conducted as part of the separate study of mangrove
landbirds. We assumed that a positive response to either the control or treatment
was more likely at points at which individuals had been detected during
the current year. Similarly, we also assumed that the presence of Mangrove
Cuckoos in previous years was also likely a positive indicator of garnering a
response during the experiment, and so we also included a dummy variable
indicating whether any Mangrove Cuckoos had been detected at the point in
surveys conducted during previous years (because of how we chose points in
2011, all points surveyed in that year had the same value for the dummy variable).
All analyses were conducted using R, version 2.12.2.
We conducted 42 playback and 21 control experiments in 2010 and 25 playback
and 22 control experiments in 2011. We surveyed control points between
9 May and 23 June in 2010 and 7 May and 15 June in 2011. We surveyed treatment
and control points between 9 May and 26 June in 2010 and 7 May and 22
June in 2011. The median date on which control points were surveyed was 27
May and 16 May in 2010 and 2011, respectively. The median date on which
experimental points were surveyed was 9 June and 23 May in 2010 and 2011,
respectively. We detected Mangrove Cuckoos at 14 of the 67 treatment points
(20.9%) and at 1 of the 44 control points (2.3%). This difference was highly
significant, indicating that using a playback (P = 0.01) increased the likelihood
of detecting a Mangrove Cuckoo. We never recorded a response by >1
Table 1. Results of a generalized linear model1 (assuming binomial errors and using a logit link
function) examining whether Mangrove Cuckoos were detected at a higher rate when vocalizations
of a conspecific were broadcast during surveys (entered as a dummy variable = 1 when playbacks
used). Three potentially confounding covariates (survey date, number of detections at the point during
passive surveys earlier in the year, and number of detections at the point during passive surveys
in preceding years) were included in the model as well.
Term Estimate SE Z-value P
Playback used? 2.76 1.09 2.51 0.01
Survey date -0.01 0.02 -0.32 0.75
Current-year detections 0.68 1.03 0.66 0.51
Previous-year detections 1.27 0.65 1.95 0.05
1Residual deviance = 69.2; residual degrees of freedom = 104.
452 Southeastern Naturalist Vol. 11, No. 3
bird at any point; males and females are not distinguishable by sight, so we do
not know the sex of birds responding to the playback. The average time of response—
the length of time before a response was observed—was 131 s (range
= 20–267 s), and all of the responding birds were either seen or heard. All of
the birds detected during the experiment were first detected within 50 m of the
observer. Of the other predictors, only the presence of Mangrove Cuckoos at
the point in past years was significantly related to the probability of detecting
a Mangrove Cuckoo (P = 0.05). Detections of Mangrove Cuckoos during other
surveys conducted in the same year was unrelated to the probability of detecting
a cuckoo during the experiment (P = 0.5), as was the date on which the
experiment was conducted (P = 0.74) (Table 1).
Employing the playback survey method increased the rate of detection of
Mangrove Cuckoos. All of the responses in this experiment included a vocal
component, although in some cases, the responding individual was also visible
to the observer. These findings may have important implications for methods
used to determine the presence of Mangrove Cuckoos. Mangrove Cuckoos are
secretive by nature, call infrequently, and inhabit dense forests that make visual
observations difficult and that may also reduce the transmission of sound,
thus decreasing the ability of observers to detect vocalizing birds. By soliciting
a vocal and visual response, call playbacks may allow researchers to gain
more reliable estimates of abundance and distribution than would be possible
using passive point-count surveys, especially given the low encounter rate
recorded at control points. Similar conclusions have been determined in past
studies of Yellow-billed Cuckoo (Halterman 2009) and breeding marsh birds
(Allen et. al. 2004).
Two caveats are warranted regarding our results. First, we may have underestimated
the effect size in this experiment if individuals vary in how they
respond to playback. All of the birds that we observed responding to playback
were vocalizing, but observers may fail to detect birds that approach the observer
without vocalizing because the habitat is incredibly dense. A study
of radio-marked Yellow-billed Cuckoos in southwestern Arizona found that
cuckoos that responded by silently approaching the observer were less likely
to be detected than birds that responded vocally (Halterman 2009). Second,
we cannot establish that the content of the playback—what we assume was a
territorial defense and advertisement vocalization—was important in driving
the response that we observed.
In addition to refining which parts of the recorded vocalization are key
to the response that we observed, more research needs to be done to establish
playback methodology. Factors such as time of day, breeding stage, and
frequency of surveys may influence detectability. Based on personal observations
made during our 2010–2011 field season, we suspect Mangrove Cuckoos
2012 R.D. Frieze, S.M. Mullin, and J.D. Lloyd 453
call more frequently just before sunrise and sunset. Breeding behaviors for
Mangrove Cuckoos in southern Florida have been observed to occur between
the months of April (courtship feeding) and July (nest building) (Hughes
1997). However, we have observed cuckoos vocalizing as early as March.
Therefore, playback surveys could potentially be effective as early as April,
thereby increasing the chances of detection. Lastly, whether the frequency of
playback surveys during the breeding stage—and perhaps beyond—influences
detectability should also be investigated. Halterman (2009) found that mated
Yellow-billed Cuckoos were more responsive to playbacks than were unmated
birds, suggesting the need to perform surveys later in the breeding season
when birds are paired and more likely to be responsive. If seasonal fluctuations
in responsiveness occur, multiple surveys may prove more useful in
estimating densities and monitoring population trends.
The mangrove forests of southern Florida support a unique avifauna that
includes several species, like Mangrove Cuckoo, that are found nowhere else
in North America. These forests are threatened by a variety of factors, including
rising sea level, invasion by exotic plants and animals, and habitat loss and
degradation as a result of urbanization, water channelization, and mosquito control
(US Fish and Wildlife Service 1999). Conserving populations of Mangrove
Cuckoos and the ecosystems they occupy in the face of these changes requires a
more complete understanding of their natural history, including basic elements
such as distribution and abundance. By improving our ability to detect Mangrove
Cuckoos, the methods that we have developed in this paper represent an important
step in filling these critical information gaps.
This work was funded in part by grants from the Florida Fish and Wildlife Conservation
Commission, the US Fish and Wildlife Service, and Everglades National Park. We
gratefully acknowledge the support and assistance provided by our agency collaborators,
especially Elsa Alvear, Sonny Bass, Terry Doyle, Joyce Palmer, and Karl Miller.
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