SOFIA - ATLSS - Wading Birds

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 and others, 1990; Walters and others, 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.

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 Across Trophic Level Systems Simulations (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, foraging behavior of wading birds at feeding sites with known prey availabilities was recorded on film. Current funding levels at SFWMD dictate that the foraging data, which will require thousands of hours to extract from the films, will require increased funding to meet the time schedule proposed for ATLSS (U.S. Geological Survey, 1997). This foraging 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.

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. Each experiment began when ponds were stocked with fish and ended when bird-use nearly ceased (a period of approximately two weeks). The maximum number of birds present in a day (all ponds pooled) was approximately 280. For the fish density experiment, two 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, three 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.

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. Following the field portion of the study, time-activity budgets of focal birds (Altmann, 1974) were constructed from videotapes. From each time-activity budget, we calculated mean prey-intake rate as the response variable.

Thus far, only the water depth and fish density experiment has been analyzed. 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 two prey densities and three water depths (table 1). 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 three depths.

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 two 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.

Effects of Hydrology on Wading Bird Foraging Parameters