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Effects of Hydrology on Wading Bird Foraging Parameters

Project Proposal for 2001

Continuation Research Plan [Year 3 of 3 Years]

Project Title: Effects of Hydrology on Wading Bird Foraging Parameters

Co-Principal Investigators: Dr. Dale Gawlik and Fred Sklar
Everglades Systems Research Division
South Florida Water Management District
West Palm Beach, Florida 33406



Declining wading bird populations in the last century represented one of the first and most visible signs that the Everglades ecosystem was being degraded. Recovery of these populations has been identified as a key indicator of successful restoration efforts. Wading birds are used as bioindicators because of their high mobility over large areas, rapid response to changes in lower trophic levels upon which they feed, and the historical record of data available for the Everglades.

One widely proposed cause for the decline in wading birds is that current hydropatterns have altered the availability of prey. Prey availability is determined by the abundance of prey and the vulnerability of prey to capture. Prey abundance is affected by factors such as nutrient levels and hydroperiod whereas vulnerability to capture is affected by such things as behavior of the prey species, water depth, vegetation density, and body size. Each component of prey availability is affected differently under various water management scenarios. For example, management for long periods without severe drydowns changes the species composition of the fish community. Different species of fishes exhibit different behavioral response to predators, thus changing their availability to capture. Moreover, these behavioral differences of the prey may be dependent on water depths. When the water column is deep, social species like the Golden Shiner (Notemigonus crysoleucas) may occur in schools and present wading birds with a very different capture probability than more solitary fishes like the Bluegill (Lepomis macrochirus). However, as the water level recedes, capture probabilities of the two species may converge.

Ongoing modeling efforts in south Florida, such as the Federal Across Trophic Level System Simulation (ATLSS) program, integrate information on hydrology and wading bird food availability to provide predictive power for future water management decisions. Currently, the biggest information gap limiting the wading bird component of ATLSS is foraging success as a function of prey availability and water depths. The District is conducting a series of experiments aimed at determining the effects of water management (manifested through changes in prey availability) on the use of foraging sites by wading birds. The first experiment examined the influence of fish density and water depth on wading bird foraging. The second experiment examined fish size, fish density and water depth. The third experiment, addressed in this proposal, examines fish species, fish density and water depth.

As part of the third experiment, the District filmed foraging behaviors of wading birds at feeding sites with known fish species, water depths, and fish densities. Current funding levels at the District dictate that the foraging data, which require thousands of hours to extract from the films, will not be available soon enough to be used in the ATLSS wading bird model. USGS’s funding of this proposal will allow the District to focus on the intensive task of quantifying the foraging behaviors of wading birds in a timely manner. These critical data will greatly aid the development of a successful wading bird component of ATLSS, as well as to contribute to effective management of the Everglades’ hydrology.


During FY99, the objectives of this study were to determine the proximate effects of water depth and fish density on wading bird foraging parameters. During FY2000, the objectives were expanded to include the effects of fish size, fish density, and water depth. This third experiment proposes to examine the effect of fish species, fish density, and water depth. The species of wading birds examined in this study are those in the ATLSS wading bird model: Wood Stork (Mycteria americana), White Ibis (Eudocimus albus), Great Egret (Casmerodius albus) and Great Blue Heron (Ardea herodias). The species of fish being compared are the Bluegill and Golden Shiner, both of which are native to the Everglades. The experimental hypotheses being tested are that fish species, fish density, and water depth, affect wading bird prey-intake rates and capture success.


Videotapes of foraging birds from the fish species experiment will be reviewed and time-activity budgets will be constructed sequentially for each focal bird visible in the field-of-view for each film session up to a maximum of 15 individuals/species/session. A time-activity budget is a record of the amount of time an animal devotes to each behavior, such as capturing prey or searching for prey, during an observation period. Data will be entered into an Oracle database using software currently under development at the SFWMD and designed specifically for time-activity budgets of wading birds.

From each time-activity budget, the District will calculate prey-intake rate and capture success as response variables. Descriptive statistics such as the mean and standard deviation will be determined for each bird species at different treatment levels. Where there are adequate data to conduct statistical analyses, the relative effects of water depth, prey density, and fish species will be determined.


Date Deliverable
2 month after CE (Contract Execution) Letter report outlining progress to date. Preliminary data on fish depletion rates for each pond and sample size estimates for bird species.
4 months after CE Letter report outlining progress to date. Report will contain means and standard deviations for foraging parameters for those species of which the District has completed time-activity budgets.
10 months after CE Draft final report. Draft report will contain descriptive statistics for the 4 species of wading birds. Statistical analyses will be presented for those species with adequate sample sizes. Comments will be solicited from USGS Biological Resources Division (BRD) staff.
12 months after CE Final Report. Comments from BRD staff on draft report will be incorporated into final revision.

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