Wading Bird Use of Managed Wetland Habitats for Foraging in the Northern Indian River Lagoon

Wading bird use of managed wetland habitats for foraging in the northern Indian River Lagoon System.

Eric D. Stolen, Dave R. Breininger, and Rebecca B. Smith Dynamac Corp., Kennedy Space Center, FL

Continuous long-term ecological monitoring of wading bird populations in the northern Indian River lagoon system has been underway since 1987. Some wading bird species appear to have benefited from changes in habitat resulting from salt marsh impounding. However, use of habitat by wading birds varies greatly between seemingly similar impoundments. The myriad conditions present in this region and the ongoing efforts to reconnect impoundments to the estuary provide a unique opportunity to investigate factors influencing wading bird habitat use under different management scenarios. This talk explores patterns of wading bird habitat use within managed impounded wetland habitat in a subtropical estuary, and the mechanisms controlling those patterns.

Study Site - The Kennedy Space Center/Merritt Island National Wildlife Refuge (KSC/MINWR) is located on the Atlantic coast of Florida, and encompasses a large portion of the northern part of the Indian River Lagoon System (IRLS). Stretching for ca. 250 km the IRLS is a sub-tropical estuary with an unusually high level of biodiversity due to its location at the junction of the warm-temperate Carolinian Province and the Tropical Caribbean Province. Historically the eastern shore of the IRLS was extensively vegetated with irregularly flood salt marsh habitat. However, almost all salt marsh in the northern part of the IRLS was impounded for mosquito control by the 1970’s. There is growing concern that isolation of wetland habitat from the IRLS has reduced ecological benefits of the system. Currently, efforts are underway which will reconnect over three-quarters of all impounded wetlands in the IRL, including the majority of impoundments at KSC/MINWR. The IRLS is an important site for wading birds on the southeastern Atlantic coast of North America. KSC/MINWR supports a large wading bird population that utilizes freshwater and salt marsh habitats for feeding, roosting, and nesting.

Methods - Wading bird habitat surveys included in this analysis were conducted between April 1987 and April 2002. Surveys were conducted by helicopter flying at an altitude of approximately 60 m, and a speed of 60 kn. The sampling schedule for surveys was one per month, although some months were missed due to constraints on use of the NASA helicopter. Roughly 20 % of the nearly 11,000 ha of impounded marsh habitat and 16% of the estuarine/river boundary occurring on KSC/MINWR was surveyed. Impoundments were flown systematically such that all area within was observed, and all individuals visible within the impoundment were counted. Estuarine edge was surveyed by flying approximately 300 m from the shore and included observations within this 300 m strip.

Between April and June 2000, nesting Great Egrets and Snowy Egrets were followed by helicopter from their nests to their foraging locations between sunrise and five hours after sunrise. The helicopter was hovered near the colony until a bird was observed leaving the colony. Then the bird was followed until it landed at a foraging location. Once a bird landed, a GPS position was recorded at the location, and the habitat type was noted. If the subject was part of a group, an attempt was made to note the landing position of all members of the group, but often this was not possible because the subject bird would continue flying. In these cases, only information about the subject bird was recorded.

Results –Sixty-one percent of wading birds followed from nesting colonies landed at foraging sites within impoundments, 15 % landed in estuarine edge habitat and 24% landed in unimpounded freshwater wetland habitat. (N=79). The average number of wading birds foraging within impoundments and along estuarine shoreline was estimated to be approximately 7000 individuals; the number of birds using the northern IRLS system is certainly greater than this, since number of birds using unimpounded freshwater wetlands was not included in this estimate. Ten of the sixteen species of wading birds known to occur in the northern IRLS were observed regularly during aerial surveys (Figure 1).

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Figure 1. Density of wading birds observed foraging within impounded wetland habitat in the northern Indian River Lagoon System. Species are: Great Blue Heron (GBHE), Great Egret (GREG), Snowy Egret (SNEG), Little Blue Heron (LBHE), Tricolored Heron, TCHE), Reddish Egret (REEG), White Ibis (WHIB), Glossy Ibis (GLIB), Roseate Spoonbill, (ROSP), and Wood Stork (WOST).

There were distinct temporal and spatial habitat use patterns within individual wading bird species foraging in impounded wetlands. Most species showed a preference for foraging within open water habitat, but use of vegetated habitats increased in fall and winter. These changes in habitat use corresponded with hydrological changes within the system. Density of wading birds within impoundments varied greatly across the landscape; the ratio of marsh to open water habitat within the impoundment explained some of this variation (Figure 2).

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Figure 2. The density of foraging wading birds within impoundments (bars) increases as the ratio of open water to marsh habitat (line) decreases. Ratio calculated as arcsine (ha open water / ha marsh). Individual impoundment names not shown on figure.

Current research efforts focusing on linking wading bird habitat use patterns to prey availability will also be discussed. Models of the relationship between hydrological and habitat changes and bird use of foraging habitat will be presented.

Eric D. Stolen, Dynamac Corp., Mail Code: DYN-2, Kennedy Space Center, FL 32899, [email protected]