Sample size and duration of time-activity budgets

The filming of birds was opportunistic in the sense that birds were not filmed in treatment combinations in which they never or rarely occurred. The result was that some treatment combinations were absent and, therefore, the range of water depths and fish densities was not always great enough to allow for a powerful comparison of the two effects.

The lack of correlation between prey intake rates and duration of time-activity budgets for the Wood Stork and White Ibis suggests that the average time of 11.1 min was adequate to quantify the prey intake rate of a typical foraging bout for these species. The strong and weak negative correlations for the Great Blue Heron and Great Egret, respectively, suggest that for these species, prey intake rates might be liberal.

Differences in length of time among species generally (excluding Great Blue Herons, for which we had small samples) reflected the amount of movement during foraging bouts because we ended a time-activity budget when a focal bird disappeared from the field of view. Wood Storks and White Ibises were moving almost constantly while foraging whereas Great Egrets were often stationary.

Giving-up density

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 GUDs in both of those depths were greater than in the shallow treatment. In some cases there was no difference between GUD_50%max and GUD_min. This was because bird abundance decreased so quickly that the first day abundance was below 50% of the max it was also the last day that birds were seen in a pond.

Factors affecting intake rates

Overall, the statistical models, which considered water depth and fish density, had low explanatory power. This may be partially related to sample size as indicated above, but it also may suggest that intake rates were mostly affected by factors other than water depth and fish density. This notion is further supported by the large individual variability within treatments. Individual variability could result from two independent sources. One source might be that birds entered the ponds in different body condition and therefore had different metabolic needs. These birds could respond similarly to environmental factors but their foraging effort is different because their bodies are in different metabolic states. Casual observation indicated that birds did alter their foraging effort in response to satiation. We observed several instances when, after a period of successful feeding, birds would repeatedly strike at and capture a fish but not consume it. But, eventually those individuals would start foraging normally again.

The other possible source of individual variation in intake rate was that birds arrived at ponds in similar body condition but they performed differently once they got there. These differences could result from differences in age, gender, phenotype, memory, or presence of conspecifics. Conspecifics can affect foraging performance because individuals gain information on the profitability of micro-sites around other flock members. It was not uncommon for birds to relocate within a pond closer to an individual that had a run of successful captures over several minutes. These "runs" occurred even after fish densities had dropped, and their distributions were probably more clumped. Thus, even when fish densities were low at the scale of a pond, it would be possible for individuals to follow fish patches, which were high density at a small scale, by regularly relocating next to successful flock members. If the two strategies (i.e., follower and searcher) differed in their profitability, then it could lead to individual variation with a pond.

The White Ibis model suggested that fish density had an effect on intake rates but there was no such effect from water depth. The lack of an effect from water depth was somewhat surprising because water depth did affect the selection of sites by White Ibis in the same experiment (Gawlik 1996). However, only four White Ibis were filmed in deep water so the range of water depths was small and possibly contributed to a lack of effect. Those four birds occurred in the deep water when fish densities were moderately high and under those conditions they had a relatively high rate of prey intake. Collectively, the site selection data (Gawlik 1996) and this study suggest that the first response to deep water by this species is to avoid the foraging location, or at least to avoid it when fish densities are below a certain level.

The Great Egret model indicated that the relationship between prey intake rate and fish density was dependent on water depth. This pattern is readily interpretable when considering data from only the shallow and medium depth ponds. In those cases, the relationship was strongly positive when water depth was shallow and fish were presumably most vulnerable to capture. At the medium depth, vulnerability to capture presumably decreased and the relationship disappeared. This pattern suggests that when prey are vulnerable to capture, prey intake rates are proportional to fish density. But, when fish are not vulnerable to capture fish density has little affect on intake rate. The pattern becomes more difficult to interpret when considering data from the deepest treatment. The mean intake rate was higher in the deep treatment than in either the shallow or medium depths.

These data illustrate that further study is needed to clearly identify factors affecting intake rates and sources of individual variation. A collaborative effort between Florida International University (Victor Apanius) and the South Florida Water Management District (DEG) is underway to identify physiological costs of foraging under different environmental conditions. This effort should shed light on sources of individual variation in feeding success under similar environmental conditions.

LITERATURE CITED

Altmann, J. 1974. Observational study of behavior: sampling methods. Behavior 49:227-267.

Bancroft, G. T., S. D. Jewell, and A. M. Strong. 1990. Foraging and nesting ecology of herons in the lower Everglades relative to water conditions. Final report to the South Florida Water Management District, West Palm Beach, Florida, USA.

Brown, J. S. 1988. Patch use as an indicator of habitat preference, predation risk, and competition. Behavioral Ecology and Sociobiology 22:37-47.

Frederick, P. C., and M. W. Collopy. 1989. Nesting success of five Ciconiiform species in relation to water conditions in the Florida Everglades. The Auk 106:625-634.

Gawlik, D. E. 1996. Influence of water depth on the selection of foraging sites by wading birds. Pp. 142-148 in T. V. Armentano (ed.) Proceedings of the Conference: Ecological Assessment of the 1994-95 High Water Conditions in the Southern Everglades. Everglades National Park, Homestead, Florida.

Kushlan, J. A. 1974. Quantitative sampling of fish populations in shallow, freshwater environments. Transactions of the American Fisheries Society 103:348-352.

Walters, C., L. Gunderson, and C. S. Holling. 1992. Experimental policies for water management in the Everglades. Ecological Applications 2:189-202.

United States Geological Survey. 1997. ATLSS: across-trophic-level system simulation: an approach to analysis of south Florida Ecosystems. Progress Report from Biological Resources Division of United States Geological Survey.

Related information:

How will research on fishes and wading birds guide and evaluate Everglades restoration? (poster from the South Florida Restoration Science Forum)

South Florida Wading Bird Report (hosted by the South Florida Water Management District)

Across Trophic Level System Simulation (ATLSS) project page (from the SOFIA website)

Across Trophic Level System Simulation (ATLSS) homepage

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