Field Notes: Great Egrets Foraging on Terrestrial Invertebrates in Everglades National Park, Florida

Tue, 31 Jan 2017 00:00:00 GMT

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Figure 2. Great Egret strikes at a terrestrial invertebrate at Eco Pond.

Great Egrets mostly forage in aquatic habitats and their primary prey consists of fish. In one study, fish constituted 95% of prey and 98% of prey weight. Invertebrate prey included primarily aquatic invertebrates but also terrestrial insects including dragonflies, damselflies, and grasshoppers (McCrimmon et al. 2001). Another study added dung beetles and earthworms (Schlorff 1978). Clearly, terrestrial invertebrates usually contribute but a tiny fraction of the Great Egret diet. If so, why did I encounter so many Great Egrets foraging in terrestrial habitats even though they were surrounded by a sea of water and sawgrass, their traditional foraging environment? The answer probably lies in the weather events that dominated South and Central Florida during January 2016. A brief Internet survey indicated that El Nino-influenced storms produced nine inches of rain in January, about five times the historical average, in what is usually the heart of the Florida dry season. It was the wettest January since 1932 (South Florida Water Management District News Release, February 1, 2016), resulting in high water levels across South and Central Florida including Everglades National Park. Conditions were so bad that Lake Okeechobee threatened to flood the area. Hence, the federal authorities lowered the lake level with a resulting increase in water levels in parts of Everglades National Park.

Figure 3. Great Egret with a prey item in its bill at Eco Pond.

Studies have indicated that Great Egret numbers tend to decrease when water depths exceed 8–16 inches and that their distribution in the Everglades is tightly linked to water depth (McCrimmon et al. 2001). Thus it seems likely that high water levels forced the Great Egrets out of their usual feeding sites and caused them to shift to foraging in terrestrial habitats. I suspect that the high water levels may also have led to a wide dispersal of small fish and that the resulting low density of prey fish may also have influenced the Egrets’ shift to an insect diet.

Literature cited

Hancock, J., and J. A. Kushlan. 1984. The herons handbook. New York: Harper & Row.
Kushlan, J. A. 1978. Feeding ecology of wading birds. in Wading Birds (A. Sprunt IV, J. C. Ogden, and S. Winckler, Eds.), p. 249–297. New York: National Audubon Society, Research Report No 7.
McCrimmon, D. A., Jr., J. C. Ogden, and G. T. Bancroft. 2001. Great Egret (Ardea alba) in The Birds of North America, No. 570 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington D.C.: The American Ornithologists’ Union.
Schlorff, R. W. 1978. Predatory ecology of the Great Egret at Humboldt Bay, California. in Wading Birds (A. Sprunt, IV, J. C. Ogden, and S. Winkler, eds.) p. 347–353. New York: National Audubon Society, Research Report, No. 7.

Field Notes: Rose-breasted Grosbeak Chase

Tue, 31 Jan 2017 00:00:00 GMT

Suddenly, as my friend was trying to find a position to also get some pictures, the grosbeak flew out of the tree and within seconds was joined by three more grosbeaks. They zoomed quickly past us toward the field, turned around, and I looked down in surprise as they circled around the lower part of my legs and then flew a bit farther off again, flying faster and faster until a few seconds later all four of them landed about 15 feet from us on a stone wall at the side of the road. My friend and I were too shocked to get any pictures while they were zooming around or circling my legs, unfortunately. It was too incredible an experience! It seemed that three male Rose-breasted Grosbeaks were chasing one female, hoping to gain her affection in order to mate with her. When they landed on the stone wall, the three males displayed and seemed to yell at one another.

After a second or two they all took off and the female flew to a crab apple tree alone. I never saw who wound up winning her affections. Since my lens was too big to capture the entire scene, the only image I managed to get was just one of a single male doing his display on the stone wall!

February 2017

Field Notes: Great Egrets Foraging on Terrestrial Invertebrates in Everglades National Park, Florida

Literature cited

Field Notes: Rose-breasted Grosbeak Chase