Download Wading Bird Colony Location, Size, and Timing on Lake

Wading Bird Colony Location, Size, and Timing on Lake Okeechobee, 2008-2009
FINAL REPORT
To
National Park Service
Everglades National Park
40001 SR 9336
Homestead, Florida 33034
Task Order No. J5297 05 0083
22 January 2010
By
Richard A. Botta
Dale E. Gawlik
Damion E. Marx1
Department of Biological Sciences
Florida Atlantic University
777 Glades Road
Boca Raton, FL 33431-0991
561-297-3333
[email protected]
[email protected]
1
Deceased
ACKNOWLEDGEMENTS
We would like to acknowledge the U.S. Army Corps of Engineers for funding this research as well
as Sonny Bass and supporting staff at the National Park Service for facilitating the research grant.
We also thank staff at the South Florida Water Management District, the Florida Fish and Wildlife
Commission, and Florida Atlantic University for their logistic support.
We express our gratitude to our research technician Justin Bredlau for his hard work in the field, and
to the rest of the Gawlik Lab for their support.
ii
TABLE OF CONTENTS
Page
TITLE PAGE ............................................................................................................................................... i
ACKNOWLEDGEMENTS ...................................................................................................................... ii
TABLE OF CONTENTS .......................................................................................................................... iii
LIST OF TABLES ....................................................................................................................................... iv
LIST OF FIGURES ..................................................................................................................................... v
ACRONYMS AND ABBREVIATIONS ................................................................................................ vi
INTRODUCTION ...................................................................................................................................... 1
Historical Wading Bird Nest effort at Lake Okeechobee ................................................................ 1
Current Monitoring Effort ................................................................................................................... 1
METHODS ................................................................................................................................................... 2
Colony Survey......................................................................................................................................... 2
Hydrology ................................................................................................................................................ 3
RESULTS AND DISCUSSION ................................................................................................................ 3
Hydrology ................................................................................................................................................ 3
Location ................................................................................................................................................... 6
Timing ...................................................................................................................................................... 8
Size ........................................................................................................................................................... 10
Wood Storks and Roseate Spoonbills ................................................................................................. 11
Environmental Conditions ................................................................................................................... 12
iii
LITERATURE CITED............................................................................................................................... 12
iv
TABLE
LIST OF TABLES
Page
1
Colony Location (NAD 83), individual colony counts of nests for each species of wading
bird detected at Lake Okeechobee during aerial surveys in the 2008 breeding season........... 8
2
Geographic coordinates (NAD 83) and species-specific peak nest efforts in detected
colonies during the 2008 breeding season at Lake Okeechobee ................................................ 8
3
Timing and nest effort for species breeding in wading bird colonies during 2009 at Lake
Okeechobee ........................................................................................................................................ 9
4
Geographic coordinates (NAD 83) and species-specific peak nest efforts in detected
colonies during the 2009 breeding season at Lake Okeechobee ................................................ 9
v
FIGURE
LIST OF FIGURES
Page
1
Map of systematic aerial transects flown over Lake Okeechobee during 2008 and 2009. ..... 3
2
Hydrographs depicting average daily rain and average lake stage for 2008 and 2009 ............. 4
3
Hydrograph comparing the difference in lake stage during the breeding season from 20052009 ..................................................................................................................................................... 5
4
Colony locations during 2008 and 2009 at Lake Okeechobee.................................................... 6
5
Regression of January lake stage on GREG nest effort. Hollow point is 2009. ................... 10
vi
ACRONYMS AND ABBREVIATIONS
CAEG – Cattle Egret
CERP – Comprehensive Everglades Restoration Plan
DOQQ – Digital Orthophotoquarterquadrangles
FAA – Federal Aviation Administration
FAU – Florida Atlantic University
GBHE – Great Blue Heron
GEW – Greater Everglades Wetlands ecosystem, which includes the Kissimmee River, Lake
Okeechobee, the Everglades, and Florida Bay
GLIB – Glossy Ibis
GREG – Great Egret
LBHE – Little Blue Heron
LOPP – Lake Okeechobee Protection Plan
NAVD88 – North American Vertical Datum 1988
NGVD29 – National Geodetic Vertical Datum 1929
MAP – Monitoring and Assessment Program
RECOVER – Restoration, Coordination and Verification program
SFWMD – South Florida Water Management District
SNEG – Snowy Egret
TRHE – Tricolored Heron
USACE – United States Army Corps of Engineers
WHIB – White Ibis
WOST – Wood Stork
vii
INTRODUCTION
Wading Bird Nest Effort at Lake Okeechobee
The first aerial surveys conducted on wading bird colonies on Lake Okeechobee (hereafter
referred to as the Lake) commenced in 1957 and proceeded until 1976 (David 1994a). Through
these surveys, nest counts were seen to range from a high of 10,400 in 1974 to a low of 130 nests in
1971. These aerial surveys, although beneficial, may have underestimated nest efforts since it was
only performed once during the nesting season. In 1977, aerial surveys shifted to a systematic
technique where monthly surveys were conducted to more accurately depict the effects of water
management on these wading bird populations (David 1994b, Smith and Collopy 1995). The main
five species of wading birds surveyed are: White Ibis (Eudocimus albus), Glossy Ibis (Plegadis falcinellus),
Great Egret (Ardea alba), Great Blue Heron (Ardea herodias), and Snowy Egret (Egretta thula). These
species of birds continued to be surveyed on a yearly basis as population sizes continued to shift
based on water level changes seen on the Lake. For example, in 2008 drought conditions reached a
breaking record for low water levels on the Lake which seemed to correlate with the lowest recorded
counts of 38 nests since surveys started in the 1970’s.
Current Monitoring Effort
In May of 2005, Florida Atlantic University received funding to document the timing, size
and location of wading bird colonies at Lake Okeechobee as part of CERP monitoring for the
GEW. On June 3, 2005, we conducted a single aerial survey just as the rainy season was beginning
and lake levels were rising. From 2006–2009, Florida Atlantic University (FAU) conducted monthly
aerial surveys of breeding wading birds to enhance the historical dataset. To our knowledge, these
efforts represent the first systematic aerial surveys at the Lake since 1992. Herein, we report results
from the 2008–2009 colony surveys. We will compare these two years with contrasting
environmental conditions that produced one of the best years and the worst nesting year on record
for the Lake.
viii
METHODS
Colony Surveys
As part of the CERP Monitoring and Assessment Plan, FAU surveyed wading bird nesting
to determine both location and size of colonies on the Lake. In 2008, during the months of January
and February, airboats were utilized to determine that potential wading bird colony locations were
not being utilized and nesting had not been initiated. March conditions were uncertain due to plane
crash and the deaths of the dedicated field crew. In April, formal aerial surveys were initiated where
two dedicated observers surveyed wading bird nests along specified aerial transects flying an altitude
of 244 m at a speed of 185 km/hr. One transect paralleled the eastern rim of the Lake from Eagle
Bay Island to the Clewiston Lock. Remaining transects were oriented East-West, spaced at an
interval of 3 km and traversed the littoral zone. Two observers searched for colonies, one from
each side of a Bell Jet Ranger 206 helicopter.
Once a colony had been confirmed, it was circled by
helicopter at a lower altitude of 122 m to enable researchers to count number of nests of each
species present within the colony as well as record photographs and geographic coordinates of the
colony itself. These were then mapped to specific stands of vegetation or islands onto 1 m
resolution digital orthoquarter quadrangles (DOQQ). Ground visits were not performed to
determine nesting success due to the lack of nests. Colonies were defined post-hoc as any
assemblage of ≥ 2 nests that were separated by ≥200 m (Erwin et al 1981, Smith and Collopy 1995).
Distances were calculated between colonies using ArcGIS.
In 2009, the surveys were conducted similarly as the prior year with minor differences.
Formal aerial surveys commenced in January, monthly, until June. Methodology remained the same,
but ground visits to accessible colonies were conducted to improve colony counts and species
composition. It should be noted, that even with the use of photographs and increased ground
surveys, it still remains difficult to accurately survey small dark colored wading birds which
ultimately reduces the overall accuracy of their nest numbers.
2
Figure 1. Map of systematic aerial transects flown over Lake Okeechobee during 2008 and 2009.
Hydrology
Rainfall and hydrology data were obtained from the South Florida Water Management
District’s DBHydro database. Lake stage is the mean of stage readings from the 4 gauges located in
the limnetic zone of the Lake (L001, L005, L006, and LZ40). Lake stages, reported as feet were
converted to meters. The National Geodetic Vertical Datum 1929 (NGVD1929) was converted to
North American Vertical Datum (NAVD1988) using VERTCON 2.0. Rainfall obtained from these
gauges was converted from inches to centimeters.
RESULTS AND DISCUSSION
Hydrology
3
6
4.5
5
4.0
4
3.5
3
3.0
2
2.5
1
2.0
Jan
daily rainfall cm
stage m NAVD88
5.0
0
Feb
Mar
Apr
May
Jun
Jul
2008
5.0
6
4.5
5
4.0
4
3.5
3
3.0
2
2.5
1
2.0
Jan
0
Feb
Mar
Apr
May
2009
Figure 2. Hydrographs depicting average daily rain and average lake stage for 2008 and 2009.
4
Jun
Jul
daily rainfall cm
stage m NAVD88
Stage m NAVD88
Daily Rain cm
In 2008, The Lake experienced both low water levels and poor recession rates, making it
very likely that hydrologic conditions led to the poor reproductive performance of wading birds.
Hydrologic patterns probably acted to reduce prey availability, but we cannot reject the hypothesis
that suitable colony sites were also limiting. Wading birds prefer to nest in trees that remain
surrounded by water throughout the nesting season and such places were scarce in 2008.
Water year 2009 was a year of extremes in terms of hydrology. It began with Tropical Storm
Fay raising the lake level almost two feet. And according to SFWMD, this was the driest dry season
and wettest May since records began. On 1 Jan 2009, the lake was at 4.3 m (14 ft). This dropped to
a low of 3.2 m (10.6 ft) on 18 May 2009 and then rose to 3.5 m (11.3 ft) by 31 May 2009 (Fig. 2).
Throughout the dry season, suitable habitat for foraging wading birds shifted across the littoral zone
toward the limnetic zone. By the end of this season, only the edge by the limnetic zone provided
foraging habitat. In June, many flight lines of wading birds from the Eagle Bay East Colony were
heading NW over the levee toward pocket marshes in more upland habitat (Marx and Gawlik 2006).
5.0
4.5
m NAVD88
4.0
3.5
3.0
2.5
2005
2006
2007
2008
2009
2.0
1.5
Jan
Feb
Mar
Apr
May
Jun
Day of Breeding Season
Figure 3. Hydrograph comparing the difference in lake stage during the breeding season from 2005-2009.
5
Jul
The relationship among hydrologic patterns, habitat, and wading bird nesting is becoming
better known (Marx and Gawlik 2006, Marx and Gawlik 2007a); however, a second year of low
water levels provided new information not evident during the first year of drought. By May of 2007,
the littoral zone was completely dry leaving only the open and unvegetated near shore and limnetic
zones as foraging habitat for wading birds (Marx and Gawlik 2007b). In 2008, the littoral zone
remained dry, but submerged vegetation was established in much of the near shore zone, probably
making it better foraging habitat than in 2007. This shift of vegetation down the elevation gradient
suggests that habitat conditions may start to improve after the first year of a lower water regime, be
it driven by reduced rain or management. However, we believe it is unlikely that near shore habitat
would ever improve to the level of the littoral zone marsh, even if it had ideal water depths and
recession rates. The elevation gradient in the near shore zone is steeper than the littoral zone marsh
and provides less area of suitable habitat when hydrologic conditions are ideal.
Location
Figure 4. Colony locations during 2008 and 2009 at Lake Okeechobee.
6
2008
In 2008, only 3 wading bird colonies were detected in the Lake Okeechobee area. Eagle Bay
Island was within The Lake proper, Lakeport was in the rim canal, and Gator Farm was outside the
lake at an alligator farm (Fig. 4). Surveys were no longer conducted at the Martin County Florida
Power and Light Reservoir because of security concerns by the company. Of the 3 colonies
detected in 2008, only the Gator Farm colony was active in 2007. The Eagle Bay Island colony was
active in 2006 and many other years, whereas the Lakeport colony may be a new colony location.
While we circled the colony, it was noticed that flight lines of birds in and out of the Lakeport
colony were to the West, suggesting that birds were foraging in wetlands outside the Lake.
2009
In 2009, eighteen colonies were located (Fig. 4), sixteen on-lake and two off-lake, at both
traditional and novel colony sites. With stage levels dropping rapidly and at 4.1 m by 1 March 2009,
colonies located in traditional littoral areas were avoided or abandoned. By the March flights, no
colony within the Moore Haven marsh was active. In contrast, the deeper Moonshine Bay
contained the largest multi-species colony. Rock Island, Clewiston Spit, and the Clewiston Channel
colonies were all active through the end of the dry season, presumably because of deeper water
surrounding these colonies. Colonies outside the levee, Lakeport and Gator Farm, were located on a
canal and pond, respectively, thereby providing permanent water around the colony regardless of
drying patterns.
With location comparisons against maps of historical nest sites (Smith et al 1995, David
1994a) and previous FAU monitoring flights, the locations within Moonshine Bay were not
traditionally used. With the lower water levels, locations may have been available for nesting
substrate that might be inaccessible with deeper water levels. Also, with the prime willow substrate
possibly selected against in the higher elevation marshes of Moore Haven because of lack of
inundation, cattails (Typha latifolia) were found to be used by Great Egrets, White Ibis, and Glossy
Ibis in Cochran’s Hole and Moonshine Bay. The colony at Cochran’s Hole was entirely in cattails,
while the largest colony, Moonshine Bay, consisted of willow heads surrounded by cattails. At
Moonshine Bay, ground visits confirmed that mostly White Ibis and Glossy Ibis were using the
cattail as substrate, while ardeid nests were found predominately in willows. At Eagle Bay East and
Eagle Bay Trail locations, bulrush (Scirpus spp.) was a major component in the nesting substrate at
the colonies for White Ibis and Glossy Ibis.
7
Size
2008
Table 1. Colony locations (NAD 83), individual colony counts of nests for each species, and maximum counts of each species of wading bird detected at Lake Okeechobee
during aerial surveys in the 2008 breeding season.
Geographic location
Colony
name
Eagle Bay
Island
Lakeport
Gator Farm
Latitude
Longitude
Survey date GBHE
27.18659
-80.83056
26.9726
-81.1144
27.02278
-81.06084
GREG
SNEG
TRHE
LBHE
WHIB
GLIB
WOST
CAEG
23-Apr-08
0
0
0
5
5
0
0
0
500
27-May-08
0
0
0
0
0
0
0
0
3500
23-Apr-08
0
0
0
0
0
0
0
0
27-May-08
0
1
0
0
0
0
0
20
2000
23-Apr-08
0
20
0
0
0
0
0
0
100
27-May-08
0
12
0
0
0
0
0
8
500
0
20
0
5
5
0
0
8
6000
Peak #
nests species
*Michael Cheek (SFWMD), surveyed the Gator Farm colony by helicopter 1 Jul 08 and counted 6 fledged juvenile WOST.
Table 2. Geographic coordinates (NAD 83) and species-specific peak nest efforts in detected colonies during the 2008 breeding season at Lake Okeechobee
ID
Peak Month¹
Lattitude
Longitude
GREG
GBHE
WHIB
SNEG
LBHE
TRHE
WOST
GLIB
ROSP
CAEG
ANHI
Lakeport Marina
5
MAY
26.97260
-81.11440
1
---
---
---
---
---
---
---
---
2000
---
92
Eagle Bay Island
11
APR
27.18659
-80.83056
---
---
---
---
5
5
---
---
---
3500
5
142
Gator Farm
3
APR
27.02278
-81.06084
20
---
---
---
---
---
8
---
---
500
---
205
Colony
Total¹
¹ Does not include CAEG or ANHI
² Species undetected during monthly survey effort
Season-wide nest effort for all wading birds peaked at 38 nests, excluding Cattle Egrets
(Table 3). Nest effort among Great Blue Herons, Great Egrets, Snowy Egrets, White Ibis, and
Glossy Ibis, the species that were consistently surveyed in the historic record (David 1994a), peaked
at 30 nests. This estimate makes 2008 the worst year on record for wading bird nesting at Lake
Okeechobee. Counts of those species from 1971, 1981, and 2007, previously the 3 worst years,
ranked slightly higher with 130, 520, and 550 nests, respectively.
2009
8
Table 3. Timing and nest effort for species breeding in wading bird colonies during 2009 at Lake
Okeechobee. Italics denote peak nest effort for species included in grand total.
Month
GREG GBHE WHIB SNEG LBHE TRHE WOST GLIB ROSP CAEG ANHI
Peak nest
effort¹
January
233
38
---²
---
---
---
---
---
---
---
15
271
February
329
10
---
---
---
---
30
---
---
---
10
369
March
325
5
500
1025
10
440
35
26
3
---
32
2369
April
228
18
2820
2967
17
961
25
2015
---
170
31
9051
May
166
1
1470
984
3
109
14
500
---
1198
13
3247
June
69
6
300
54
1
---
---
---
---
750
---
430
¹ Does not include CAEG or ANHI
² Species not detected during monthly survey
Table 4. Geographic coordinates (NAD 83) and species-specific peak nest efforts in detected colonies during the 2009 breeding season at Lake
Okeechobee
Longitude GREG GBHE WHIB SNEG LBHE TRHE WOST GLIB ROSP CAEG ANHI
Total¹
Colony
ID Peak Month¹
Lattitude
Clewiston Channel
12
APR
26.78420
-80.89423
50
---²
---
60
---
20
---
Clewiston Marsh
15
MAR
26.76599
-80.89796
30
---
---
320
---
20
---
Clewiston Spit
10
APR
26.77591
-80.90939
60
---
---
80
1
80
---
10
---
---
---
231
Cochran's Hole
9
APR
26.89073
-81.01295
40
---
500
---
---
---
---
500
---
---
---
1040
Eagle Bay East
16
MAY
27.17987
-80.83080
5
3
1200
800
10
80
---
300
---
---
---
2398
Eagle Bay South
7
JAN
27.17064
-80.84643
85
25
---
---
---
---
---
---
---
---
20
110
Eagle Bay Trail
11
APR
27.18659
-80.83056
10
2
20
90
10
10
---
---
---
50
5
142
Gator Farm
3
MAR
27.02278
-81.06084
130
---
---
40
---
35
---
---
200
---
205
Indian Prairie North
14
MAR
27.08482
-80.88620
50
---
150
400
---
400
---
---
3
---
30
1003
Lakeport Marina
5
APR
26.97260
-81.11440
12
---
---
70
---
10
---
---
---
350
---
92
Moonshine 2
17
APR
26.92233
-81.03053
2
4
---
18
---
4
---
---
---
---
6
28
Moonshine 3
19
APR
26.92755
-81.03479
9
---
---
320
---
40
---
500
---
---
8
869
Moonshine Bay
13
APR
26.91117
-81.02514
25
4
1300
1200
---
700
---
1000
---
---
---
4229
Moore Haven
1
FEB
26.88525
-81.09517
53
8
---
---
---
---
---
---
---
---
---
61
Moore Haven East2
8
JUN
26.88410
-81.05237
30
---
---
---
---
---
---
---
---
---
---
30
Moore Haven East3
2
JAN
26.86238
-81.00704
---
4
---
---
---
---
---
---
---
---
---
4
Moore Haven NW
4
FEB
26.95153
-81.05730
50
8
---
---
Rock Island 4
18
APR
26.97227
-81.03672
12
3
---
38
---
---
---
2
---
5
---
---
---
---
135
---
---
2
370
---
---
---
58
---
---
5
55
¹ Does not include CAEG or ANHI
² Species undetected during monthly survey effort
In 2009, the eighteen colonies (Fig. 4) contained 9185 estimated nests. This number was derived by
summing the peak nesting month for all species except for Anhingas and Cattle Egrets (Table 1).
For historical comparisons, one nest total was tallied for Great Egrets, Great Blue Herons, White
Ibises, and Snowy Egrets, and one for those species plus Glossy Ibises. These totals were 8169 and
6154 respectively. These nest counts ranked as the fourth highest since first aerial surveys began in
1957 by the Florida Audubon Society (David 1994a). However, we counted only 329 Great Egret
9
nests, which was 48% of the historic mean for all survey years. This was not surprising because
Great Egret numbers tend to be low when the January stage is <4 m (Fig. 5). Given that this year’s
January stage was 3.78 m, the numbers of Great Egret nests we observed matched what could be
expected for similar hydrology.
In this attempt to create links between factors affecting wading bird nest effort, Marx and
Gawlik (2007a) analyzed this link between hydrology and nest effort. More directly, they analyzed
the relationship between the effects of initial dry season lake stage, dry season recession, and the
interaction between both. The initial lake stage was determined as the median lake stage during the
month of January. Based on historic averages, the interquartile ranges for low, medium, and high
lake stages were < 3.77 m, 3.77-4.36 m, and > 4.36 m, respectively. It was shown that a medium
initial stage with an extended recession maximizes nest effort. The previous two years of 2008 and
2009 corroborated this model. In 2008, a total of 30 nests were attempted with a low initial lake
stage of 2.71 m and no extended recession. On the other hand, 2009 produced the fourth highest
nest effort with a medium lake stage of 3.78 m and an extremely dry year that produced an extended
recession that lasted until the middle of May. With limited inundated marsh, this limited the
carrying capacity of the Lake to provide optimal foraging conditions (Smith et al 1995). Although
this basic wading bird nesting model is currently useful for management, a further refinement under
a wider range of hydrologic conditions will provide more powerful and robust predictions.
3500
3000
GREG nest effort
2500
2000
1500
1000
500
0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Median January Lake Stage (m NAVD 888)
Figure 5. Regression of January lake stage on GREG nest effort. Hollow point is 2009.
10
Timing
2008
Prior to March, wading birds were not yet nesting. By the 23 April surveys, it was confirmed
that both herons and egrets were at the nesting/incubation stage while Wood Storks (Mycteria
americana) were still building nests. The 27 May surveys indicated that Wood Storks contained small
chicks. Great Egret nest numbers had a slight increase and small herons were no longer seen in
these colonies that these nests had failed. Typical for Cattle Egrets (Bubulcus ibis), the number of
their nests had increased from April to May, though these nest numbers were higher than observed
in past years.
2009
Aerial surveys conducted on 28 January 2009 confirmed nesting for Great Egrets and Great
Blue Herons at Moore Haven and Eagle Bay Island in the Lake as well as two locations outside of
the Lake; Lakeport and Gator Farm. February surveys showed Great Egret colonies had been
abandoned in Moore Haven, most likely due to the drying out of the marsh in this area. On 26
February, 3 Great Blue Heron and 3 Great Egret nests were found in Indian prairie.
By March, there was a large increase of small ardeids and ibis nesting on the Lake. During
this month, there was a discovery of 3 Roseate Spoonbill nests within the Indian Prairie colony (see
below). Moonshine Bay, a colony located in a lower elevation section of the Lake, had increased
nesting activity for Great Egrets, Snowy Egrets, White Ibis, Great Blue Herons, and Glossy Ibis.
This colony, being the largest colony at 4229 estimated nests, continued to be active with fledglings
during the last survey in June. East of Eagle Bay, nesting was confirmed during March and activity
continued to be seen in this area through the last survey in June.
Wood Storks and Roseate Spoonbills Reproductive Success
In 2008, a small colony (Gator Farm) containing Wood Storks and 2 other species developed
in cypress trees on an alligator farm about 4 km north of Harney Pond along Highway 721, in the
same location as the previous year. On 23 April, there were 22 birds perched in the colony and a
number of nest platforms already completed, but no eggs had been laid. By 27 May, there were 8
stork nests with small chicks, indicating that the adults must have laid eggs shortly after our April
survey. On July 1, Michael Cheek (SFWMD) surveyed the colony and noted 6 fledged juvenile
storks perched in trees and flying within the colony. This number is down from the 22 chicks that
are thought to have fledged the colony in 2007 (Marx and Gawlik 2007b).
11
In 2009, with improved hydrology the Gator Farm was once again detected. Storks were
present but not nesting on the Jan 28 survey; however 30 nests were evident by the 18 February
survey. A maximum of 35 nests were seen during March Surveys. By May, approximately 55 older
young were seen in the vicinity. By the June surveys, only 7 were left within the colony.
Another interesting development in 2009 was Roseate Spoonbill (Platalea ajaja) nests at the
Indian Prairie colony. Although Roseate Spoonbills were reported nesting at the Lake in 1874 (Oder
1874), we are not aware of any modern records of Roseate Spoonbills nests on Lake Okeechobee.
Ground visits on April 7 showed that one nest on the NW edge of the colony contained one
nestling and two eggs. The following week three young were present, but by 21 April the nest had
been abandoned. We did not track in detail the status of the other two Roseate Spoonbill nests
because the colony became inaccessible by airboat, but on 23 April they were no longer present and
were assumed to have failed.
Environmental Conditions
The location and timing of colonies during this study seemed to be related to stage at the
onset of the dry season and subsequent drying rate as in other years (Marx and Gawlik 2007a).
Receding water levels have shown to be beneficial for concentration of prey for wading bird
foraging (Kushlan 1976, Gawlik 2002), but initial lake levels are also important for submersion of
the littoral zone for initial prey production (Havens and Gawlik 2005). Although nesting occurred at
colonies located across most of the hydrologic gradient in the littoral zone, the most successful
colonies were those at the lower elevations that were surrounded by deeper water.
A number of nests this year were not in the typical willow vegetation, largely because willow
was absent from many colony locations. The smaller ardeids nested in phragmites at the Rock Island,
Clewiston Spit, and the Clewiston Channel colonies. Some of this vegetation was in areas that would
normally be unusable for nesting during higher lake stages. Cochran’s Hole, a colony initiated by
Great Egrets and later colonized by White Ibis, formed in substrate comprised of only cattails.
Small ardeids (2398 nests) nested in willow and bulrush east of the traditional Eagle Bay Island
colony site. Locations in Eagle Bay and Moore Haven, which had willow vegetation, were dry by
the end of February. Rock Islands and Clewiston Spit were missing woody vegetation due to
previous storms. Normally, years with drier conditions can help regenerate and expand the willows
that are most suiting for nesting on the Lake (David 1994a); however, severe fires during the recent
dry years may have reversed this expansion, at least temporarily.
12
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Waterbirds 17:69-77.
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13
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84-9. South Florida Water Management District, West Palm Beach, FL.
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