RESOURCE 

EVALUATION 

REPORT

SFNRC Technical Series

2017:1

Behavioral Observations of the

Cape Sable Seaside Sparrow from

an Acoustic Array

National Park Service

U.S. Department of the Interior

South Florida Natural Resources Center

Everglades National Park 

Behavioral Observations of the

Cape Sable Seaside Sparrow

from an Acoustic Array

SFNRC Technical Series 2017:1

South Florida Natural Resources Center

Everglades National Park

Homestead, Florida

National Park Service

U.S. Department of the Interior

Behavioral 0bservations of the Cape Sable Seaside Sparrow i

Behavioral 0bservations of the Cape Sable Seaside Sparrow

from an Acoustic Array

SFNRC Technical Series 2017:1

EXECUTIVE SUMMARY

The Cape Sable seaside sparrow is an endangered subspecies endemic to critical habitats

of the Florida Everglades and there is concern that environmental change has potential to

reduce suitable habitat. Difficulties observing the sparrow limit data informing the sparrows

range, population, and importantly its response to environmental conditions. We present

analysis of a 5 month monitoring effort using passive acoustics to observe sparrow behavior

finding that sparrows avoid treelines while engaged in territory maintenance, continue to

vocalize while habitat is inundated, do not vocalize on a day-to-day basis, that individuals can

be uniquely identified from acoustic spectral signatures, and quantify the hourly distribution

of song. This study demonstrates that the ability to remotely localize animals in both

space and time is a significant advantage of array-based monitoring over single microphone

recordings, and, the passive nature of the sensing poses no threat of injury to the animals.

Application of this technology is well-suited to address questions of animal habitat usage

and behavioral responses.

Restoration or expansion of suitable habitat through ecological management has been identified as a viable tactic for survival of the subspecies. These results indicate that removal

of artificially introduced woody vegetation to expand suitable habitat may provide such a

transition.
ii South Florida Natural Resources Center (2017:1)
Behavioral 0bservations of the Cape Sable Seaside Sparrow iii

TABLE OF CONTENTS

CONTRIBUTING AUTHORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Recording Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Song Identification and Source Localization . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 Water Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.4 Territory Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1 Source Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Spectral Uniqueness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.3 Behavioral Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.3.1 Territory Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.3.2 Helicopter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3.3 Red-shouldered Hawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3.4 Daily Song Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3.5 Water Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3.6 Daily Song Clustering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3.7 Treeline Avoidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
iv South Florida Natural Resources Center (2017:1)

CONTRIBUTING AUTHORS

Joseph Park1, Jed Redwine2, Tom Virzi3 and Kevin Kotun1

1 Physical Resources Branch, South Florida Natural Resources Center, Everglades National

Park. 950 N. Krome Avenue, Homestead, FL 33030-4443

2 Science Coordination Branch, South Florida Natural Resources Center, Everglades National Park. 950 N. Krome Avenue, Homestead, FL 33030-4443

3 Ecostudies Institute. P.O. Box 735 East Olympia, WA 98540

Comments and Questions: Joseph Park@nps.gov, Jed Redwine@nps.gov, Kevin Kotun@nps.gov

ACKNOWLEDGEMENTS

We gratefully acknowledge the guidance of Tylan Dean for suggesting the deployment location and educating us about the sparrow.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply

endorsement by the National Park Service. Although this report is in the public domain,

permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report.

Please reference this report as follows:

Park, J., Redwine J., Virzi T. and Kotun K., (2017). Behavioral 0bservations of the Cape

Sable Seaside Sparrow: from an Acoustic Array. U. S. Department of Interior, Everglades National Park, South Florida Natural Resources Center, SFNRC 2017:1. 13 pp.
Behavioral 0bservations of the Cape Sable Seaside Sparrow v

FOREWORD

Everglades National Park is internationally recognized for its unique biological diversity and

beauty, classified both as an International Biosphere Reserve and UNESCO World Heritage

Site. Roughly 86 percent of the park is designated as wilderness, housing thirty-six federally

protected animal species. One species in danger of extinction is the Cape Sable seaside

sparrow whose range is restricted to Everglades National Park and Big Cypress National

Preserve. The lifecycle of the sparrow is intimately tied to marl-prairie grasslands and the

hydrologic cycle, where they thrive at the interface of seasonal inundations and the lower

reaches of waist-high sedges and grasses.

The natural hydrologic cycle within the park has been significantly impacted by 20th century water control infrastructure, with continuing tension between goals prescribed in the

Comprehensive Everglades Restoration Plan (CERP) and the expected regional hydrologic

cycles in some sparrow subpopulations. The ability of the sparrow to colonize new habitats

may hold a key to species survival.

This report documents an innovative technical synthesis of acoustic remote sensing and signal

processing enabling tracking of male sparrows without the presence of human observers. The

primary question posed for this research was whether or not male sparrows avoid artificially

introduced woody vegetation, with results supportive of the hypothesis. This has immediate

consequence for ecological management since the results suggest that removal of the artificially introduced woody vegetation has potential to expand viable sparrow habitat. Aside

from this primary finding, it was also discovered that the birds have individual voices, and

can be reliably identified from the spectral features of their song. The report also quantifies

daily vocalization behavior and statistics over an entire breeding season providing valuable

information for the design and improvement of sparrow monitoring and survey efforts.

Research such as presented here demonstrates the transference of science into actionable

ecological management for an endangered species, work that must continue in earnest to

afford the best chance of survival for our endangered cousins.

Robert Johnson

Director

South Florida Natural Resources Center

Everglades National Park

November 2016
vi South Florida Natural Resources Center (2017:1)
Behavioral 0bservations of the Cape Sable Seaside Sparrow 1

1 Introduction

Globally, extinction risks appear to be accelerated by climate change and industrialization

with concern for up to one sixth of species (Urban, 2005; Bellar et al., 2012; Li et al., 2016).

Subsequent landscape changes can influence species-specific habitat usage through complex

interactions that need to be understood in order to facilitate meaningful assessments of

extinction risk (Keith, 2008). The Florida Everglades encompasses several such habitats

and species including the Cape Sable seaside sparrow (Ammodramus maritimus mirabilis) a

non–migratory endangered subspecies endemic to open wetlands (Howell, 1919; Nicholson,

1928; USFWS, 1967). Sparrow habitat is classified as critical (USDOI, 2007) with concern

that environmental change is leading to a reduction in suitable habitat (Nott et al., 1998;

Dean and Morrison, 2001; SEI, 2007).

The sparrow is found predominantly in marl prairies characterized by clumped grasses consisting of muhly grass (Muhlenbergia filipes) and short sawgrass (Cladium jamaicense). These

freshwater plant communities are sustained by hydroperiods that are not too wet, such as

those found in freshwater sloughs which may be perennially inundated favoring floating

aquatic vegetation, or conditions that are too dry allowing establishment of woody vegetation (USFWS, 1999, 2016). Sustenance of these habitats is directly influenced by interannual

variations in hydroperiod and water depth such that long-period hydrologic conditions and

their associated landscape responses are determinants of sparrow habitat suitability. As

such, efforts to influence regional hydrology currently form the basis of ecological management strategies aimed at species preservation (USFWS, 2016).

The sparrow is thought to have once proliferated near the Cape Sable region along the

southwestern extremity of the Florida peninsula (Howell, 1919; Nicholson, 1928). However,

in response to rising sea level and episodic extreme saltwater inundation from storm surge,

this region has changed from freshwater prairie to salt–tolerant floral communities. Contemporary ranges encompass marl prairies between the northern boundaries of Everglades

National Park and coastal wetlands. These modern settlements are part of the greater Everglades ecosystem, a system that is modified through control and redistribution of water

resources feeding the park as well as by interior landscape features introduced before park

establishment. The remarkably flat and uniform topography of the Everglades coupled with

subtropical hydrologic cycles imposes a narrow range of habitat suitability such that relatively small changes in hydrologic conditions or topography can allow the emergence and

dominance of different ecological communities. One such example is the introduction of treelines along abandoned agricultural drainage ditches. Although elevation differences between

ditch excavation spoil banks and the surrounding marsh are less than a meter, it is sufficient

to allow woody vegetation to establish and sustain treelines along these ditches. Observations of sparrow behavior suggest that they avoid areas with woody vegetation, perhaps

as a mechanism to avoid areas where predators can easily establish (Lockwood et al., 1997;

Dean and Morrison, 2001) such that these artificially introduced treelines may be reducing

suitable sparrow habitat.

Observing the sparrow is difficult. The bird is small, its habitat remote and often difficult

to access, only males sing, and only during the breeding season. These difficulties limit data
2 South Florida Natural Resources Center (2017:1)

informing the sparrows’ range, population, and importantly their response to environmental

conditions. For example, although interannual hydrologic conditions control habitat suitability, relationships between bird abundance and hydrologic variables on daily or weekly time

scales are less clear with an apparent insensitivity to hydroperiod (Beerens and Roma ̃nach,

2016) but preference for drier conditions (Beerens et al., 2016). It has also been suggested

that sparrows are behaviorally impacted by inundation (Dean and Morrison, 2001; Beerens

and Roma ̃nach, 2016), do not sing when their territory is flooded (Nott et al., 1998) or when

water depth exceeds 10 cm (Lockwood et al., 1997). On interannual time scales survival rates

are not associated with water levels (Boulton et al., 2009), but within intraannual periods

wetter conditions preceding the breeding season favor earlier breeding and larger clutch sizes,

presumably from abundant food sources, while nest survival is negatively associated with

high average rainfall late in the breeding season (Baiser et al., 2008; Boulton et al., 2011).

The song of a male sparrow provides important conspecific information to other individuals as

he establishes a breeding territory early in the season and continues to defend boundaries of

his territory throughout the season broadcasting song from grass perches around the territory

(USFWS, 1999; Virzi et al., 2012). It is also a primary auditory cue for human observers

engaged in demographic and census studies. The vocal nature of the male lends itself to

remote sensing based on passive acoustics (Virzi and Davis, 2012), as well as to experiments

using conspecific acoustic cues to influence behavior (Virzi et al., 2012). Virzi and Davis

(2012) developed a general sparrow song recognizer with an automated spectral classifier

capable of detecting the presence of sparrows with a 95% success rate and false-negative

(failure to correctly classify a sparrow song) rate of 14%. Based on manual observations

and acoustic studies it is thought that males vocalize primarily in the early morning, while

little is known about the day-to-day repeatability of an individual’s efforts to maintain its

territory through vocalization and how such variations are related to specific behaviors.

Data informing questions of sparrow behavior such as song frequency and timing, influences

of water levels, as well as questions of habitat suitability have traditionally been answered

with human observers conducting field-studies requiring visual, aural, or capture techniques.

Here, we augment these observations with long-term observations from a passive acoustic

monitor. The monitoring system provides an automated, daily monitoring presence with

the ability to localize sparrow songs, allowing us to characterize spatially-explicit sparrow

behavior while engaged in territory defense and maintenance. Restoration of suitable habitat

through ecological management may be a viable tactic for the survival of the species. These

results indicate that removal of artificially introduced woody vegetation to expand suitable

habitat may provide the desired restorative effect.

2 Methods

A passive acoustic monitoring system was deployed within Everglades National Park from

March 23 through September 2, 2016 in sparrow subpopulation B, coinciding with the sparrow breeding season (March through August) and annual wet-season. The system consisted

of a 4-microphone array deployed at 25.33754◦ N, 80.79918◦ W in an equilateral triangular

arrangement with one microphone at each vertex, and one in the center as shown in Figure
Behavioral 0bservations of the Cape Sable Seaside Sparrow 3

1. Nominal sensor spacing was 1.1 m between the central and peripheral microphones, 2

m between vertex sensors. Sensor coordinates are listed in Table 1. The microphones were

model SMM-A1 manufactured by Wildlife Acoustics (Wildlife Acoustics, 2016), two automated digital recorders, model SM3BAT, recorded two microphone channels per recorder.

Audio recordings were synchronized with GPS timing signals to a 1 millisecond accuracy.

Details on the acoustic performance of this system are described in Park and Kotun (2017).

Figure 1. Acoustic array on July 8, 2016 consisting of (4) SMM-A1 microphones denoted S1, S2, S3 and

S4, (2) SM3BAT digital recorders, and (2) SM3 GPS antenna. The S1 - S2 arm is aligned east - west.

Sensor S1 is located at 25.33754◦ N, 80.79918◦ W.

Table 1. Sensor coordinates in local reference frame with origin at sensor S1.

Sensor S1 is geolocated at 25.33754◦ N

80.79918◦ W.

Sensor East (m) North (m)

S1 0.000 0.000

S2 -1.103 1.058

S3 -1.630 -1.042

S4 -1.815 0.952

2.1 Recording Schedule

Audio recordings were made 14 times each day in 10 minute blocks. The first five recordings

followed a 10-minute-on, 10-minute-off format with the first recording starting 10 minutes

before sunrise. The next five recordings were made at 09:00, 10:00, 13:00, 17:00 and 18:00

local time. The final four recordings followed the 10-minute-on, 10-minute-off format with

the last recording ending 10 minutes after local sunset.
4 South Florida Natural Resources Center (2017:1)

2.2 Song Identification and Source Localization

Recorded sparrow songs were manually identified with Wildlife Acoustics Songscope software

and locations estimated with an automated 3-step processing algorithm. The first step

is verification of coherent energy across the acoustic array with cross-spectral coherence

computed over 0.5 s windows. If the coherence across the frequency band of 2.5 to 5 kHz

at all sensor pairs exceeds a threshold, typically 0.4, then the signals are passed to step

2. The second step is determination of relative sound arrival times at each microphone by

cross-correlation, subjected to limits imposed by the known sensor separations. Lastly, an

algebraic solution to the GPS navigation equations from the inter-sensor time delays provides

a source location estimate (Wilson et al., 2014).

2.3 Water Depth

Water depth is estimated from data observed at the Everglades National Park hydrologic

monitoring station NP46 located 2 km south of the acoustic monitoring location. The

estimate is derived from the water depth difference between the observed data at NP46 and

water depth estimated at the acoustic monitor by the Everglades Depth Estimation Network

(EDEN) (Telis et al., 2015). The difference is 3.0 cm so that reported depths are the observed

depth at NP46 gauge minus 3.0 cm.

2.4 Territory Mapping

Cape Sable seaside sparrow territory boundaries were mapped in ArcMap 10.2.2 using the

Geospatial Modeling Environment spatial analysis tools for home range estimation. Territory

data were collected on a 68 ha demographic study plot located southwest of the acoustic array.

Four replicated line transect surveys were conducted on the study plot between April 5 and

June 27, 2016 to detect breeding sparrows. The study plot was subsequently revisited to

collect perching points for all male sparrows detected on the surveys. Points were collected

using a handheld GPS device and later entered into ArcMap for analysis. The first step

in the analysis used the kernel density estimation (KDE) tool to provide an estimate for a

probability density function of perching points. Model parameters were applied consistently

among KDE estimates for all individuals: bandwidth = SCV; cell size = 10 m; extent =

study plot boundary. Once the KDE estimate for each individual was calculated, the isopleth

analysis tool was used to calculate the 95% home range isopleth for each territory.

3 Results

A total of 3373 songs were recorded over the period March 23 through September 2, 2016

of which 1417 were localized, although no songs were detected after July 24, 2016. A song

of exceptional clarity recorded at close range on May 23, 2016 exemplifies the sparrow song

as shown in the spectrogram of Figure 2. Previous recordings allow one to distinguish a

two-note preamble followed by a shrill trill. Here we find a three-note preamble followed
Behavioral 0bservations of the Cape Sable Seaside Sparrow 5

by the trill, with the interesting observation that the three-note sequence follows a musical

major scale harmonic sequence of octave - fifth - root. Features above 5 kHz are normally not

present in sparrow recordings at larger distances from the microphone as these components

are relatively weak in amplitude, but here we observe high-frequency components of the

initial note and trill between 5 and 6 kHz.

Figure 2. a) Power spectral density of a song recorded at close range on May 23, 2016 at 07:25:00 EDT.

b) Spectrogram of the song.

3.1 Source Level

The traditional method to assess sparrow abundance consists of transporting an observer to

a study plot either on foot or by helicopter where the observer listens for songs or visually

sights birds over a period of 7 to 10 minutes with specific plots visited once per season. More

detailed demographic studies establish plots that are visited daily throughout the breeding

season, again relying on aural or visual detections. Population estimates derived from these

point or line-transect surveys require an estimate of the maximum detection range (Buckland,

2006), and in the auditory case this estimate can be analytically determined if the animal

radiated acoustic power, or source strength, is known along with detection parameters such

as the noise level and observer auditory acuity. We were able to estimate the source strength

of the May 23 song from power spectral densities at the 4 microphones exemplified in Figure

2 where we note the bulk of acoustic power is located in the frequency band from 3.5 to 5 kHz.

Integration of the spectral sound pressure levels and addition of a transmission loss estimate

from the source location to the microphones results in an estimated sound pressure level of

85 ±3 dB//20μPa. This compares well to the mean maximum broadcast amplitude of 82

dB//20μPa determined by Rindy et al. (2008) for the song sparrow (Melospiza melodia).

3.2 Spectral Uniqueness

Virzi and Davis (2012) developed a general song recognizer based on an ensemble of individual songs. Here we find that individual birds present a unique spectral signature. Figure

3 shows a spectrogram of three individuals denoted A, B and C, competing simultaneously
6 South Florida Natural Resources Center (2017:1)

on June 10, 2016. Sparrow C consistently vocalizes with emphasis in the upper frequency

range from 4 to 5 kHz with a distinctive frequency modulation of the trill near 4.7 kHz. Bird

A had perhaps the most robust vocal presentation with emphasis on the preamble note near

2.8 kHz. Bird B emphasizes the 3.7 kHz note at the beginning of the trill, often produced a

preamble note near 5 kHz, and exhibited a declining frequency modulation during the trill.

These unique spectral signatures were first noticed on June 10, 2016 and last recognized on

July 22, 2016 indicating that individual spectral signatures are stable over a period of at

least six weeks and that spectrographic identification can be a viable complement to capture

and banding techniques.

Figure 3. Spectrogram of three individuals denoted C, A and B respectively

from left-to-right recorded June 10, 2016 at 06:44:49 EDT.

3.3 Behavioral Observations

3.3.1 Territory Defense

On June 10, 2016 between 06:43 and 06:52 there was an intense vocal exchange between

sparrows A, B and C. Initially at 06:43, C was the sole sparrow observed vocalizing from a

bearing of 268 ±12◦. At 06:43:42, A was detected at a bearing of 280 ±7

◦

, wherein C and

A subsequently engaged in counter-singing. At 06:44:37 B was first detected at a bearing

of 273 ±9

◦

, and all three continued counter singing until 06:45:08, after which C was not

detected. At 06:45:29 A moved to a bearing of 253 ±18◦, remaining there until 06:46:45.

From 06:50:15 until 06:50:57 B moved to a bearing 250 ±10◦, followed by repositioning of

A to a bearing of 199 ±5

◦

from 06:51:07 until 06:51:57. At 06:51:08, B moved to a bearing

of 200 ±6

◦

, both A and B continued counter-singing until 06:52 after which no songs were

recorded. We interpret these movements as evidence that bird A or B maintains a territory

to the west or southwest of the array, and that this encounter could represent the efforts

of B to defend his range. These observations are consistent with independently conducted
Behavioral 0bservations of the Cape Sable Seaside Sparrow 7

demographic surveys (below) identifying nest locations to the west (265◦) and southwest

(230◦).

3.3.2 Helicopter

Current survey methods assume that the birds are indifferent to the presence of a helicopter

such that the arrival and departure of the machine produces no change in behavior. Survey

periods from helicopter observers are typically 7 or 10 minutes. On May 25, 2016 a helicopter was recorded in close proximity from 07:04 to 07:09. Prior to aural evidence of the

helicopter sparrows were singing until 07:06 when the helicopter noise reached peak intensity

at which time all sparrow songs ceased. The helicopter moved away after 07:08, with the

last detectable sounds arriving at 07:09. Sparrow songs resumed approximately two minutes

after the helicopter was inaudible, indicating that the birds ceased singing for approximately

5 minutes in response to the helicopter incursion.

3.3.3 Red-shouldered Hawk

On May 4, 2016 from 07:36:00 to 07:36:14, and from 07:42:26 to 07:43:08, a red-shouldered

hawk was recorded near the monitor. Sparrow songs were also recorded during these intervals

allowing us to examine their relationship. During the first interval the hawk approached from

the southwest traveling to the northwest. During the second interval the hawk continued to

the northeast, then flew back around to the southwest and west. Examination of the first

interval found continuous sparrow songs at approximately 5 second intervals before, during,

and continuing after the hawk calls for a period of 2 minutes 15 seconds. The second period

(07:42:26–07:43:08) was more challenging to aurally detect and classify sparrow songs as

their amplitudes were weaker and background noise levels had increased, however, the hawk

calls are much clearer. A sparrow vocalization was evident 1.5 seconds before the hawk

call initiates, however, it is not clear if they continue during the hawk calls. Sparrow songs

were found again 1 minute 15 seconds after the hawk calls end. We interpret these data as

suggestive that male sparrows engaged in territory defense are indifferent to the presence of

a red-shouldered hawk.

3.3.4 Daily Song Distribution

Sparrow population surveys based on aural detection are dependent upon the time of day

which the birds are most likely to sing. The continuous presence of the monitoring system

allowed us to assess the hourly distribution of song detection with results presented in Figure

4. Although the recording schedule did not record equal intervals throughout the day, there

is evidence of a bi-modal song distribution with the majority of songs between 6 AM and 8

AM local time, and another less-intense period from 6 PM to 9 PM local time.
8 South Florida Natural Resources Center (2017:1)

Figure 4. Hourly distribution of song detections. Note that data were not recorded

every hour of the day. The recording schedule is detailed in the methods section.

3.3.5 Water Levels

As discussed above, long-term hydrologic conditions dominate habitat suitability for the

sparrow, while behavioral responses to short-term conditions are less-clear. In Figure 5 we

compare the number of songs recorded each day with water level. The water table was generally below ground until May 18th, when a large rain event (9.1 cm) abruptly inundated the

marsh with the area remaining submerged for the remainder of the study. 3025 (90%) of the

songs were recorded with inundated habitat, and 635 (19%) when the water depth exceeded

10 cm indicating that sparrows continue to engage in song and territory maintenance during

inundated conditions. There does not appear to be an obvious relationship between number

of songs recorded per day and water depth.

Figure 5. Total daily number of songs and water depth.
Behavioral 0bservations of the Cape Sable Seaside Sparrow 9

3.3.6 Daily Song Clustering

To our knowledge the counts shown in Figure 5 are the longest continuous, autonomous

observations of sparrow songs and reveal that song activity tends to occur in clusters. Days

of peak activity are often preceded by days with an increasing number of songs, and these

clusters are separated by days with no songs. It may be that the observed clustering is

related to initiation of multiple clutches, as sparrows are known to engage 1 to 3 nesting

attempts per season (Boulton et al., 2011). The episodic nature of these observations should

be considered when conducting aural surveys.

3.3.7 Treeline Avoidance

A particular strength of the monitoring system is the ability to localize songs with the

resultant probability density of azimuthal bearings from all detections shown in Figure 6.

Also shown are 95th percentile isopleths of territories and two nest locations determined

from independent line-transect surveys of demographic study plots. Territories west and

southwest of the array are consistently localized by the array, with evidence of an additional

territory northwest of the array outside of the line-transect survey boundary (green line).

The azimuthal distribution of songs indicates that males avoided the treeline with 88% of

songs localized to bearings outside of the sector from 20◦to 160◦ and 94% outside the sector

from 40◦to 140◦.

Figure 6. Aerial photograph of deployment location (25.33754◦ N 80.79918◦

W) with polar plot of detected bearing probability (blue). Polar grid radial

azimuths are aligned to bearings with respect to north. The linear landscape

features east of the deployment are treelines introduced by abandoned agricultural drainage canals. Each circle of the polar grid corresponds to a radial

interval of probability density 0.1% and linear dimension of 36 m. Black

and yellow circles mark nest locations, yellow contours are 95th percentile

isopleths of territories derived from territory mapping on demographic study

plots, diagonal green lines indicate survey boundaries.
10 South Florida Natural Resources Center (2017:1)

4 Discussion

Observation of the Cape Sable seaside sparrow traditionally relies on manual field surveys to

visually sight, aurally identify, or capture the bird for demographic and behavioral studies.

Virzi and Davis (2012) first applied passive acoustic remote sensing to sparrow monitoring,

and here we extend their work with the addition of a multi-element acoustic array capable

of localizing sparrow songs. The array autonomously records a preprogrammed sequence

of time blocks based on local sunrise and sunset times, is continuously present, and once

deployed provides minimal habitat disruption.

The acoustic nature of the sparrow song has been clarified with a high-resolution spectral

analysis revealing a descending three-note preamble followed by the trill. We were also able

to estimate the radiated acoustic source strength of a sparrow song as 85 ±3 dB//20μPa.

The source level is required to estimate maximum aurally observable distances, a necessary

component of distance-sampling techniques for population estimates. The data also suggest

that male vocalization generally follows a bimodal daily distribution with most activity between 6-8 AM, and a secondary peak between 6-9 PM. Inspection of song spectral properties

suggests that individual males can be uniquely identified and that these unique signatures

are stable over a period of at least six weeks. Combining spectral uniqueness with temporal

changes in song location provides a method to passively record behavioral dynamics without

direct, real time manual observation.

The known range of the sparrow is within the Florida Everglades, a landscape that is ecologically driven by hydrologic conditions governing the spatiotemporal extent of suitable

habitats. Specifically, interannual hydrologic conditions control the development and sustenance of open grasslands favored by the sparrow, and the birds have adapted their breeding

cycle to precede and coincide with the annual wet season. Intraannual variations in water

level can have positive or negative impacts on nesting success depending on the timing of

hydrologic conditions, while short-term water levels have not been clearly associated with

sparrow song behavior. Our data find that male sparrows continue to vocalize when their

territory is inundated, with no obvious dependence on water depths between 3 and 13 cm.

Examination of the number of daily songs over the deployment period finds that song activity

was recorded in clusters with days of peak activity preceded by days with increasing numbers

of songs. We also note that these clusters are separated by days where no songs were recorded.

Identification of environmental covariates with these clusters may reveal useful information

regarding sparrow vocalization behavior. It further suggests that abundance surveys should

visit survey sites more than one day per season.

The ability to localize songs both in space and time represents a significant advantage of an

array-based monitoring station over single microphone recordings with the resultant spatial

distribution of songs indicating a territory exists to the north of the array. This area was

not manually surveyed in the demographic study suggesting that acoustic source localization

deployments early in the breeding season may inform subsequent manual field-observations.

Along with identification of an apparent indifference to water levels and a red-shouldered

hawk when male sparrows are engaged in vocalization, perhaps the most ecologically pertinent finding is that male sparrows avoided an artificially introduced treeline suggesting that

ecological management aimed a species preservation should investigate remediation methods

to return these habitats to their natural condition.
Behavioral 0bservations of the Cape Sable Seaside Sparrow 11

5 Conclusion

Owing to its small population size, limited distribution, and concerns over habitat sustainability, the Cape Sable seaside sparrow was one of the first species listed under the Endangered Species Preservation Act of 1967, a status that remains in place today. Expansion

of suitable habitat is recognized as an important strategy for preservation of the subspecies

(SEI, 2007) and we find that male sparrows overwhelmingly avoid treelines composed of

woody vegetation when vocalizing in defense of their breeding territories. Since there have

been past successes in ecological restoration by the removal of invasive woody vegetation

(Ewel, 2013; NRC , 2007), this work suggests that the removal of woody vegetation and

treelines introduced by remnant agricultural features may provide opportunities for habitat

expansion.

This work also demonstrates the utility of passive acoustic source localization to monitor

animal behavior over an entire season. Relevant findings are that sparrows do not sing every

day during the breeding season, an important factor in the design and conduct of manual demographic and population surveys, and, that acoustic arrays have potential to detect

sparrow territories that were not captured by manual surveys. Further, it is shown that individuals can be uniquely identified by the spectral signature of their song, that vocalizations

do not exhibit a clear dependence on water depth, that there is an approximately 2-5 minute

refactory period of vocalization in response to a helicopter, and, an apparent indifference of

male sparrow vocalization to the aerial presence of a red-shouldered hawk.
12 South Florida Natural Resources Center (2017:1)

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