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2009 SOUTHEASTERN NATURALIST 8(2):277–292
Effectiveness of Call-broadcast Surveys for Breeding
Marsh Birds Along Coastal Alabama
Eric C. Soehren1,*, James W. Tucker, Jr.2, and D. Garth Crow1
Abstract - Point-count surveys targeting Ixobrychus exilis (Least Bittern), Laterallus
jamaicensis (Black Rail), Rallus longirostris (Clapper Rail), Rallus elegans
(King Rail), Porphyrio martinica (Purple Gallinule), and Gallinula chloropus
(Common Moorhen) were performed using a standardized marsh-bird monitoring
protocol along tidally infl uenced emergent marshes of coastal Alabama during the
2004 breeding season. We compared the number of target species detected during
an initial passive-listening period to those detected during a subsequent multiplespecies
call-broadcast period to evaluate the effectiveness of the call-broadcast
period for increasing detections. Additionally, we examined the number of responses
of each target species to conspecific and heterospecific calls to determine how marsh
birds respond to multiple species call-broadcast sequences. Numbers of new individuals
detected during call-broadcast periods were significantly greater (P < 0.05) than
those detected during the initial passive-listening period for Clapper Rail, King Rail,
and Purple Gallinule, but not for Least Bittern and Common Moorhen. Black Rails
were not detected during this study. Conspecific calls were more effective at eliciting
responses than heterospecific calls for each of the target species except Least Bittern.
Although the calls of Clapper Rail and King Rail are very similar, our data indicate
that the advertisement calls of both species should be incorporated within multiple
species call-broadcast sequences where populations are sympatric. We also recommend
that Purple Gallinule call-broadcasts be applied to marsh-bird inventories or
monitoring programs throughout this species’ range as a result of its effectiveness,
which was previously uncertain.
North American marsh-bird species, particularly bitterns and rails, are
habitat specialists that depend primarily upon emergent wetlands. Because
of their dependence on marshes, documented losses of wetlands across
North America have raised concerns about the current welfare of this group
(Conway 2002, Eddleman et al. 1988, Ford et al. 1997, Ribic et al. 1999).
In addition, marsh birds are under-sampled by current long-term, landscapescale
monitoring programs such as the USGS Breeding Bird Survey (Conway
2002) because this group of birds is generally reclusive, inhabit dense
marshes, vocalize infrequently, and seldom fl y. These limitations have resulted
in a paucity of baseline population information for marsh-bird species
across the continent (Ribic et al. 1999). To address these data deficiencies,
a marsh-bird monitoring protocol was proposed for North America (Ribic
1Alabama Department of Conservation and Natural Resources, State Lands Division,
Natural Heritage Section, 64 North Union Street, Suite 464, Montgomery, AL 36104.
2Archbold Biological Station, Avian Ecology Laboratory, 475 Easy Street, Avon
Park, FL 33825. *Corresponding author - email@example.com.
278 Southeastern Naturalist Vol. 8, No. 2
et al. 1999) and later developed (Conway 2002) to adequately sample marsh
birds to determine population trends at local, regional, and continental scales
(Conway and Gibbs 2005).
Detecting marsh-bird presence in wetlands usually requires aural surveys
because of their reclusive habits (Conway and Gibbs 2005). Many marshbird
studies have used call-broadcast approaches under the assumption
that this method increases the number of detections over passive-listening.
However, little is known about the degree to which call-broadcasts increase
detections compared to passive-listening surveys for many marsh-bird species
(Conway and Gibbs 2005) and has been identified as a subject needing
further investigation (Ribic et al. 1999). Although numerous studies using
call-broadcasts have been performed on marsh birds in other regions
of North America (e.g., Allen et al. 2004, Bogner and Baldassarre 2002,
Conway et al. 2004, Erwin et al. 2002, Evens et al. 1991, Gibbs and Melvin
1993, Glahn 1974, Lor and Malecki 2002, Manci and Rusch 1988, Rehm and
Baldassarre 2007, Swift et al. 1988, Tomlinson and Todd 1973), very few
have been conducted in the Southeast (Holliman 1978, Legare et al. 1999,
Runde et al. 1990) or have yet been published, especially those focusing on
2 or more species (Cely et al. 1993).
Therefore, as part of a broader study along Alabama’s coastline,
we compared the number of individuals detected during an initial
passive-listening period to those detected during a subsequent multiplespecies
call-broadcast period following Conway (2002) protocols to
evaluate the effectiveness of the 2 survey techniques for 6 northern Gulf
Coast breeding marsh birds: Ixobrychus exilis (J.F. Gmelin) (Least Bittern),
Laterallus jamaicensis (Gmelin) (Black Rail), Rallus longirostris Boddaert
(Clapper Rail), Rallus elegans Audubon (King Rail), Porphyrio martinica
(L.) (Purple Gallinule), and Gallinula chloropus (L.) (Common Moorhen).
Furthermore, we examined the responses detected during the multiple-species
call-broadcast period to determine how each target species responded to
conspecific and heterospecific call-broadcasts (Conway and Gibbs 2005).
This study targeted the tidally infl uenced marshes occurring along
the shoreline of Baldwin and Mobile counties within the jurisdiction
of the Alabama Coastal Area Management Program (ACAMP; Fig. 1).
Alabama’s coastal emergent marshes are infl uenced primarily by a salinity
gradient that progressively transitions from polyhaline inundation (saline)
along the Gulf of Mexico proper to mesohaline and oligohaline inundation
(brackish) within portions of Mobile Bay inward to the lower reaches
of the Mobile-Tensaw River Delta, respectively (Tiner 1999). For the
purpose of this study, we classified coastal marsh-types as either “saline”
or “brackish” emergent marshes based on representative plant communities
described in Stout and Lelong (1981). Saline emergent marshes were
characterized primarily by dominant stands of Juncus roemerianus Scheele
2009 E.C. Soehren, J.W. Tucker, Jr., and D.G. Crow 279
(Black Needlerush), Spartina alternifl ora Loiseleur (Smooth Cordgrass),
or mixtures of both found in vast expanses or fringing along the mainland
coastline, islands, and inlets of Mobile Bay. These marshes are regularly
inundated by tidal fl ooding where salinities exceed 15 ppt (Holliman 1978).
Brackish emergent marshes were typically characterized by one or several
combinations of Typha domingensis Persoon (Southern Cattail), Sagittaria
lancifolia L. (Bull-tongue Arrowhead), Cladium mariscus ssp. jamaicense
(Crantz) Kükenth (Jamaica Swamp Sawgrass), Spartina cynosuroides (L.)
Roth (Big Cordgrass), Peltandra virginica (L.) Schott (Green Arrow Arum),
Phragmites australis (Cavanilles) Trinius ex Steudel (Common Reed), Alternanthera
philoxeriodes (Martius) Grisebach (Alligatorweed), or other
less frequently encountered dominants occurring in the lower portions of
the Mobile-Tensaw River Delta (hereafter referred to as the lower Delta) and
around Shelby Lakes in southern Baldwin County (Soehren 2005). These
estuary marshes occur where freshwater meets saltwater tides and salinities
fl uctuate between 5 to 15 ppt or higher depending on tidal stage, weather,
Figure 1. Marsh-bird survey point locations along coastal Alabama, April–July 2004.
Hollow dots represent points within saline emergent marsh, and solid dots represent
survey points within brackish emergent marsh. Regional sampling areas and landmarks
denoting sampling area boundaries are represented in boldface font.
280 Southeastern Naturalist Vol. 8, No. 2
and watershed drainage (Holliman 1978). Within both marsh-types, shrubby
stands of Baccharis halimifolia L. (Groundsel Bush) and Iva frutescens L.
(High-tide Bush) often occurred where elevations were slightly higher than
the surrounding marsh (Holliman 1978).
Survey route selection
Prior to initiating surveys, we identified emergent marsh habitat by
reviewing recent USGS black-and-white aerial imagery (1997 or 2001; TerraServer
®, Raleigh, NC) of the entire Alabama shoreline which includes the
outer coast, offshore islands, inlets, bays, rivers, and creeks combining for
977 km (607 mi) in total length (NOAA 1979). We then reconciled all identified marshes with described marsh-types mapped south of US Interstate
10 (Stout and Lelong 1981) to designate either saline or brackish emergent
marsh classifications. Following marsh designation, we manually marked
potential survey points over 1:24,000 digitized USGS topographical maps
using ArcGIS software (ESRI®, Redlands, CA) at 800-m increments along
the open water-emergent vegetation interface. We placed points at 800-m
intervals instead of the recommended 400-m intervals (Conway 2002) to
accommodate for the large size of the sampling area. We then linked survey
points into groups of 10 and assigned each group a route name and number
generating a total of 86 possible survey routes along coastal Alabama (Soehren
and Crow 2004). Logistic constraints made it infeasible to survey all 86
routes, so we selected a random subset of 34 routes (21 saline and 13 brackish
routes) consisting of 317 total sample points for surveys (Fig. 1). Lastly,
we divided the entire coastal Alabama study area into 3 primary sampling
regions to better coordinate survey scheduling. Sampling regions included:
1) lower Mobile County (Mississippi state line to Upper Dog River); 2)
lower Delta (Blakeley Island to Ducker Bay); and 3) lower Baldwin County
(Weeks Bay to Perdido Bay) (Fig. 1).
Our sampling methodology followed the North American marsh-bird
monitoring protocols (Conway 2002). We performed all surveys once by
boat along the water-emergent vegetation interface during the breeding season
from late April to late July when adults on territory are most likely to be
vocal. We randomly determined the sampling order and starting location (either
survey point 1 or 10) for all 34 routes prior to initiating field surveys. We
conducted both morning and evening routes to maximize survey opportunity.
Morning surveys began 30-min before sunrise or shortly thereafter and were
completed by 10:00 AM CST. Evening surveys began 4 hrs before sunset
and were completed by dark. We approached each survey point slowly until
the bow of the boat rested firmly over marsh vegetation. In cases when tides
were low, we lifted the boat motor and used a push pole to quietly move the
boat to the marsh edge. We postponed surveys when wind speeds exceeded
a Beaufort scale of 3 or during periods of steady rain.
2009 E.C. Soehren, J.W. Tucker, Jr., and D.G. Crow 281
We recorded all marsh birds detected during an initial 5-min passivelistening
period followed by a call-broadcast period (Conway 2002). To
reduce observer bias, we practiced bird identification, the sampling protocol,
and distance estimation prior to initiation of the surveys (Kepler and Scott
1981). To elicit call responses from target species, we used a portable compact
disc player equipped with 2 external, amplified speakers. We placed this
setup at the bow of the boat facing the marsh. We obtained 2 compact discs
with the primary advertising calls of each target species from the national
Marsh Bird Survey Coordinator (Dr. Courtney Conway, USGS Arizona Cooperative
Fish and Wildlife Research Unit). One compact disc contained the
calls of Black Rail, Least Bittern, King Rail, and Clapper Rail for broadcasting
in saline emergent marsh habitats (9-min survey), while the other disc
contained calls of Least Bittern, King Rail, Clapper Rail, Common Moorhen,
and Purple Gallinule for brackish emergent marsh habitats (10-min survey).
Both compact discs began with 5-min of silence (for the passive-listening
period) before the advertising calls started. Each broadcast consisted of
30-sec of calls followed by 30-sec of silence to listen for responses and to
separate subsequent species’ calls. Primary advertising calls were broadcast
in order starting from the least intrusive species to the loudest. All calls
were broadcast between 80–90 decibels at approximately 1-m in front of the
speakers (Conway 2002).
Differentiating between Clapper and King Rail calls
A challenging aspect of performing coastal marsh-bird surveys in the
Southeast is distinguishing between the similar calls of Clapper and King
Rails where populations are sympatric. Although both species are identifi-
able when observed, distinction between their respective calls can be very
difficult, especially at lengthy distances. To reduce the likelihood of misidentification, we performed several trial runs using call-broadcasts in emergent
saline and brackish marshes focusing on identifying both species aurally
before surveys were initiated. Moreover during surveys, the repetitiveness
of listening to the broadcast of both species’ calls over the course of the field
season added to our abilities and confidence of distinguishing the subtle
differences between the 2 species. Typically, Clapper Rails are restricted to
tidal saline marshes (Holliman 1978) and usually do not overlap with King
Rails, which occur in both inland fresh and tidal brackish marshes (Meanley
1969). However, both species are occasionally found together in transitional
areas where tidal saline marshes converge with tidal brackish marshes, particularly
where Uca spp. (fiddler crab) populations persist (Meanley 1969,
Meanley and Wetherbee 1962). This transitional condition exists along
coastal Alabama within a poorly defined zone ranging from the US Interstate
10 Causeway at the northern edge and extending southward along the eastern
and western shores of Mobile Bay (including the inner confl uences of rivers
and creeks) to Weeks Bay in lower Baldwin County and Mon Louis Island
in lower Mobile County, respectively (E.C. Soehren, pers. observ.). It also
282 Southeastern Naturalist Vol. 8, No. 2
occurs where the Perdido River empties into the Perdido Bay. Within transitional
zones, we differentiated King Rails aurally from Clapper Rails by the
former having an overall richer-quality sound of slightly deeper and more
evenly-pitched notes given during the “kik” and “clatter” call bouts, the 2
most often detected call-types. Occasionally, we were able to validate some
aural identifications when we later observed calling individuals emerging
from marsh vegetation after our call-broadcast periods were completed. Although
we made pre-survey preparations to address this aural identification
challenge (i.e., within the transitional zone), it is possible that some of the
individuals we detected aurally may have been misidentified as a result of
the variability in calls that exists among individuals of both species. Hence,
differentiation between both species can be very difficult and potentially
unreliable. Nevertheless, we identified all confusing Rallus spp. calls within
the transitional zone as either Clapper or King Rails based upon our initial
We compared the effectiveness of the passive-listening and call-broadcast
methods for each of the 6 focal species by calculating the proportion
of individuals detected by each method-type. We assigned individuals
initially detected during the passive-listening period that later responded
during the call-broadcast period as passive-listening detections. Therefore,
the proportions responding to call-broadcasts refl ected only the increase in
number of individuals detected. We calculated the percent difference in the
number of first detections between the 2 periods (i.e., [call-broadcast period
– passive-listening period]/passive-listening period x 100; Gibbs and Melvin
1993) to show effectiveness of the call-broadcast period for increasing
detections over the passive-listening period. To compare species detections
between the two periods in saline marshes, we excluded detections from the
5th-min of passive-listening to standardize sampling effort with the 4-min
call-broadcast period. As an additional measure of detection in response
to call-broadcasts, we calculated the proportion of individuals detected by
passive-listening that later responded to call-broadcasts. This additional
measure of detection is the proportion of individuals known present from
the passive-listening period that later responded to call-broadcasts; thus, it
is a simple estimate of detection probability in response to call-broadcasts.
We included all individuals that responded to any call-broadcast, including
those first detected during the passive-listening period, to examine responses
of individual species to conspecific and heterospecific calls. However, we
excluded individuals if they were detected only during the passive-listening
period. Some individual birds responded to more than 1 type of call-broadcast;
thus, responses across call types often were greater than the number
of individuals, and summation of proportions were greater than unity. For
statistical analyses, we compared proportions by calculating 95% confidence
intervals around the binomial proportions (Zar 1984:378) and concluded
significant differences when confidence intervals did not overlap.
2009 E.C. Soehren, J.W. Tucker, Jr., and D.G. Crow 283
We surveyed a total of 195 sites in saline emergent marshes and 122 sites in
brackish emergent marshes using point-counts between 27 April and 23 July
2004 (Fig. 1). Overall, we detected 604 individuals consisting of 385 Clapper
Rails, 50 Least Bitterns, 38 King Rails, 89 Purple Gallinules, and 42 Common
Moorhens (Table 1). An additional 16 Clapper Rails and 1 Least Bittern were
detected only during the fifth minute of passive-listening in saline marshes,
but we excluded these individuals to standardize sampling effort with the latter
4-min call-broadcast period. No Black Rails were detected.
Aside from a single detection in a brackish emergent marsh of the lower
Delta sampling region, we detected Clapper Rails consistently in saline
emergent marshes, and they were the most numerous of the target species encountered
(Table 1). Of the 384 Clapper Rails detected in saline marshes, 267
(70%) were initially detected during the call-broadcast period and 117 (30%)
were detected during the first 4-min of the passive-listening period (Fig. 2),
accounting for a 128% increase in individuals detected (Table 1). For those
individuals detected within the multiple-species call-broadcast sequence,
the proportion of Clapper Rails responding to conspecific calls was signifi-
cantly greater than the proportion responding to King Rail calls. Furthermore,
the proportion of individuals responding to King Rail calls was significantly
greater than the proportion responding to Least Bittern and Black Rail calls
(Fig. 2). Of the 117 Clapper Rails detected passively in saline marshes, 76
(65%) later responded to call-broadcasts.
We detected Least Bitterns in both saline (n = 15, 0.08 birds per point)
and brackish (n = 35, 0.29 birds per point) emergent marshes at nearly a
1:3.6 ratio between the 2 marsh types. We did not find differences between
Table 1. Number of individuals detected by point counts for targeted species of marsh birds
in brackish and saline marshes of coastal Alabama. Numbers include individuals detected
during an initial passive-listening period (Passive) and a subsequent call-broadcast period
(Broadcast only). Also included are number of individuals detected by passive-listening that
later responded to call-broadcasts (Both), total number detected during call-broadcasts (Broadcast
total = Broadcast only + Both), total number of individuals detected (Total = Passive
+ Broadcast only), and the percent difference between new individuals detected during the
call-broadcast and passive-listening periods (% difference). Targeted species were Common
Moorhen (COMO), King Rail (KIRA), Least Bittern (LEBI), Purple Gallinule (PUGA), and
Clapper Rail1 (CLRA).
Brackish marshes Saline marshes
Targeted species COMO KIRA LEBI PUGA CLRA KIRA LEBI
Passive 25 7 17 30 117 0 7
Broadcast only 17 23 18 59 267 8 8
Both 13 5 8 15 76 0 2
Broadcast total 30 28 26 74 343 8 10
Total 42 30 35 89 384 8 15
% difference2 -32% 229% 6% 97% 128% - 14%
1One Clapper Rail detected in brackish marshes is not shown.
2Calculated as ([Broadcast only - Passive]/Passive) x 100 (Gibbs and Melvin 1993).
284 Southeastern Naturalist Vol. 8, No. 2
the proportion of Least Bitterns detected by passive-listening and call-broadcasts
in either saline (Fig. 3A) or brackish marshes (Fig. 3B). Although no
differences were found between the proportions, call-broadcasts collectively
increased the number of individuals detected after the passive-listening
period by 14% and 6% in saline and brackish marshes, respectively. However,
Least Bitterns appeared as likely to respond to heterospecific calls as to
conspecific calls (Figs. 3A and B). Of the Least Bitterns detected passively,
2 of 7 (29%) in saline marshes and 8 of 17 (47%) in brackish marshes later
responded to call-broadcasts.
King Rails were the least numerous of all target species encountered, but
were detected in both saline (n = 8, 0.04 birds per point) and brackish (n =
30, 0.25 birds per point) emergent marshes at approximately a 1:6.2 ratio
between marsh-types. Within brackish marshes, only 7 (23%) individuals
were detected during passive-listening periods while the other 23 (77%) were
detected after responding to call-broadcasts (Fig. 4), which substantially
increased the number of individuals detected after passive-listening by
Figure 2. Proportions (± 95% CI) of Clapper Rails detected in saline marshes of
Alabama during point-counts by passive-listening and broadcasting recorded calls.
Species are coded as follows: BLRA = Black Rail, LEBI = Least Bittern, KIRA =
King Rail, CLRA = Clapper Rail.
2009 E.C. Soehren, J.W. Tucker, Jr., and D.G. Crow 285
Figure 3. Proportions (± 95% CI) of Least Bitterns detected during point-counts by
passive-listening (P) and call-broadcasts (C-b) in: A) saline marshes and B) brackish
marshes of Alabama. Call-broadcasts of species are coded as follows: BLRA =
Black Rail, LEBI = Least Bittern, KIRA = King Rail, CLRA = Clapper Rail, COMO
= Common Moorhen, and PUGA = Purple Gallinule.
286 Southeastern Naturalist Vol. 8, No. 2
229% (Table 1). King Rails responded to conspecific calls significantly more
often than Least Bittern calls, but not significantly more than the calls of
Clapper Rail, Common Moorhen, or Purple Gallinule (Fig. 4). Five (71%)
of the 7 King Rails detected during passive-listening in brackish marshes
later responded to call-broadcasts. Within saline marshes, all 8 King Rails
were detected at survey points within the “transitional” zone between saline
and brackish tidal marshes and all were detected only during the call-broadcast
period (Table 1). Of these, 7 (88%) responded to conspecific calls (95%
CI = 0.473–0.997), while the other individual (12%) responded to Clapper
Rail calls (95% CI = 0.003–0.527).
We detected Purple Gallinules (n = 89) only in brackish emergent marshes.
They were the most-encountered target species in brackish marshes and
the second-most detected overall. Two-thirds of the individual Purple Gallinules
detected (n = 59, 66%) were first detected during the call-broadcast
period (Fig. 5A), accounting for a 97% increase in the number of individuals
detected after passive-listening (n = 30, 34%; Table 1). Moreover, 15
Figure 4. Proportions (± 95% CI) of King Rails detected during point-counts in
brackish marshes of Alabama by passive-listening and broadcasting recorded calls.
Species are coded as follows: LEBI = Least Bittern, KIRA = King Rail, CLRA =
Clapper Rail, COMO = Common Moorhen, and PUGA = Purple Gallinule.
2009 E.C. Soehren, J.W. Tucker, Jr., and D.G. Crow 287
(50%) Purple Gallinules that were initially detected during passive-listening
later responded to call-broadcasts. For all individuals that responded to
call-broadcasts (n = 74; Table 1), 65 (88%) responded to conspecific calls,
which was a significantly greater proportion than those responding to heterospecific calls (Fig 5A). Like Purple Gallinules, Common Moorhens (n =
42) were detected only in brackish marshes. Interestingly, more individuals
(n = 25, 60%) were detected during the passive-listening period than during
the call-broadcast period (n = 17, 40%; Table 1), but this difference was not
significant (Fig. 5B). Of the 25 individuals detected during the passive-listening
period, 13 (52%) later responded to call-broadcasts. For those detected
within the multiple-species call-broadcast sequence (n = 30, Table 1), Common
Moorhens responded to conspecific calls significantly more than King
Rail calls, but not more than the calls of the other target species (Fig. 5B).
Along coastal Alabama, the call-broadcast period significantly increased
detections for Clapper Rail (128%), King Rail (229%), and Purple Gallinule
(97%) (Figs. 2, 4, and 5A, respectively), but was less effective for Least Bittern
(6–14%; Figs. 3A and B) and Common Moorhen (-32%; Fig. 5B). We
acknowledge that the sequential sampling protocol we used did not allow for
a direct analysis of the effects of call-broadcasts on detection rates of targeted
species. For example, our estimates of percent increase assume that an equal
number of individuals were detected during the two survey periods and the
additional individuals detected during the call-broadcast period resulted
from responses to the call-broadcasts. As a general rule, the rate that new
birds are detected by passive-listening rapidly declines after approximately
5 min of sampling (Bibby et al. 1992, Lynch 1995, Petit et al. 1995, Scott and
Ramsey 1981). Although we cannot conclude that all individuals detected
during call-broadcast periods strictly resulted from their response to the
broadcast of calls, the increase in the number of individuals detected using
call-broadcasts suggests an improvement in the effectiveness of sampling
over passive-listening. The percent differences we report should be viewed
as conservative estimates because we did not include individuals responding
to call-broadcasts if they were detected during the passive-listening period
(see Table 1). We propose a stronger and more direct approach to compare the
effectiveness of the two survey methods would consist of repeated sampling
and alternating between methods at adjacent points within sampling dates, returning
to those points as quickly as practical at approximately the same time
of day on a subsequent date, and repeating the survey using the method not
used previously. This approach would minimize confounding effects of temporal
variation, and using the same observer and equipment to conduct the
repeated surveys would minimize observer bias to produce a paired-sample
design to allow direct testing for the effects of call-broadcasts.
Despite the aforementioned shortcomings of the sequential sampling
protocol we used to measure the effects of call-broadcasts on detection
288 Southeastern Naturalist Vol. 8, No. 2
Figure 5. Proportions (± 95% CI) of detections during point-counts by passive-listening
(P) and broadcasting recorded calls (C-b) for A) Purple Gallinule and B) Common
Moorhen in brackish marshes of Alabama. Species are coded as follows: LEBI = Least
Bittern, KIRA = King Rail, CLRA = Clapper Rail, COMO = Common Moorhen, and
PUGA = Purple Gallinule.
2009 E.C. Soehren, J.W. Tucker, Jr., and D.G. Crow 289
rates, the protocol enabled us to compare the responses of targeted species
to conspecific and heterospecific call-broadcasts. Both Clapper and King
Rails responded more often to their conspecific calls than to each other’s
calls within the multiple-species call-broadcast sequences (Figs. 2 and 4).
Prior to this study, there had been a question on whether use of 1 of these
2 species’ calls would be sufficient to elicit responses from both species
where populations are sympatric (Ribic et al. 1999:7). Our findings suggest
that both species’ calls should be incorporated into multiple-species callbroadcast
sequences during the breeding season along the northern Gulf
Coast, but additional study is probably needed to better evaluate responses
in other regions. In addition, the effectiveness of using call-broadcasts for
detecting Purple Gallinules was classified as uncertain prior to this study
due to a paucity of information about it (Ribic et al. 1999). Our data clearly
indicate that call-broadcasts significantly improved detections for Purple
Gallinules over passive-listening, and they responded to conspecific calls
consistently more often than to heterospecific calls placed within a multiplespecies
call sequence (Fig. 5A). Hence, we recommend that Purple Gallinule
call-broadcasts should be applied to marsh-bird inventories or monitoring
programs throughout this species’ range.
Although call-broadcasts appeared less effective for Least Bittern and
Common Moorhen than for the other target species, other studies have
shown that Least Bittern (Bogner and Baldassarre 2002, Gibbs and Melvin
1993, Swift et al. 1988) and Common Moorhen (Conway and Gibbs 2005,
Ribic et al. 1999:50) are detected more effectively using call-broadcasts
than by passive-listening. The lower responses from Least Bittern and
Common Moorhen during the call-broadcast period that we observed may
have resulted from several factors including the conservative approach we
used to calculate percent differences. For example, we did not include 13
Common Moorhens that responded to call-broadcasts in our calculations
because those individuals had been detected previously by passive-listening
(Table 1). Thus, we actually detected 30 Common Moorhens during callbroadcasts
versus 25 during passive-listening, but those 13 individuals
detected via call-broadcasts did not contribute to increased efficiency by addition
of call-broadcasts to the sampling protocol. Similarly, our calculations
for Least Bitten also appear conservative (Table 1).
Other factors that possibly limited the responses of Least Bitterns
and Common Moorhens to call-broadcasts include the timing of surveys
within the breeding season (Rehm and Baldassarre 2007) and the length of
time each call-broadcast segment was played within the multiple-species
call-broadcast sequence. For example, Bogner and Baldassarre (2002)
recommend 5-min of broadcasting vocalizations with 15-sec bouts of calls
separated by at least 15-sec of silence to survey for Least Bitterns due to
their slow responses to call-broadcasts. Conversely, our call-broadcast segments
lasted only 30-sec for each target species and may have been too short
to elicit adequate responses from Least Bitterns and Common Moorhens.
290 Southeastern Naturalist Vol. 8, No. 2
Furthermore, broadcasting calls of multiple species in succession may increase
or decrease the probability of detecting the targeted species (Conway
and Gibbs 2005). Although we found that King Rail, Clapper Rail, and
Purple Gallinule were detected much more frequently during call-broadcasts
than during passive-listening, Least Bittern and Common Moorhen simply
may not have responded to call-broadcasts as readily for reasons unknown.
Subtle factors such as the number and order of species on a call-broadcast
sequence, dialects of the calls broadcasted, and time of day may have played
a role in the limited responses. Such factors will require more study to better
understand how multiple-species call-broadcasts affect targeted marsh-bird
responses in different regions of North America (Conway and Gibbs 2005).
Lastly, although call-broadcasts increased the number of individual
marsh birds detected, only 71% or fewer of the individuals detected during
the passive-listening period later responded to call-broadcasts. Therefore,
detection probability for target species was less than perfect using callbroadcasts,
attesting to the secretive nature of these species.
Initial funding for this project was provided in part by ACAMP Project Award
#NA17OZ2324 through the Coastal Zone Management Act of 1972, as amended,
administered by the Office of Ocean and Coastal Resource Management, National
Oceanic and Atmospheric Administration and in conjunction with the Alabama Department
of Conservation and Natural Resources (ADCNR), State Lands Division,
Coastal Section. Additional matching funds and equipment use was provided by the
ADCNR, State Lands Division. We gratefully acknowledge the efforts of Courtney
Graydon for assisting with surveys and preparing the study area figure. Additional
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