Functional Response of Three Wading Bird Species to Prey Density
Erynn M. Call
South Florida Water Management District, West Palm Beach, FL
Dale E. Gawlik,
South Florida Water Management District, West Palm Beach, FL
Declining wading bird populations in the last century have been considered one of the most prominent signs of the degradation of the Everglades ecosystem. Consequently, recovery of these populations will be a key indicator of successful restoration efforts. It has been hypothesized that the specific mechanism by which Everglades degradation has led to declining bird populations is related to changes in hydropatterns. These changes have most likely altered the availability of prey to wading birds.
Prey availability is determined by both 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 USGS 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. We conducted 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. Here we present the preliminary analysis and results of the experiment addressing the effect of fish species, fish density and water depth on wading bird foraging behavior.
The study was conducted in the Everglades Nutrient Removal Project adjacent to the northwest boarder of A.R.M. Loxahatchee National Wildlife Refuge, Palm Beach County, Florida (fig. 1).
The experiment was initiated on 10 March 1997 when ponds were stocked with golden shiners (Notemigonus crysoleucas) and bluegill (Lepomis macrochirus), and was completed on 15 March 1997 when bird use nearly ceased. Treatments were assigned randomly among 12 ponds using a 2x2x2 (water depth 10 cm, 28 cm; fish density low and high; fish species golden shiner and bluegill) factorial arrangement with two replicates.
Foraging flocks were monitored with a video camera. Following the video monitoring, time-activity budgets of focal birds were constructed from videotapes. Three bird species were observed during this experiment: Great Egret (Ardea alba; visual feeder), White Ibis (Eudocimus albus; tactile feeder) and Wood Stork (Mycteria americana; tactile feeder). Two measures of prey consumption were calculated, mean prey capture per minute and the mean time interval between prey captures. The relationship between these two measure of prey consumption and predicted fish density (fish/m2) was investigated. We wanted to determine if capture rates were affected by low versus high fish density treatments.
Capture rate (fish/min) and the mean time interval between prey captures were plotted against predicted fish density for visual (figs. 2, 3) and tactile (figs. 4, 5) feeding wading birds. After visually inspecting the data, it appeared that prey density did not influence capture rates for visually feeding birds. In other words, wading birds had similar capture rates at low and high predicted fish densities. Sample sizes were too small to draw conclusions about the tactile feeding species.
Several of our experiments showed a strong numerical response to prey density by wading birds, in contrast to the apparent lack of functional response shown here. In other words, once these birds were at the ponds, capture rates were similar. One possible explanation for constant capture rates is that wading birds will leave a patch before intake rates decrease substantially. As a result, no decreasing intake rates can be detected. The fish density at which an individual wading bird will leave a pond in search of a new foraging locale is termed the giving up density (GUD). Further investigation of the relationship between prey density and wading bird foraging behavior will be conducted. The GUD will be calculated for this experiment and offer useful insight into the linkage among drydowns, fish populations and wading birds.
Call, Erynn M. and Dale E. Gawlik.
South Florida Water Management District, 3301 Gun Club Road,
West Palm Beach, FL 33406,
Phone: 561-686-8800, Fax: 561-681-6310,