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).
Figure 1. Location and arrangement of experimental
The experiment was initiated on 10
ponds.
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.
Figure 2. Predicted fish density vs. capture rate for visual
feeding wading birds at 10 and 28 cm water depths.
Water depth 10 cm, Bluegill
Water depth 28 cm, Shiner
Capture rate (fish/min)
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
10
Mean time interval (min) between prey captures
0.6
Figure 3. Predicted fish density vs. mean interval between captures
for visual feeding wading birds at 10 and 28 cm water depths.
12
Water depth 10, Bluegill
Water depth 28, Shiner
10
8
6
4
2
0
0
2
Predicted fish density (fish/m2)
Figure 4. Predicted fish density vs. capture rate for tactile
feeding wading birds at 10 and 28 cm water depths.
Water depth 10 cm, Bluegill
Water depth 28 cm, Shiner
Capture rate (fish/min)
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
2
Predicted fish density (fish/m )
6
8
10
Figure 5. Predicted fish density vs. mean inteval between captures
for tactile feeding wading birds at 10 and 28 cm water depths.
10
Mean time inteval (min) between prey captures
0.6
4
Predicted fish density (fish/m2)
2.5
Water depth 10 cm, Bluegill
Water depth 28 cm, Shiner
2.0
1.5
1.0
0.5
0.0
0
1
2
3
Predicted fish density (fish/m2)
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, ecall@sfwmd.gov
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