THE FOOD HABITS AND NESTING SUCCESS 

i OF WOOD STORKS 

IN EVERGLADES NATIONAL PARK IN 1974 

I. 

I 

I 

Natural ~esouices Report 1 

I Number 16 

U.S. Department of the Interior 

National Park Service 
THE FOOD HABITS AND NESTING SUCCESS OF k!OOD STORKS 

IN EVERGLADES MATIONAL PARK 1974 

JOHN C. OGDEN 

Research Department 

National Audubon Society 

Taverni er , Florida 

JAMES A. KUSHLAN 

U.S. National Park Service 

Everglades National Park 

Homestead, Florida 

JAMES T. TILMANT 

U.S. National Park Service 

~iscayne National Monument 

Homestead, Florida 

U.S. Department of the Interior, National Park Service 

Natural Resources Report Number 16 1978 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-005-00717-2 
ABSTRACT 

The Wood Stork (Igycteria americana) is a wading bird adapted to fluctuating water levels 

such as those found in the Everglades of Everglades National Park, The population of 

Everglades Wood Storks declined through the 1960s and they did not nest successfully from 

1967 through 1973. This paper describes a study conducted in 1974 on the factors affecting 

food supplies and successful nesting of Wood Storks in Everglades National Park. Since 

Wood Storks nested successfully in 1974, this study provided the first opportunity in 7 

years to obtain information necessary for the preservation of the species in Everglades 

National Park. As a result of this initial study, we reconunend areas of future research 

required for proper management of the Everglades ecosystem. 

Wood Storks began nesting in late January 1974 and the 2,000 nesting storks successfully 

reared 1,900 young. Since 1974 was a dry year, it continued the recent but unnatural 

pattern of storks nesting successfully in dry years, which began after the institution of 

water control in 1962. An important result of this year's nesting was that the time of 

nesting correlated with the rate of drying in the Everglades, substantiating a previously 

derived relation. The 1974 results proved that sufficient food can still be produced in 

the highly altered southern Florida environment to permit successful nesting of the remnant 

population of Everglades Wood Storks, although with loss of habitat and artificial impoundment of water, storks were forced to fly 130 Ian from the colony to feed young late in the 

nesting season. 

Preliminary information was obtained on the abundance of Wood Stork prey in the southern 

Everglades and estuarine areas of the park. The first substantial understanding of food 

habits of Everglades Wood Storks also was acquired through collection of food items from 

nestling and adult storks. The diet of storks was composed almost entirely of fish and 

only a few species made up most of the food consumed. Storks fed where food was relatively 

concentrated and selectively consumed the relatively larger fish of those available. 

The rapid population decline and changes in Wood Stork nesting success of recent years 

coincided with the institution of water control. Analyzing these changes, as well as deriving the proper management to preserve the natural Everglades ecosystem are complicated by 

diversion of surface water to the western part of the southern Everglades rather thh 

through the natural drainage basin. 

Cover photo Wood Stork in Typical Feeding Posture by Frank Mazzotti 

As the Nation's principal conservation agency, the Department of 

the Interior has responsibility for most of our nationally owned 

public lands and natural resources. This includes fostering the 

wisest use of our land and water resources, protecting our fish 

and wildlife, preserving the environmental and cultural values 

of our national parks and historical places, and providing for 

the enjoyment of life through outdoor recreation. The Department 

assesses our energy and mine~al resources and works to assure 

that their development is in the best interests of all our 

people. The Department also has a major responsibility for 

American Indian reservation cononunities and for people who live 

in Island TerritoKes under U.S. achzinistration. 
REFERENCES 

IVLEV, V. S. 1961. Experimental Ecology of the Feeding of Fishes. Yale Univ. Press, 

New Haven, Conn, 302 p. 

KAHL, M. P. 1962. Bioenergetic of growth in nestling Wood Storks. Condor 64:169-183. 

. 1963. Food ecology of the Wood Stork in Florida: A study of behavioral and 

physiological adaptations to seasonal drought. Ph.D. Thesis, Univ. Georgia, Athens. 

. 1964. Food ecology of the Wood Stork (Mycteria arnericana) in Florida. Ecoz. 

Monogr. 34:97-117. 

, and L. J. PEACOCK. 1963. The bill-snap reflex: A feeding mechanism in the 

American Wood Stork. Nature 199:505-506. 

KUSHLAN, J. A. 1974. Observations on the role of the American Alligator (AIZigator 

mississippiensis) in the southern Florida wetlands. Copeia 1974:993-996. 

. 1976. Wading bird predation in a seasonally fluctuating pond. Auk 93:464-476. , J. C. OGDEN, and A. L. HIGER. 1975. Relation of variation of water level and fish 

availability to Wood Stork reproduction in the southern Everglades, Florida. U.S. 

Geological Survey, Tallahassee, Florida. Open-file Rept. 75-434. 56 p. 

OGDEN, J. C., J. A. KUSHLAN, and J. T. TILMANT. 1976. Prey selectivity by Everglades 

Wood Storks. Condor 78 :324-330. 

PALMER, R. S. 1962. Handbook of North American Birds, Vol 1. Yale Univ. Press, New 

Haven, Conn. 

ACKNOWLEDGMENTS 

This study was supported by the U.S. National Park Service. 

We thank Dr. William B. Robertson, Jr., Gary E. Davis, and L. 

Lee Purkerson for comments on and assistance in setting up the 

study. William Robertson, Frank Nix, Gary Davis, Roy Johnson, . 

Thomas Schmidt, Ronald Hermance, and Charles Hilsenbeck reviewed 

and commented on the report. Marilyn Kushlan drew the figures. 

We also thank the Ranger Division of Everglades National Park, 

particularly Mike Warren, Fred Dayhoff, Stan Robbins, and 

Fred Whiteheaqi, for assistance. 

Natural Resources Report No. 16 



THE FOOD HABITS AND NESTING SUCCESS OF WOOD STORKS 

IN EVERGLADES NATIONAL PARK IN 1974 

INTRODUCTION 

This paper is a report on a study of the 

ecology of the Wood Stork (Mycteria wnericana) in Everglades National Park, conducted from September 1973 to June 1974. The 

Wood Stork is an important link in the food 

chains of the Everglades and estuarine 

ecosystems of southern Florida. It is 

adapted to the fluctuating water conditions 

which typify southern Florida (Kahl 1964) . Its nonvisual feeding behavior (Kahl and 

Peacock 1963) requires high prey density 

(Kahl 1964). The rapid decrease in population over the past decades suggests that it 

is the wading bird species most susceptible 

to adverse alteration of natural ecological 

conditions. Because of wading birds' ecological importance, historically large 

populations, and adaptations to fluctuating 

water marshlands, understanding the ecology 

of the Wood Stork and of other less sensitive species is necessary for the management and preservation of the natural Everglades ecosystem. 

The breeding status of the Wood Stork in 

Everglades National Park was especially 

critical at the start of this study. The 

last previous successful nesting season was 

1967, and during the 1960s and early 1970s 

the breeding population fell from approximately 5,000 to 2,000 birds. Thus, the 

successful 1974 nesting season provided the 

first opportunity in 7 years to gather information necessary for the preservation of 

this species in Everglades National Park. 

The background of the current program 

was a report by Kushlan et al. (1975) which 

attempted to discern historic relationships 

between surface water changes, fish populations, and the nesting success of Wood 

Storks in Everglades National Park. The 

results showed that before the completion 

of the levee system north of the park in 

1962, Wood Storks nested successfully in 

those years of high water levels, high surface water discharge, and high rates of 

drying late in the nesting season. Kushlan 

et al. (1975) concluded that after 1962 the 

above pattern of water fluctuation no longer permitted successful nesting in the 

present, highly altered system and recognized that future management must be devised within the constraints imposed by the 

altered system. Considering information 

after 1962, Kushlan et al. (1975) found 

that nesting failed primarily because of 

desertion, which correlated with rising 

water levels, either a small water level 

rise early in the nesting season or a large 

rise at the beginning of the rainy season. 

They also found that the latter type of 
desertion resulted from late nesting and 

that, after 1962, early nesting occurred 

with rapid drying rates early in the dry 

season. They noted that since nesting by 

Wood Storks would be initiated and sustained only through adequate food supplies, 

the key to nesting success must lie in the 

production and timely availability of fish, 

about which little was hown. Kushlan et 

a1 . (1975) hypothesized that freshwater 

fish may move long distances as water levels fell, emigrating ahead of progressively 

drying conditions and thereby increasing 

population densities and availability to 

Wood Storks. 

In the present study, we attempted to 

further investigate the findings and test 

hypotheses that reducing surface water discharge into the southern Everglades, known 

as Shark River Slough, would increase drying rates and bring about earlier nesting 

of Wood Storks. To accomplish this, it was 

proposed that gates in the two eastern 

spillways that release water into the park 

be closed in November and December and 

water be diverted through two western spillways. Discharge through the western spillways apparently has little effect on the 

slough since it drains mostly to the southwest (F. A. Nix pers. comm.) . 

STUDY AREA 

Everglades National Park is located on 

the subtropical southern tip of Florida 

(Fig. 1). The central feature and primary 

watershed of the park are the freshwater 

marshes of the southern Everglades, known 

as Shark River Slough. A dominant factor 

within the Everglades ecosystem is a seasonal rainfall pattern and the resulting 

seasonal fluctuation of water levels. 

Water levels are high in summer and fall 

and drop through the winter and spring. 

FIGURE 1. Map of southern Florida. 

National Park Service 
1 

----- 

ging Stations 

r Lake Site V Rainf all stat ions 

OPermanent fish trap sites 

AFish samples at stork 

feeding sites 

Feeding site where stork 

regurgitated 

A Feeding site where both 

fish sample & regurgitation 

obtained 

Stork rookery, 1974 

0 Unused rookery site 

FIGURE 2. ~ocation of fish samples and other sites in southern Florida. 

The Everglades is composed primarily of Everglades was impounded by levees to form 

sawgrass marsh and marsh prairie. Water three Water Conservation Areas (Fig. 1) and 

from the Everglades flows into the estuar- surface water discharge into the southern 

ies where mangrove swamps and coastal flats Everglades became controlled. Water is presare the predominant habitats. The natural ently discharged into the park through spillEverglades extended from Lake Okeechobee ways (called the S-12 structures) on the 

to the mangrove smrfps of the southwestern northern park boundary (Fig. 2). Water from 

coast. In the early 1960s the northern the two eastern structures is conducted into 

' Natural Resources Report No. 16 3 
the Shark River Slough through the marsh 

and along a canal (L-67 Canal) on the eastern park boundary (Fig. 2) . 

METHODS 

Hydrologic data were collected by the 

U.S. Geological Survey (USGS) and park personnel. Water levels were measured at reference station P-33 (Fig. 2) and at staff 

gage stations along a transect down Shark 

River Slough (Fig. 2). Discharge was measured periodically at S-12 water control 

structures along the northern park boundary 

and total discharge through all the structures was then calculated by the USGS. 

Drying rate is defined as the rate of water 

level change from the highest water level 

in one month to the lowest level 3 months 

later as measured at reference station P-33 

(see Kushlan et al. 1975 for details). 

Two long-term fish trap sites used by 

the USGS from 1965-72 were reestablished. 

Monthly fish samples were taken with two 

4.6 m2 pull-up traps from November until 

the site dried in May. Our procedures followed those of the previous study (Kushlan 

et al. 1975) except that nets were left 

submerged in the water rather than exposed 

to air during the period between samples, 

and sampling was done during the day rather 

than at night. The latter change was justified because statistical analyses of data 

collected by the USGS showed no significant 

differences were detectable in numbers, 

weights, or kinds of fish caught between 

day and night trapping. 

Nine other fish-trapping stations were 

also established along a transect down 

Shark River Slough and sanrpled at monthly 

intervals (Fig. 2). All but one were 

located next to permanent staff gage stations. Elonthly fish samples along this 

transect were taken with 1-m2 drop-traps or 

1-m2 throw-traps, unless a st'ation was dry. 

Fish sampling stations also were established 

at three sites on Cape Sable (Fig. 2). 

Food of Wood Storks was determined by 

obtaining regurgitation samples from nestlings in colonies and from adults at feeding 

sites. Nestling storks regurgitate readily 

when handled. We found that when we used a 

helicopter to fly close to feeding adult 

storks, they would regurgitate food onto 

the ground before flying away. We then 

landed to collect the regurgitation and took 

several drop-trap or throw-trap samples of 

fish at the same place where regurgitation 

samples were obtained (Fig. 2). Thus we 

were able to compare the species and sizes 

of fish available to those taken by storks. 

The data collected at feeding sites were 

combined for analysis into three habitats: 

coastal, mangrove headwaters, and Everglades. 

Fishes collected during the study were 

preserved and later identified, counted, 

and measured for total length and dry 

weight. Aerial surveys, conducted two to 

four times per month, were used to locate 

feeding storks, follow storks from colony 

sites to feeding grounds, observe nesting 

colonies, census nests, and monitor stage 

of nesting. More detailed observations on 

the development of young, number of young 

in nests, survival of nestlings, and daily 

activity of adults were obtained by ground 

visits to the rookeries. 

In addition to other aspects of the 

Wood Stork study, a color-marking and banding program was begun to obtain information 

on movements of storks outside the park and 

to gain vital information on mortality of 

nestlings and young. 

National Park Service 
RESULTS AND DISCUSSION 

Hydrologic Conditions 

During the 1974 hydrobiological year, 

1 June 1973 through 31 May 1974, rainfall 

was 15% below normal at the Everglades reference station but 30% above normal at the 

coastal reference station (locations in 

Fig. 2). Figure 3 indicates the above normal rainfall for the year on the coast was 

due primarily to high rainfall in August 

and December. December was also a month of 

above normal rainfall in the Everglades. 

Rainfall between January and May was well 

below normal at both locations, only 19% of 

normal in the Everglades, and only 39% of 

normal along the coast. Thus, despite 

periods of high rainfall, the 1974 hydrobiological year was characterized, overall, 

by below normal rainfall. 

Monthly calculated water discharge 

through the control structures into the 

park fell below the minimum annually scheduled amount from October through January. 

These low discharges were caused by low 

rainfall, gate manipulation, and nonacceptance by the park of makeup discharge from 

the U.S. Corps of Engineers. The two easternmost control structures were kept closed 

for 25 days in November and December resulting in only 81% of the minimum schedule 

discharge during that period. 

Reporting the total discharge through 

all 5-12 structures, as is presently cal- ,/ 

culated by the USGS (and presented in Fig. 

4), does not show differences among individual spillways. Such information is 

critical because flows through different 

structures could have different impacts 

on Shark River Slough. Total flow cannot 

be used to show all effects of gate manipulation, and data on discharge through 

individual structures were not available. 

In an attempt to estimate individual flows, 

semimonthly USGS measurements of flow 

through each structure were used to determine proportional discharge at each structure. We applied these proportions to the 

total daily discharge calculated by the 

USGS and thus obtained an estimate of discharge through each structure. This estimate is especially inaccurate for periods 

when more than one gate change occurred 

!\ 

I \ / \f-- Coast 

J J A S 0 N D J FM A M 

FIGURE 3. Percentage of normal rainfall each month in the Everglades and on the coast 

June 1973 to May 1974. 

Natural Resources Report No. 16 5 
nimum 

ount : 

scheduled 

received 

JJASONDJFMAM 

FIGURE 4. Total discharge through S-12 spillways in 1973-74. 

between semimonthly measurements. 

Our calculated discharge through each 

S-12 structure from October through 

December 1973 is shown in Fig. 5. During 

the November-December manipulation, flow 

through S-12C ceased while flows through 

S-12A and B increased. S-12D remained 

closed. Thus, the below-minimum discharge 

of November and December was released 

through structures A and B and presumably 

most of it did not enter the slough. 

Water level fluctuations monitored at 

the Shark River Slough gaging station P-33 

(Fig. 6) declined rapid1 y through November, 

rose in December, and fell at an increasing 

rate through spring. Water at P-33 reached 

ground level in April and fell to a low of 

0.4 m (3.0 ft) above mean sea level (msl) 

in May. Water level remained below ground 

surface until mid-June. 

6 National Park Service 
No conclusion is possible as to whether 

the short period of gate manipulation had 

any effect on the early drying rate. As 

noted above, discharge from October through 

January was below schedule, particularly so 

in November because of the manipulation 

(Fig. 4). Rainfall was also below normal 

in October and November (Fig. 3) and during 

this period water level fell rapidly. However, the high December rainfall (Fig. 3) 

raised water levels which decreased the 

overall November-January drying rate. 

Therefore, in 1974 below schedule discharges were not in themselves enough to 

provide a very high drying rate. The early 

drying rate (November-January) at P-33 was 

FIGURE 5. isc charges through each S-12 spillway from September through December 1973. 

(S-12~ was closed with no discharge during this period.) 

Natural Resources Report No. 16 
was 0.14 &day; the late drying rate 

(January-March) was similar at 0.15 cm/day. 

Some information also was gathered on 

changes in water levels elsewhere in the 

slough. Figure 7 shows, to the best of current information, the general pattern of 

drying in the southern Everglades. In 

general, the slough first dried around its 

middle and then toward the northeast and 

southwest from this area. As the dry season progressed, these two tongues of water 

retracted until May when the remaining 

pools were northeast of Rookery Branch and 

southwest of the L-67 extension canal 

where water was continuously replenished 

through the spring by discharge through 

S-12D. This general pattern was derived 

from intermittent aerial surveys and is 

not precise in many details. More de2 

tailed information is required on the important questions of how the slough dried. 

Two of the present study results are of 

interest in respect to the analyses and 

conclusions of KusNan et al. (1975). 

First, drying rates observed are similar 

to those that, before 1962, were associated 

with failure of Wood Stork nesting. Successful stork nesting during the study year 

supports the conclusion that the Everglades 

system has changed in recent years. Second, 

the suggestion that early drying rate may 

be correlated with time of colony formation 

is strongly supported by the events of1974. 

The regression derived by Kushlan et al. 

(1975) predicts that a drying rate of 0.14 

cm/day would be associated with colony 

formation around 26 January, which is precisely what happened (see Nesting Success). 

I I I I I I I I I i 

J J A S 0 N DJFMA MJ 

FIGURE 6. Water level changes at Everglades gaging station, P-33 from June 1973 through 

June 1974. 

8 National Park Service 
FIGURE 7. Pattern of surficial drying in Shark River Slough, 1973-74. 

Natural Resources Report No. 16 
Movement Patterns and Feeding Areas 

Most storks arrived in the park from 

northern summering areas between early 

November and early December 1973. Storks 

first concentrated on coastal feeding sites 

on the west coast and Cape Sable (Fig. 8). 

A maximum count of 1,568 birds on 4 December was nearly the exact count expected on 

aerial censuses based on estimated 1973 

production and mortality. Based on nest 

counts, the 1974 breeding population was 

2,000 storks (1,000 pairs). 

Storks continued to feed in typical 

sites along the coast through December, 

with a gradual movement into mangrove headwaters. Storks in breeding condition were 

first observed in early December, but the 

unusually heavy rainfall and subsequent 

water level rise of that month were correlated with a dispersal of birds that 

apparently delayed rookery formation. Only 

890 storks were counted on the early 

January survey and these were widely dispersed. By 24 January, storks had again 

concentrated at coastal feeding sites and 

two of the three rookeries were formed. 

During February and March, high numbers 

of storks were feeding in lower Conservation Area 3A, with lesser numbers at feeding locations in mangrove headwaters and 

in Shark River Slough. Their preference 

for feeding in the Conservation Area 

rather than in the mangroves is not clear. 

At that time, we sampled higher densities 

of important prey species in drying creeks 

along mangrove fringes, particularly near 

Rookery Branch, than in Conservation Area 

3A, and storks using the Conservation Area 

had to fly 50 km, many spending the night 

at the feeding area. 

By early April, Conservation Area 3 was 

almost completely dry, and the major concentration of feeding storks shifted to the 

drying lower end of the Shark River Slough 

and adjacent mangrove fringes. A drying 

pool in Upper Shark ~iver Slough was also 

used by feeding storks. 

By May nearly all of the Shark River 

Slough and most of the mangrove areas had 

dried. ~ew' feeding locations were apparently available in the park and storks made 

extremely longiflights to feeding grounds 

well north of the park. Adult storks only 

returned to rookeries to feed young, remaining there an hour or less. Mid-May 

surveys located feeding storks in Conservation Area 2 (Fig. 1), 130 km north of the 

colonies, and lesser numbers on drying 

ponds and in mangrove swamps northeast of 

Everglades City, 75 km from the colonies. 

Nesting Success 

Three nesting colonies were established 

in the park in 1974 (Fig. 2). Lane River 

and East River rookeries formed between 15 

and 24 January; Madeira Rookery formed 

during the first week of February. There 

were approximately 250 nests at Madeira 

(50 of which were deserted during incubation in late April), 150 at East River, 

and 600 at Lane River. A total of approximately 950 nests produced young. No storks 

nested on Cuthbert Island, an island often 

used by nesting storks in past years. East 

River Rookery has been much larger in past 

years. Storks and egrets, which park files 

show formerly nested at East River, are 

apparently shifting to the growing Lane 

River site, and it is possible that a 

similar shift has occurred in recent years 

from Cuthbert to Madeira. 

.Madeira was the most synchronized rookery, forming last and finishing by 27 May. 

Both East River and Lane River rookeries 

Nati'orial Park Service 
were relatively unsynchronized, with egglaying at Lane River occurring over a 5- 

week period. It is not known why the synchrony differed in the colonies, but it 

may be the result of late-nesting birds 

merely being attracted to the largest 

colony. Fledging young remained in East 

River until the second week of June and 

in Lane River until the third week of 

June. 

Counts early in the nesting season 

showed an average of 2.5 young per nest. 

Survival during the first month after 

hatching was not documented. Exact counts 

of older young that died prior to fledging 

showed that these losses were 0.25 young 

per nest at Madeira, 0.20 young per nest 

at East River, and 0.18 young per nest at 

Lane River. Overall, adults fledged about 

2.0 young per nest, producing about 1,900 

juveniles. 

This production is a substantial recruitment to the previously decreasing 

population of Everglades Wood Storks. If 

nesting is considered at least marginally 

successful when production is sufficient 

to permit a stable population from one 

nesting season to the nest, 1974 was the 

first successful nesting in 7 years and 

only the third successful season since 

the institution of surface water control 

in 1962. A question of concern is the 

mortality of these young over the 6 months 

following fledging. If late nesting and 

a presumably lowered food availability 

during the last month of nesting resulted 

in the production of suboptimally nourished young, mortality could be severe. 

If normal mortality occurs, presently 

estimated to be 40% during the first year 

(Palmer 1962), aerial censuses of birds 

retuning in the winter of 1974-75 should 

locate about 2,300 birds. A much lower 

return would suggest higher post-fledging 

mortality. 

Three hundred nestlings were banded, 

tagged with red nasal saddles, and sprayed 

on the wing with yellow-gold picric acid 

dye. Five reports of marked birds were 

received later. The two most distant 

recoveries were one marked juvenile found 

dead in early June at Sebastian Inlet near 

Pelican Island National Wildlife Refuge in 

Brevard County, Florida, and one found 

alive on 1 July near Ft. Myers, Florida. 

Everglades Wood Storks nested later in 

1974 than any previous successful year and 

near the limit of having sufficient time 

to complete the nesting cycle. Nesting 

was completed successfully because of a 

late start of the wet season and an unusually slow rise in water level. 

During most of the season, nestling 

storks appeared to grow normally and in 

many cases the usual brood of two to three 

birds survived until late in the nestling 

period. By 14 May, however, 7 nestlings 

33-39 days old averaged 35% below the expected weight from Kahl's (1962) study, 

20 nestlings 40-44 days old averaged 27% 

below expected weight, and 6 nestlings 

45-47 days old averaged 20% below expected 

weight. It is believed nestling weights 

were subnormal late in the season because 

of lowered availability of food supplies 

as water levels began to rise. 

The discovery that storks continued to 

forage and feed young despite slowly rising water levels in late May and June, 

and that they traveled over 130 lan from 

the colony to find remaining pools of 

water is extremely important. A late 

start of the rainy season and the advanced 

stage of nesting in early summer made 

Natural Resources Report No. 16 
December 20 

January 9 & 15 

FIGURE 8. Feeding sites of Everglades Wood Storks in 1973-74. Each dot represents t 

five storks. 

12 National Park Service 
Natural Resources Report No. 16 
continued nesting possible. The eventual 

success of nesting proves that sufficient 

food can still be produced in the highly 

altered south Florida environment to permit 

reproduction. Prior to this study, it was 

suspected that this might not be the case. 

Perhaps it is not so much the food supply 

that runs out at the end of the dry season, 

but time in which to locate it. It remains 

probable, however, that the overall fish 

production in the southern Florida ecosystem had declined in recent years due, 

at least, to loss of habitat. Before 

1962, successful stork nesting occurred 

in high water years, suggesting that there 

was a greater total biomass of fish available then than now, assuming that similar 

fish densities were necessary for stork 

feeding under such high water conditions. 

Fish Populations 

Reestablishment of previous USGS monitoring stations provided an opportunity 

to compare fish densities in 1974 to those 

from the period of 1965-72. Figure 9 

shows changes in fish populations in the 

Everglades as indexed by the two monitoring sites. Populations of fish and prawns 

were generally high in 1973-74. Maximum 

recorded fish density was higher than that 

of 1967, the last year when Wood Storks 

nested successfully. 

Fish densities in other parts of the 

slough and on Cape Sable were measured 

monthly at the permanent drop-trap stations shown in Fig. 2. Two trappings per 

month at Cape Sable sites were too variable to statistically detect small changes 

in density. With a mean density of 16.8 

fish/m2, there were no significant differences in fish density at the three Cape 

Sable sites (analysis of variance, P>0.05). 

Similarly, no significant differences were 

discerned among trappings in Shark River 

Slough during November and December (analysis of variance, ~>0.05). This indicates 

that within the resolution of the trapping 

technique fish were distributed homogeneously throughout the slough prior to January, 

except for station 17% which had higher 

densities (studentized range test, R0.05) , 

probably because of its location on the 

edge of a stream. Excluding station 174, 

the average density in the slough during 

this period was 6.9 f ish/m2. 

Changes began to appear in the slough in 

January (Fig. 10). Average January fish 

density of 9.0 fish/m2 in the upper slough 

was similar to previous densities. However, 

density in the lower slough had decreased 

significantly to 3.6 fish/m2 (comparisons 

by t-test, P<0.05). This difference continued through February. No samples were 

taken in March. By April, the lower slough 

had dried, and densities at the two intermediate stations in the upper slough were 

higher than previous levels while the other 

two still-flooded stations were unchanged. 

The high mean values of 71.0 and 35.3 fish/ 

m2 for the intermediate stations could only 

have come about by movement of fish into 

these trap sites. All trap sites were dry 

in May. 

The sampling program in the Shark River 

Slough detected increases in two stations 

late in the dry season, which reflect fish 

movement, but was not sufficiently sensitive to detect changes at lower fish density. This shortcoming and lack of sampling in March preclude any quantitative 

statement on fish movements in the slough. 

In addition, it should be noted that the 

pattern of drying in the slough (Fig. 7) 

suggests that fish movement is much more 

National Park Service 
ALL FISH AND F'RAWNS 

FIGURE 9. Long-term record of fish populations in Shark River Slough showing how 

conditions during the current study 1973-74 compared to previous years. 

complex than would be shown adequately by 

a single transect. The program (which, because of few previous attempts at this sort 

of study, must be considered a preliminary 

one) did show that the methodology used can 

detect tenfold changes in density. However, 

detecting differences among the low densities of fish that prevail in the Everglades 

for most of the year will require the development of more sensitive sampling procedures. 

Prey Density 

There was a sharp contrast between fish 

densities at permanent sampling sites and 

at locations where storks actually fed. 

Both in the slough and on Cape Sable fish 

densities were higher at locations where 

Natural Resources Report No. 16 15 
UPPER SLOUGH LOWER SLOUGH 

January 

20 1 I February 

April 

L-67 2 5 7 9 P-33 14 16 17s 

FIGURE 10. Fish density at stations on northeast to southeast transect down Shark River 

Slough ('Fig. 2). Means are plotted. Ranges are indicated by vertical lines. 

storks fed (t-tests, p<O.t~j). Along the 

coast, average density was 40 fish/m2 as 

contrasted with 16.8 fish/m2 at permanent 

trap sites. In the Everglades, average 

density at feeding sites was 141 fish/m2 

as contrasted with 10.3 fish/m2 at the 

permanent sites. These data of fish density quantify and extend to the Everglades 

and coastal habitats Kahl's (1964) finding 

that storks in the Big Cypress Swamp feed 

on high densities of fish and, furthemre, 

show that storks foraged in locations where 

fish densities were relatively high. 

A notable exception occurred in the 

northern Everglades. Fish densities at 

feeding sites in Conservation Area 3A were 

lower than in Everglades marshes within 

the park (means 12.2 vs . 144.5 fish/m2) . 

16 National Park Service 
Despite this, there was heavy use of Area 

3A by storks. More information from the 

Conservation Area is essential, especially 

since storks attracted to this area in 

March were passing up high concentrations 

of fish closer to the colony. Although 

there were no significant differences in 

the mean fish densities among coastal, mangrove, and Everglades feeding sites (t-test, 

Do. 05) , the trend of increasing mean density 48.8, 82.9, and 144.5 fish/m2, respectively, suggests that storks fed on higher 

densities later in the season. 

~ood Studies 

Little was known previously about food 

consumed by Wood Storks in southern Florida 

or elsewhere. Kahl (1963), who surnnarized 

all existing information on food of Wood 

Storks, reported on stomach contents of 

seven southern Florida storks that had been 

examined, including two he collected in the 

Big Cypress, one found dead in the Big 

Cypress, and four collected at Cape Sable 

Gator Lake in 1924. All seven contained 

only fish. Thus, other than four 50-year 

old specimens from Gator Lake, no data was 

available on food of Everglades Wood Storks. 

Previously, our understanding of foraging 

relationships of Everglades storks was 

extrapolated from Kahl's study. Samples of 

adult and nestling regurgitation and data 

on fish availability collected in this 

study provide important information on this 

previously little known aspect of stork 

ecology. 

Table 1 lists the prey taken by Wood 

Storks in three foraging areas and food 

brought to nestlings at two colonies. (All 

scientific names of prey are also shown in 

the table.) Fish comprised practically all 

of the Wood Stork's diet in extreme southNatural Resources Report No. 16 

ern Florida. Overall, 27 species of fish 

were consumed by Wood Storks. However, 

only a relatively few kinds of fish made 

up most of the total. Flagfish, sailfin 

mollies , marsh killifish, and the combined 

species of sunfish accounted for 83% of the 

individuals and 72% of the biomass. One 

additional species, the yellow bullhead 

which made up less than 2% of the individuals, comprised 12% of the biomass. Thus, 

these five groups of fish made up 85% of 

the number and 84% of the biomass of fish 

consumed by Wood Storks. 

The only vertebrates other than fishes 

were one red-spotted newt, three tadpoles, 

and one adult frog. Seven freshwater 

prawns were found among the 3,000 food 

items analyzed. This is surprising in 

view of' the extemely high densities 

attained by this minnow-size$ crustacean, 

as can be seen by comparing the two graphs 

of Fig. 9, one of which includes the 

prawn. Even though densities of prawns 

at stork feeding sites reached 1,242 

prawns/m2, they were not taken in any 

quantity. It is highly probable that 

most of the prawns were ingested secondarily by storks because prawns are also 

eaten by fish, especially sunfish, and 

some of the specimens were found in the 

mouth of such fish disgorged by storks. 

Collections of regurgitation from nestling storks were made at Lane River Rookery on 17-18 April and at Madeira Rookery 

on 24 April. Food brought to nestlings 

was similar to the two colony sites studied (Table I), with two notable exceptions. 

Sailfin mollies were more important at 

Lane River. There was a significant' dif - 

ference in the size of food brought to 

young at the ;two rookeries (t-test, P<0.05). 

Fish from Madeira Rookery averaged 4.1 cm 

17 
P 

m TABLE 1. PREY CONSUMPTION BY WOOD STORKS 

Percentage of Prey Items Percentage of Biomass 

Madeira Lane River All Madeira Lane River All Coastal Mangrove Everglades Rookery Rookery Samples Coastal Mangrove Everglades Rookery Rookery Samples 

Fish 

Florida gar Lepisosteus pZatyrkincus 0 0 0.4 0 0.4 0.2 0 0 9.2 0 3.2 2.8 

Bowfin Amia calva 0 0 0 0 0.1 cO.1 0 0 0 0 0.3 0.1 

Chain pickerel Esox niger 0 0 0 0 0.1 cO.1 0 0 0 0 0.3 0.1 

Golden shiner Notemigonus crysoteucas 0 0 0 0 0.3 ~0.1 0 0 0 0 0.4 0.2 

Taillight shiner Notropis macuZatus 0 0.4 0 cO.1 0.4 0.2 0 0.2 0 <0.1 <0.1 <0.1 

Yellow bullhead IctaZurms netulis 0 0 1.8 0.8 4.2 1.7 0 0 8.5 11.5 16.4 11.8 

Tadpole madtom Noturus gyrinus 0 0.4 0 0.3 0.3 0.2 0 0.2 0 0.2 0.1 0.1 

Sheepshead minnow Cyprinodon variegatus 18.1 6.7 2.0 0.5 0.7 4.1 18.2 4.8 1.0 0.2 <0.1 2.7 

Golden topminnow FunduZus ckrysotus ' 0 0 1.5 1.7 1.8 1.3 0 0 1.6 1.8 0.4 0.8 

Marsh killifish FunduZus confluentus 7.5 28.0 25.1 18.4 17.4 18.0 8.9 31.7 22.0 14.9 5.2 10.7 

Seminole killifish FunduZus seminozis 2.7 1.8 0 0.3 0.1 0.7 15.2 22.6 0 0.8 0.4 3.1 

Flagfish JordanelZa floridae 0 22.7 46.2 41.1 34.0 32.0 0 9.3 11.0 15.6 4.0 7.0 

Bluefin killifish Lucania goodei 

Rainwater killifish Lucania pama 

Rivulus RivuZus marmoratus 0.2 0 0 0 0 0.1 0.1 0 0 0 0 cO.1 

Mosquitofish Gambusia affinis 16.4 3.1 2.8 4.7 4.8 6.3 0.7 0.5 0.2 0.7 0.4 0.5 

Least killifish Heterandria formosa 0 0 1.8 0.6 0.1 0.5 0 0 <O. 1 c0. 1 cO.1 <0.1 

Sailfin molly PoeciZZia latipinnu 54.4 23.1 8.3 21.4 1.8 19.8 50.9 , 14.0 5.4 12.9 0.5 10.6 

Brook silverside Labidesthes siccuZus 0 0 0 0 0.3 0.1 0 0 0 0 <0.1 <0.1 

Tidewater silverside Menidia beryllim 0 0.4 0 0 0 <O. 1 0 0.2 0 0 0 <0.1 

,%?fish .- 

Bluespotted sunfish Ennescanthus gloriosus o 0 0.8 0.2 2.2 0.8 0 0 0.9 0.3 1.5 0.9 

llarmouth Lepomis quZosus 0 3.1 2.5 2.1 12.3 4.8 0 11.6 21.9 21.5 38.8 27.2 

Bluegill Lepomis macrockirus 0 0 0 0 1.1 0.3 0 0 0 0 1.4 0.7 

Redear sunfish Lepomis microlopkus 0 0 0 0.6 7.2 2.3 0 0 0 3.1 9.4 5.4 

Spotted sunfish Lepomis punctatus 0 0 4.5 0.8 6.7 2.8 0 0 17.3 8.7 9.3 8.7 

Unidentified sunfish Lepomis spp. 0 4.4 1.5 3.4 2.7 2.5 0 4.0 1.0 2.3 0.6 1.0 

rt 

P. Total Sunfish 0 0 7.5 9.3 7.1 32.2 13.5 0 15.6 40.2 35 ..6 61.0 43.9 

3 

Iil Largemouth bass Micropterus salmoides 0 0 0 0.2 1.0 0.3 0 0 0 3.6 7.2 4.4 P 

White mullet MugiZ curema 0.4 0 0 0 0 0.1 6.0 0 0 0 0 0.8 

e~ther 

Freshwater prawn PaZaemonetes paZudosus o 0.4 0.2 0 0.3 0.2 0 ~0.1 ~0.1 0 cO.1 <0.1 vl 

n, Red-spotted newt DiemictyLus viridescens 0 0.4 0 0 0 c0.1 0 0.1 0 0 0 0.1 

$. Bullfrog Rana grylio 0 0 0 0.5 0 0.2 0 0 0 1.7 0 0.4 

Total Number/Weight 518 225 398 1,150 907 3,198 249.4 63.2 245.2 415.2 978.8 1,952 
LANE RIVER 

Size (cm) 

FIGURE 11. Size frequency distribution of 

compared to 5.4 cm at Lane River Rookery 

(Fig. 11) . The difference was due to a 

greater proportion of larger fish species 

5-9 an) obtained in the Lane River samples 

(Table 1) and not to differences in size 

of similar species. 

Nestlings consumed larger fish on the 

Size (cm) 

fish from nestling Wood Storks at two colonies. 

average than did adults (t- test, P<0.05) . The differences were not great (4.6 cm VS. 

4.3 cm) and may not be meaningful. 

To determine whether there were any 

differences in the size of fish fed to 

different-aged young, regurgitation samples 

from nestlings were broken down by nestling 

Natural Resources Report No. 16 19 
age classes. There were no consistent differences in size of fish fed to young of 

various ages. In fact, the second largest 

fish found in this study, a 22-an bass, was 

from a stork less than 20 days old. These 

results do not confirm the observation of 

Kahl (1962) that smaller fish were brought 

to younger storks. 

Prey selection by Wood Storks is passive in that storks do not visually select 

and pursue prey but rely on nonvisual 

foraging behavior (Kahl and Peacock 1963). 

Particular species consumed may be captured 

selectively because of availability (i. e., 

density) , size, behavior, or other characteristics. The relationship between availability and prey actually taken can be 

quantified by a selectivity (electivity) 

index: 

e-a E = ---- 

e+a 

where e is the proportion of a species or 

length of fish among all prey eaten and a 

is the proportion of a species or length of 

fish available to foraging Wood Storks as 

determined by the samples taken at stork 

feeding sites. This index, proposed by 

Ivlev (1961), quantifies prey preference 

by a number ranging from +1 to -1. A prey 

with an index near +1 is consumed selectively in much greater proportion than it 

is available. A prey with an index near 

zero is consumed in proportion to its 

availability. A prey with an index near 

-1 is consumed below its relative abundance 

in the environment. 

Figure 12 shows the relative abundance 

of various species of fishes, their relative importance in the diet of Everglades 

Wood Stork, and selectivety of storks for 

each species of fish. Of the more important species in the diet, sunfish, marsh 

killifish, sailfin mollies, and sheepshead 

minnows were highly selected. The mosquito 

fish, abundant in feeding areas, was underrepresented in food samples obtained. Similarly, bluefin and least killifish, although relatively abundant, are seldom 

taken by storks. 

"1 1n A. FREQUENCY IN DIET 

n 0. RElAllYE ABllDkNCE AT 

FEEDING SITES 

FIGURE 12. Relative abundance of various species 

of fish, their frequency in the Wood Stork diet, 

and the selectivity of storks for each species. 

Selectivity by storks for various 

sizes of fish can be measured in the same 

way as selection for species, by calculating selectivity indexes for various 

length classes, as shown in Fig. 13, for 

all samples. Selectivity was zero for 

fish about 3.5 cm in length and rose 

rapidly for larger sizes. This pattern 

was similar in all three habitats. This 

means that relative to the available sizes 

of fish, Wood Storks selectively consumed 

2 0 National Park Service 

+1- - - - 

X 

I 

- = 

= 0- 

7 - 

.- 

.- 

4. 

- * 

vr w 

- -I- C. SELECTIVITY OF STORKS 
the larger fish, generally those over 3.5 

an. Other aspects of prey selectivity of 

Wood Storks are discussed elsewhere (Ogden 

et a1 . 1976) . 

FIGURE 13; Selectivity of Wood Storks for 

different sizes of prey. 

SUMMARY AND CONCLUSIONS 

Rainfall was below normal during most 

months of the 1974 hydrobiological year. 

Discharge into Shark River Slough was 

below the scheduled minimum from October 

through January because of low rainfall, 

management diversion of discharge west of 

the slough in November, and nonacceptance 

by the park of make-up discharges. Thus, 

the 1974 hydrobiological year was one of 

low water supply to the park. However, 

high rainfall in December dispersed feeding Wood Storks and delayed the initiation 

of colony formation. The drying rate, 

which is one way of quantifying the pattern of water level decline, was rapid 

.despite the December rainfall, and the 

overall drying pattern for this year was 

similar to years before 1962 when Wood 

Stork nesting failed. Nonetheless, 1974 

turned out to be a successful one for the 

Wood Storks, strengthening the conclusion 

of Kushlan et a1 (1975) that successful 

nesting years after 1962 do not follow the 

historic pattern. These authors also 

derived an empirical relation between drying rate in the Shark Slough and the time 

of colony formation using data from 1962 

through 1973. If this relation continued 

to hold in 1974, Wood Storks should have 

begun nesting around 26 January. In fact, 

storks began nesting between 15 and 24 

January. This extremely close agreement 

with the previously derived relation 

strongly supports its validity. 

Because of the relatively small amount 

of water manipulation, by closing of S-12 

structures, no conclusions are possible 

on impact of such actions on fish populations of the Everglades. Since no significant water manipulation took place, the 

most important contribution of the present 

program was the detailed study of the 

food ecology of Everglades Wood Storks. 

This information is basic to understanding 

the ecological relationships of this species and provides a step toward management 

of the Everglades ecosystem. 

The study showed fish populations were 

high in the Shark River Slough in 1974 and 

that movement of fish did occur as the 

slough dried, although details of such 

movement remain obscure. Wood storks fed 

on relatively high densities of fish 

except, apparently, in Conservation Area 

3, where much future study needs to be 

directed. The Wood Storks in the coastal, 

mangrove, and Everglades regions of southern Florida ate fish almost exclusively, 

consuming 27 species. However, marsh 

killifish, sheepshead minnows, flagfish, 

sunfish, and yellow bullhead made up 84% 

of the diet. Some of these species were 

highly selected by Wood Storks and consumed far in excess of their relative 

Natwal Resources Report No. 16 
abundance at feeding sites, whereas others 

were less highly selected. Abundant potential food species such as mosquitofish, 

least killifish, and freshwater prawns 

were insignificant in the stork's diet. 

Wood Storks also selectively consumed fish 

over 3.5 an, the relatively larger fish 

and therefore larger species among those 

available. Thus, these data show that the 

Wood Stork is a highly selective feeder 

despite its nonvisual, tactile hunting 

technique. 

The ecological implications and management applications of the results of this 

study are considerable. It is believed 

stork nesting in recent years has failed 

primarily because nesting began too late. 

Previously, no nesting had succeeded which 

began after December. Nesting began this 

year in late Janary, near the latest date 

at which nesting could successfully finish 

before the rainy season. The attempt to 

initiate early nesting by water management 

was inconclusive. However, nesting would 

probably have begun earlier if water levels 

had not risen in December. 

It is apparent from this year's data 

that the general rate of water decline at 

the P-33 reference gaging station does 

correlate with time of colony formation, 

but why it does is not clear. It appears 

that storks were not feeding in the area 

which is most probably influenced directly 

by small changes in the discharges at the 

5-12 structures during the early drying 

period and that less than scheduled water 

deliveries did not in themselves provide 

a rapid drying rate this year. It is 

possible that relatively small manipulation 

of dry season discharge may have little 

impact. However, it remains down how 

S-12 discharges affect water levels either 

at P-33 or elsewhere in the slough. AS 

noted by Kushlan et al. (1975), the value 

of P-33 may be as an index of the pattern 

of drying over the entire region, including 

areas where Wood Storks are feeding during 

colony formation. Thus, water level relations at P-33 are probably 'a valid indication of water conditions in relation to 

Wood Storks, but the relation between P-33 

and S- 12 discharge remains unquantif ied 

in any way applicable to management. It 

is reasonable to suspect that high discharges in the drying season might prevent 

rapid drying. Thus it remains reasonable 

that manipulation of early dry season discharges may indeed affect drying. However, 

without more detailed information on the 

amount of discharge through each structure 

and the impact of such discharge on water 

levels throughout the slough, it is not 

possible to predict the effect of such 

manipulation. 

Since Wood Storks consume predominantly 

a small array of species, it is the production and availability of these few species, 

as opposed to the availability of the whole 

array of species, which probably determines 

whether sufficient food is available for 

successful nesting. In addition, storks 

consume primarily larger fish, the ages 

of which are not presently known. This 

information is important because if the 

fish are the result of production during 

the current hydrobiological year, the 

biological and physical conditions which 

control their availability may be relatively straightforward. If they are in fact 

over one year old and have survived a 

previous dry season, the situation becomes 

more complex. Unfortunately, little is 

known about the productivity, growth, or 

ecology of these species or of the aquatic 

National Park Service 
communities of which they are a part. 

The general nonpredictability of successful Wood Stork nesting after 1962 continued. It is particularly notable that 

successful years prior to 1962 were wet 

years while the 3 successful years after 

1962 were dry years. However, other recent 

dry years and also recent high-water years 

were unsuccessful. The breakdown of predictability coincides with the institution 

of water control, although other concurrent 

environmental changes are undoubtedly in 

part responsible. Both the analysis of 

such changes and the proper management of 

the Everglades ecosystem are confounded by 

the delivery of surface water discharge to 

the western part of Shark River Slough 

rather than through its natural drainage 

east of the park. It seems highly probable 

that the breakdown of the historical pattern of wading bird nesting my only be 

rectified if the historic pattern of water 

delivery to the entire slough is again 

realized. 

RECOMMENDATIONS 

This project provided a beginning toward 

an urgently needed and long overdue study 

of the Shark Slough ecosystem. We strongly 

recommend that a unified research program 

be initiated to continue investigating the 

biological and hydrological dynamics of 

this vital but failing ecosystem, the central feature of Everglades National Park. 

We specifically recommend the following 

program as the first stage in the needed 

studies. 

HydroZogy--Data on monthly discharge 

through the S-12 structures should include 

daily discharge for each structure as a 

necessary requisite of future management 

Natural Resources Report No. 16 

decisions regarding this discharge. 

A program to determine the pattern of 

drying in Shark River Slough and adjacent 

areas of importance to wading birds should 

be established. 

Fish Studies--A program should be undertaken to determine the distribution, community structure, breeding cycles, longevity, movement, food, and productivity of 

fish populations in the southern Everglades. 

A necessary first step in this program 

must be further refinement of the sampling 

technique used to measure low densities of 

fish. Studies of the abundance and productivity of fish in Shark River Slough should 

then be contrasted with similar information 

from the Conservation Areas and eastern 

Everglades to determine the impact of water 

management on fish populations and their 

availability to higher levels of the food 

chain. These studies should be part of an 

overall effort to develop a systems model 

of the Everglades region which will provide 

the basis for management of these ecosystems. 

Studies should also be undertaken to 

determine productivity and the environmental determinates of prey availability in 

the estuarine habitats which serve as the 

prkry feeding site of Wood Storks in 

early winter. 

Sampling should be continued at the 

two previous USGS trap sites reestablished 

this year in order to perpetuate this longterm record of fish populations in Shark 

River Slough. 

Wood Stork Studies--The present study 

obtained information on the distribution, 

movements, food, and prey selectivity of 

the Wood Stork in the Everglades. Studies 

now should be undertaken to examine more 

closely its nesting biology, foraging 
behavior, and parental care in the Everglades while expanding the study of its 

ecology and biology to other parts of its 

range so that the environmental factors 

that lead to successful production in different habitats can be cornpaced and general patterns uncovered. In addition, 

banding and marking programs in the park 

and elsewhere should be continued to provide information on mortality and distribution in the nonbreeding season. 

Wading bird studies should be expanded 

to include more precise information on 

the movements, food habits, and populations of other wading bird species. Surveys should give better coverage of important feeding areas for wading birds outside the boundaries of the park, especially in the Big Cypress Swamp, the Conservation Areas, and eastern boundary area. 

AZZigator Studies--Preliminary studies 

(Kushlan 1974, 1976) have shown the importance of this species and its role in providing foraging habitat for wading birds 

and survival ponds for fish. A program 

should be begun to study the ecology of 

the American alligator in the southern 

Everglades, specifically in relation to 

water levels, fish populations, and wading 

bird predation. 

Future Considerations--Hydrobiological 

research in the southern Everglades should 

have as its ultimate goal determining 

whether the current discharge structures 

and discharge schedule adequately reproduce the natural hydrologic cycle-in the 

Shark Slough watershed and, if so, whether 

this has maintained the natural biological 

system. An effort should be undertaken 

to determine if surface flow should be 

restored to the entire Shark Slough basin. 

National Park Service