projects > across trophic level system simulation (atlss) > wading birds > work plan
Project Work Plan
Greater Everglades Science Program: Place-Based Studies
Project Work Plan FY 2003
A. GENERAL INFORMATION:
Project Title: Effects of Hydrology on Wading Bird Foraging Parameters
Other Investigator(s): Dr. Fred Sklar
The recovery of wading bird populations has been identified as a key component of successful Everglades restoration. Proposed causes for the decline in wading bird numbers have in common the notion that current hydropatterns have altered the availability of prey. The relative importance of each component of food availability (i.e., food abundance and vulnerability to capture) is a precursor to understanding the effects of specific hydrologic regimes on wading birds. The Across Trophic Level System Simulation (ATLSS) program integrates such information and provides predictive power for future 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 purpose of this project is to examine the response of wading birds to various components of prey availability using experimental ponds and treatments of water depth and various prey community characteristics. Prey community characteristics include fish density, fish size, and fish species. The species of wading birds examined in this study are those in the ATLSS wading bird model: the Wood Stork, White Ibis, Great Egret, and Great Blue Heron.
Project Objectives and Strategy:
The recovery of wading bird populations has been identified as a key component of successful Everglades restoration. Proposed causes for the decline in wading bird numbers (Frederick and Collopy 1989, Bancroft et al. 1990, Walters et al. 1992) have in common the notion that current hydropatterns have altered the availability of prey. Indeed, food availability may be the single most important factor limiting populations of wading birds in the Everglades (Frederick and Spalding 1994). Food 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 water depth, vegetation density, and body size. Each component of prey availability is affected differently under various water management scenarios. For example, long periods without severe drydowns are thought to increase the abundance of fish (Loftus and Eklund 1994). In direct contrast, these fish become most vulnerable to capture by birds when water depth is shallow. Thus, the subtle, and as of yet undefined, interaction between water depth and hydroperiod may be critical for supporting healthy populations of both fish and birds.
Integration of this field study in the ATLSS program
In the face of such conflicting management scenarios, knowing the relative importance of each component of food availability is a precursor to understanding the effects of specific water management regimes on wading birds. Ongoing modeling efforts in south Florida such as the ATLSS program, integrate such information and provide predictive power for future management decisions. Currently, the biggest information gap limiting the wading bird model of ATLSS is foraging success as a function of prey availability (United States Geological Survey 1997). The South Florida Water Management District (SFWMD) is currently conducting a series of experiments aimed at determining the effects of water management on the use of foraging sites by wading birds. Site-use data are available immediately after each experiment and thus allow for quick analyses and write-up. However, also as part of those experiments, we recorded on film, foraging behavior of wading birds at feeding sites with known prey availabilities. Current funding levels at SFWMD dictate that the foraging data, which will require 1000's of hours to extract from the films, will not be available under the time schedule proposed for ATLSS (United States Geological Survey 1997). Funding this proposal will allow us 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.
The conceptual model for this study is based on the idea that hydroperiod is a long-term process that primarily influences the abundance, body size, and species composition of the prey community whereas water depth has immediate effects on individual birds by influencing their ability to capture prey. This study seeks to determine through field experiments, the proximate effects of water depth, prey density, prey size, and prey species on wading bird foraging parameters. The species of wading birds examined in this study are those in the ATLSS wading bird model: the Wood Stork, White Ibis, Great Egret, and Great Blue Heron.
Potential Impacts and Major Products:
Information from this project directly supports the development of the ATLSS Wading Bird Model. The ATLSS Wading Bird Model will be used for a variety of CERP projects to assess their impacts on various wading bird species. Other specific uses and information needs for the ATLSS wading bird model are addressed elsewhere.
Key products from years 1 and 2 were
B. WORK PLAN
Title of Task 1: Effects of Hydrology on Wading Bird Foraging Parameters
Field experiments were conducted in a set of 15 0.2-ha ponds directly adjacent to, and NW of, Arthur R. Marshall Loxahatchee National Wildlife Refuge in Palm Beach County, Florida. Three experiments (water depth and fish density, water depth and fish size, water depth and fish species) were conducted between March, 1996 and March 1997 (see Gawlik In Press for detailed methods for the water depth and fish density experiment). Each experiment began when ponds were stocked with fish and ended when bird-use nearly ceased (a period of approximately 2 weeks). The maximum number of birds present in a day (all ponds pooled) was approximately 280. For the fish density experiment, 2 treatments were assigned randomly among 12 ponds using a 3x2x2 factorial arrangement (water depth: 10 cm, 19 cm, 28 cm; fish density: 3 fish/m2, 10 fish/m2; replicates: 2). For the fish size experiment, 2 treatments were assigned randomly among 12 ponds using a 3x2x2 factorial arrangement (water depth: 19 cm, 28 cm, 37 cm; fish size: 3 cm, 8 cm; replicates: 2). All ponds were stocked at a density of 8 fish/m2. For the fish species experiment, 3 treatments (water depth: 10 cm, 28 cm; fish density: 4 fish/m2, 16 fish/m2; fish species: bluegill, golden shiner; replicates: 2) were assigned randomly among 12 ponds. This 2x2x2x2 arrangement was more complex because the treatment of high fish density and low water was eliminated from the design.
For all experiments, ponds were initially stocked at known fish densities; however, those densities decreased quickly as a result of bird predation. Thus, we treated fish density as a continuous variable, which we monitored regularly. We determined fish densities during at least 4 sampling periods based on 1-m2 throw-trap samples (Kushlan 1974). Sampling ceased in individual ponds if no stocked fish were captured in any of the throw-trap samples. Throw-trap samples were distributed evenly within a pond by dividing each pond into 16 10x8 m plots and conducting one throw-trap sample in each plot during each sampling period. A test of sampling efficiency indicated the numbers of fish captured in throw traps represents an average of 55% of the actual fish density in the experimental ponds. All fish density values reported here are the number of fish captured in throw traps uncorrected for sampling efficiency. A linear regression model was fitted to the data from each pond such that the response was the number of fish/m2 transformed as y(ln +1). The predicted values were back-transformed to get estimated densities of fish for each pond each day of the experiment with the constraint that no predicted value could be less than zero. The estimated values were used in subsequent analyses of bird foraging success.
To measure wading bird foraging responses, we filmed feeding flocks for 5-45 minutes from a vehicle with a Hi-8 mm video camera and 8-120 mm zoom lens. A pilot study indicated that filming from a parked vehicle with cloth-covered windows disturbed birds less than a portable blind. Selection of a flock to film was based on where the largest group of birds were foraging and whether data were lacking for a given treatment combination. Flocks were allowed several minutes to re-settle and resume feeding before filming was initiated. Filming was concluded if flock size changed by more than 25%. Our aim was to get 15 min of film on each bird (with a minimum time limit of 5 min) and a maximum of 15 birds per species per film session. If too few individuals in a film session met that criterion then the minimum time limit was reduced to 2 min. During feeding activities, birds would sometimes travel to the edge of a pond or leave the pond momentarily. Because of the sloped edge of the pond, water depths in those areas were less than treatment levels. Thus, we excluded from time-activity budgets any period where a bird was at the edge or outside of a pond. This criterion resulted in a further shortening of some time-activity budgets.
Following the field portion of the study, time-activity budgets of focal birds (Altmann 1974) were constructed from videotapes. Tapes were viewed using a Hi-8 VCR connected to a high-resolution video monitor. Data were entered into an Oracle® database through a personal computer connected to an interoffice-network.
To eliminate the possibility of constructing time-activity budgets on the same individual more than once in a session, we did not use as focal birds, individuals of the focal species that appeared in view after the first focal bird left the screen. However, before a focal bird left the field of view, any new birds that appeared could have been used as focal birds. A foraging bout ended when a focal bird left the field of view, became obscured in a flock, or the film session ended (usually about 15 minutes). Capture rates were calculated as the number of prey consumed divided by the length of time (min) of the time-activity budget. Birds that never consumed a prey item were excluded from the analysis.
From each time-activity budget, we calculated mean prey-intake rate as the response variable. Descriptive statistics such as the mean and standard deviation are presented for each bird species at different treatment levels. This is the format most useful for incorporating parameter values into the ATLSS wading bird model (W. Wolff, Univ. of Miami, pers. commun.). For species of which we had adequate data to conduct statistical analyses, we determined the relative effects of the treatment variables on the response variables. Tests were conducted using PROC GLM in SAS® version 6.12 for Windows® with water depth, fish size, and fish species as class variables and fish density as a continuous variable. We specified an initial full model containing main effects and interactions. Non-significant (p > 0.05) interactions indicated that a model was over-specified and contained more terms than necessary (Littel et al. 1991, Freund and Wilson 1993). In that case, we constructed a subsequent set of reduced models containing the main effects and significant interactions only.
Thus far, only the water depth and fish density experiment has been completely analyzed. The results from the water depth and fish size experiment are nearing completion. Key results include the development and refinement of a conceptual model of wading bird foraging behavior . This model provided the rules for quantifying time-activity budgets for all species, including tactile and visual foragers, under the circumstance encountered during the experiments. We calculated prey-intake rates and their associated variability for 2 prey densities and 3 water depths. These numbers can be used to refine parameter estimates currently in the ATLSS wading bird model. We reported foraging costs (giving-up densities of fish) and their associated variability for each species at 3 depths. Note that fish density estimates and giving-up densities were not corrected for sampling efficiency (see above), thus resulting in an underestimate of true density. The adjusted numbers can be used in the ATLSS model because it reflects the lower threshold of fish abundance where wading birds will no longer use the site.
Foraging costs for all species generally increased as a function of water depth. Prey intake rates were higher for the Wood Stork and White Ibis than for the Great Egret and Great Blue Heron, however; so were their foraging costs. The higher foraging costs for White Ibises and Wood Storks relative to the other 2 species suggests that Wood Storks and White Ibises need to be in habitat that provides higher prey intake rates, and therefore may be affected more by a degradation in habitat quality. Because the giving-up density at individual ponds is a measure of habitat quality relative to the surrounding ponds, the results indicate that birds perceived deeper water ponds to be equal in quality to shallow water ponds only when they contained more fish. In other words, all species perceived foraging costs to be higher in the two deeper treatments but those costs were offset by fish density. In addition, the higher giving-up density for Wood Storks and White Ibises suggest that these species would be more negatively affected by deep water than would Great Egrets and Great Blue Herons. Great Blue Herons did not appear to perceive greater foraging costs in the deepest treatment as compared to the medium depth treatment, but giving-up densities in both of those depths were greater than in the shallow treatment. Finally, we found that the rate of fish depletion in ponds increased with decreased water depth, confirming that fish were more vulnerable in shallow water.
Work to be undertaken during the proposal year and a description of the methods and procedures:
As part of the third experiment (Year 3), 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. USGSs 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. 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.