Foraging Habitat Use by Wood Storks Nesting in the ...

Foraging Habitat Use by Wood Storks Nesting in the Coastal Zone of Georgia, USA Author(s): Karen F. Gaines, A. Lawrence Bryan, Jr., Philip M. Dixon, Michael J. Harris Source: Colonial Waterbirds, Vol. 21, No. 1 (1998), pp. 43-52 Published by: Waterbird Society Stable URL: http://www.jstor.org/stable/1521729 Accessed: 07/12/2010 14:38 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=waterbird. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

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Foraging Habitat Use by Wood Storks Nesting in the Coastal Zone of Georgia, USA KARENF. GAINES1'3, A. LAWRENCE BRYAN,JR.1,PHILIPM. DIXON1AND MICHAELJ.HARRIS2 'Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802 USA 2Georgia Department of Natural Resources, One Conservation Way,Brunswick, GA 31523 USA 3Internet: [email protected] Abstract.-We studied foraging habitat use of Wood Storks (Mycteriaamericana)from three coastal colonies using United States Fish and Wildlife Service National Wetlands Inventory data within a geographic information system (GIS). Observers followed storks from breeding colonies to foraging sites in a fixed-winged aircraft. The main objectives of the study were to estimate the foraging range of each Wood Stork colony, determine what wetland types were used in relation to their availability and spatial distribution, and determine how foraging habitat use was related to tidal stage. Storks foraged in tidal creeks during lower tide levels when prey were concentrated in shallower water, and foraged more in palustrine (freshwater) wetlands when tide levels were high. Predictability of foraging habitat use based on habitat distribution varied among colonies and depended on how wetland types were aggregated. Foraging locations were spatially clustered, in some cases by habitat type (estuarine vs. palustrine). These spatial clusterings may be explained by the proximity of a foraging location to the colony and by the habitat types around the colony. Storks also flew longer distances to forage in palustrine sites than in estuarine sites. Received29 July 1997, accepted1January 1998. Key words.-Coastal, endangered species management, foraging habitat, geographic information systems, Mycteriaamericana,National Wetland Inventory, Wood Stork. Colonial Waterbirds 21(1): 43-52, 1998

The American Wood Stork (Mycteria americana) was classified as an endangered species in the United States in 1984 because of population declines caused by loss of foraging habitat (U.S. Fish and Wildlife Service 1986). Wood Storks forage for prey, primarily fish, by tactilocation and require high densities of prey in shallow wetlands to forage efficiently (Kahl 1964). In 1986, the United States Fish and Wildlife Service (USFWS) established a recovery plan for the U.S. breeding population of Wood Storks (USFWS 1986). The top priority of this plan was to establish secure habitat for Wood Storks by (1) providing adequate feeding habitat for existing colonies, (2) determining feeding areas essential to support rookeries and nonbreeding assemblages of Wood Storks, and (3) developing techniques for identifying areas with high potential as feeding areas. The coastal environment appears to be an ideal setting for storks, given the proximity and abundance of both freshwater and brackishsaltwater wetlands, which presumably function as foraging habitat. Historically, Wood Storks have been fairly common on the Georgia coastal plain dur43

ing the summer months (Burleigh 1958), but evidence of nesting in this region prior to 1965 is scarce (Harris 1995). Wood Stork nesting in Georgia has increased steadily since 1980, and in 1993 Georgia's 11 stork colonies accounted for one-fourth of the total U.S. nesting population of this species. Seven of these colonies were in coastal counties, indicating the importance of this area to the species. Concurrent with increased use of coastal habitats by storks is an increasing rate of wetland loss, especially freshwater wetlands, in the coastal zone because of conversion to agriculture, timber, and other land uses (Hefner et al. 1994). Habitat use by storks in coastal systems has not been examined in detail. A few investigations focused on habitat use of storks in the vicinity of Cumberland Island, a barrier island off the southern coast of Georgia (Ruckdeschel and Shoop 1987; Bratton and Hendricks 1988). These studies indicated that storks use estuarine habitats as foraging sites, but they did not examine the spatial patterns of habitat use nor the effects of other factors, such as tidal stage, on foraging. Bryan (1995) found that stork presence at a

COLONIALWATERBIRDS

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coastal roost was linked to tidal stage. Other studies have focused on the spatial patterns of Wood Stork foraging in both coastal and non-coastal settings (Coulter et al. 1987; Hodgson et al. 1987, 1988; Pearson et al. 1992). Hodgson et al. (1987, 1988) used remotely sensed data to determine the change in foraging habitat with respect to season and distance from the colony. That study evaluated potential foraging area by using remotely sensed signatures for known foraging sites but did not explore habitat use in relation to other wetland types. Pearson et al. (1992) evaluated stork foraging habitat in relation to a foraging location using digitized U.S. Fish and Wildlife National Wetland Inventory (NWI) 7.5 min quad sheets within a Geographic Information System (GIS). That study did look at other potential foraging sites but restricted it to areas around the foraging point. In the current study, we examined stork foraging habitat use in relation to other known foraging points and the entire foraging range of stork colonies using NWI data, which identify very explicit wetland types (Cowardin et al. 1979). We looked at these relationships for three coastal colonies in Georgia. One colony was of particular interest because a foraging impoundment had been created for storks next to the colony. The specific objectives of the study were to (1) determine the potential foraging range of each Wood Stork colony, (2) determine what wetland types were important to Wood Storks, (3) determine how individual wetland types were used in relation to their absolute and relative availability within the foraging range for storks from each colony, and (4) determine how foraging habitat use related to tidal stage. METHODS

StudyArea The study was conducted during the 1995 and 1996 Wood Stork breeding seasons in the Sea Island coastal region of Georgia, USA (Sandifer et aL 1980). This region, which contains maritime, estuarine, freshwater, and upland ecosystems, is characterized by forested barrier islands bordered on their inland side by tidal marshes and creeks. The tidal marshes (estuarine) are bordered by river drainages and the mainland, which

support associated palustrine (non-tidal) wetlands such as swamps, drainages, and marshes. Three Wood Stork colonies in the coastal zone of Georgia were included in this study (Fig. 1). The Harris Neck colony (31?37.79'N, 81?16.50'W) is in McIntosh County in the Harris Neck National Wildlife Refuge (NWR) on a large estuarine island between the Sapelo and South Newport rivers. The St. Simons Island colony (31?16.40'N, 81?21.20'W) is in Glynn County in a freshwater impoundment on the St. Simons barrier island. The Black Hammock colony (31?02.23'N, 81?30.82'W) is in Camden County on a large estuarine island between the Satilla River and Dover Creek. The colony on the Harris Neck NWR is in a manmade impoundment (Woody Pond) that is managed to enhance successful breeding of Wood Storks. This management includes the manipulation of water levels to ensure deep water during the nesting season, thus reducing the likelihood of predation, and the addition of artificial nest structures to the wetland to increase the number of breeding pairs using the site (Robinette et al. 1995). Snipe Pond, a 9.7-ha impoundment adjacent to the Harris Neck colony, was stocked with 2,000 black bullhead (Ameiurusmelas) in 1989 to provide additional food for storks. Fish sampling in this impoundment in 1995 found this species to be both present and reproducing. Approximately 330,000 bluegill sunfish (Lepomis macrochirus)were stocked as additional forage in this site in the fall of 1994 in preparation for the 1995 breeding season. This wetland has shallow areas of appropriate depth for foraging throughout the year and has also been modified to allow water level manipulations to make it more suitable (shallower depths) as a foraging habitat. The water level in Snipe Pond was not manipulated during the study. ForagingHabitat An observer followed adult Wood Storks (N = 72) in a fixed-wing aircraft (Cessna 152 or 172) from the colonies to foraging sites by methods described in Bryan and Coulter (1987). All followed storks were assumed to be breeding birds. The locations of the foraging sites were logged into a Global Positioning System (GPS) and/or plotted on 1:100 000 scale United States Geological Survey (USGS) topographic maps. Since this method (plotting) supplied only a general area of the wetland, the observer took detailed notes describing the relative position, and habitat type (impoundments, forested drainages, forested wetlands (non-flowing), tidal creeks or pools, etc.) of each foraging site to "truth"each foraging location for future analyses. The observer also recorded the number of storks and other wading birds already present when the focal individual arrived. Wading birds already present at the site could not be determined for some sites due to the degree of canopy closure. DATA ANALYSIS

GeographicInformationSystemAnalyses Foraging sites were digitized from the topographic maps into a GIS and made into point coverages. Additional foraging locations taken with a GPS were added to this point coverage. National Wetland Inventory (NWI) 7.5 min. coverages (classification based on Cow-

WOOD STORKFORAGINGHABITATUSE

45

LEGEND * Colony * ForagingLocation -I Minimrum Convex Polygon

0 St Simosopen

_

-

Black Hammock

-

75%/ForagingZone water clear cul/ young pine pasture earth cultivatedf/exposed urban enwrgenl wetland scrub/shrubwetland forested wetland coliferous forest ffmixed forest forest hardwood h dwood forest

saltmarsh

brackdshmarsh dalflats/beacltes

20

0

20

40 Kilometers

Figure 1. Stork foraging areas and locations for each colony. Each circle represents the zone encompassing the dclosest 75% of the foraging points within a minimum convex polygon (MCP). ardin et aL (1979)) were downloaded from the USFWS internet site and used as the base habitat data within the GIS. For the purposes of this study, habitat data were pooled to the class level of the Cowardin et al. (1979) classification scheme. We verified foraging points as within the correct habitat type by comparing the field notes taken while flying over the actual foraging site. Because "tidal creek" habitat is not a classified wetland type within the NWI system, we classified it as estuarine subtidal unconsolidated bottom/tidal marsh (Cowardin et aL 1979) for the purposes of this study. When compared to field notes, only two of the 72 foraging points did not appear to be the same habitat type as shown on the NWI coverage. These two foraging points were classified based on the field notes (using the NWI classification system) and not on the NWI coverage. Rainfall, temperature, and stork breeding success were similar between years (unpubl. data) and no other evidence indicated that bird observations could not be considered independent between years. Therefore, we pooled data points for individual colonies across years for all GIS and statistical analyses. Using all the foraging points for each colony, we created minimum convex polygons (MCP) within the GIS to represent the maximum boundary of stork foraging habitat. A few widely scattered foraging points can bias a MCP such that it may not adequately represent a colony's foraging area. To correct this, a circular foraging zone was designated

within each MCP in which (approximately) 75% of the closest foraging points to the colony occurred. We determined the area of every wetland type (from NWI coverages) within the foraging zone and the entire MCP of each colony in order to determine habitat availability. StatisticalAnalyses Two series of chi-square tests were used to determine whether storks foraged in different wetland types in proportion to their availability within each colony's MCP, and within the 75% foraging zone. The first series of chisquare tests examined wetland types in two broad classifications: freshwater and estuarine. The second series looked at specific wetland habitat types (class level) using the NWI classification system. Because many expected values were less than one for this analysis, a randomization test was used to determine the significance level of the observed chi-square statistic (Edgington 1995). We used a Ripley's K statistic analysis (Ripley 1981,1987; Upton and Fingleton 1985; Rowlinson and Diggle 1992; Statistical Sciences 1993) to determine if foraging points were found more often within a distance t (radii of 0.5-10 km) of other foraging points than would be expected based on chance alone. Since foraging points were significantly clustered for all three colonies, two analyses of spatial segregation using bivariate

COLONIALWATERBIRDS

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K-functions (Lotwick and Silverman 1982; Stoyan 1987; Duncan 1991; Dixon 1996) were used to determine if foraging points tended to be clustered with other points of the same habitat type. Due to limited sample size, habitat type was classified as either estuarine or palustrine for this analysis. To evaluate whether estuarine foraging locations were clustered with other estuarine foraging locations, the number of estuarine and the number of palustrine foraging locations within distance t of each estuarine point were counted. Clustering of estuarine locations was indicated by a higher than expected frequency of estuarine points in the vicinity of other estuarine points. This analysis was repeated for clustering of palustrine foraging locations. Monte Carlo simulations (950th of 999 replicates used as the upper bound) were used for all K-function analyses to determine the upper 95% confidence bound of the test statistic (Diggle and Chetwynd 1991). This provides a onesided test for clustering of points into similar habitats. Kruskal-WallisANOVA tests were used to determine if foraging distances were significantly different between colonies. A Mann-Whitney U-test was used to determine if foraging distance to estuarine habitat was significantly different from that to palustrine habitat. Finally, a chi-square test of independence was used to determine if foraging site selection was dependent upon tidal stage. For this model, all colony points were combined and tidal stage was classified as low (three hr block surrounding low tide), or high (three h block surrounding high tide) and habitat type was classified as estuarine or palustrine. RESULTS

Foraging Habitat We followed 41 storks from their colonies to foraging sites in 1995, and 31 in 1996. Single storks from Harris Neck also flew to and roosted in two other stork colonies 7.0 km and 14.5 km away.Total foraging areas used by storks from each colony varied considerably. Black Hammock had the smallest foraging area followed by Harris Neck and St. Simons (Table 1). Direct distances to foraging points between colonies did not differ significantly when all foraging points were

considered (Kruskal-Wallis ANOVA, X22= 3.265, P = 0.195) or when 75% of the closest foraging points were considered (Kruskal= 4.048, P = 0.132). Habitat WallisANOVA, X22 type (estuarine vs. palustrine) was not a significant factor in foraging site distance for storks from either the St. Simons (Mann-WhitneyUtest, U1= 73.00; P = 0.738) or Harris Neck colony (Mann-Whitney U-test, U1 = 94.00; P = 0.160) (Table 2). However, storks from Black Hammock flew longer distances to palustrine habitats than to estuarine habitats (Mann Whitney U-test, U1 = 18.00; P = 0.030) (Table 2). Additionally, storks (all colonies combined) used palustrine sites more during higher tide levels and estuarine sites more during lower tide levels (Chi-square test of in= 14.07, N = 72, P = 0.0001). dependence, X12 HABITAT USE BY COLONY

BlackHammock A total of 25 storks were followed from the Black Hammock colony to foraging sites, including 20 to estuarine habitats, primarily tidal creeks, and five to palustrine habitats (Tables 1-2). This was not significantly different than would be expected based on the distribution of individual wetland types within the colony's foraging MCP, but was significantly different when freshwater and estuarine wedands were pooled together (Table 3). Additionally, stork foraging habitat use was not significantly different for either series of chi-square tests when the locations within the 75% foraging zone were considered (Table 3). In all cases, storks foraged in tidal creek (estuarine) habitat more fre-

Table 1. Median foraging distance, range and total wetland area for the 75% foraging zone and 100% MCP for each colony. Kruskal-WallisANOVA's testing direct distances to foraging points between each colony did not differ significantly for the 75% foraging zone or the 100% MCP.

Colony

Foraging Zone

N

Median Distance (km)

Range (km)

Total Wetland Area (ha)

Black Hammock

75% Foraging Zone 100% MCP 75% Foraging Zone 100% MCP 75% Foraging Zone 100% MCP

19 25 18 24 18 23

3.9 4.1 5.7 6.8 2.3 4.4

1.1-3.9 1.1-38.5 0.3-9.4 0.3-34.8 0.6-14.4 0.6-24.1

3,045 10,728 16,300 46,600 21,170 24,595

St. Simons Harris Neck

47

WOOD STORKFORAGINGHABITATUSE Table 2. Median foraging distance and range for each colony based on habitat type (estuarine vs. palustrine).

Colony

Habitat Type

N

Black Hammocka

Estuarine Palustrine Estuarine Palustrine Estuarine Palustrine Estuarine Palustrine

20 5 10 14 8 15 38 34

St. Simons Harris Neck Total

Median Distance (km)

Range (km)

4.0 10.1 7.7 4.5 4.5 0.6

1.1-6.3 4.0-38.5 5.1-21.0 0.3-34.8 1.6-14.4 0.6-24.1 1.1-21.0 0.6-38.5

aForaging distances from the colony were significantly different based on habitat type (Mann Whitney U-test, U, = 18.00; P - 0.030).

quently than in any other habitat type or combination of habitats (Table 3). There was the tendency for estuarine foraging points to be surrounded by other estuarine foraging points (separated by any distance between one-ten km) more often than expected by random chance (Fig. 2). There were no significant clusterings of foraging points based on palustrine habitat (Fig. 2). Harris Neck

cluding ten to tidal creeks (estuarine) and 14 to palustrine wetlands (Table 3). This was significantly different than would be expected based on the distribution of wetlands within the colony's foraging MCP, but was not significantly different when freshwater and estuarine wetlands were pooled into two categories (Table 3). Storks foraged differently than expected within the 75% foraging zone regardless of whether or not habitats were pooled (Table 3). There were no significant clusterings of points based on estuarine habitat (Fig. 4). However, there was the tendency for palustrine foraging points to be surrounded by other palustrine foraging points (separated by any distance between 1.5-5.0 km) more often than expected by random chance (Fig. 4).

Twenty-three storks were followed from the Harris Neck colony to foraging sites. Eight of these flights were to tidal creeks (estuarine) and 15 were to palustrine habitats, including 8 to Snipe Pond, the foraging impoundment adjacent to the colony (Tables 13). This pattern of habitat use was significantly different than expected based on the distriDISCUSSION bution of wetlands within the foraging MCP of the colony for both series of tests (Table Foraging habitats used by Wood Storks 3). When foraging points within the 75% from three coastal colonies differed from zone were considered, chi-square tests were one another by varying degrees, particularly significant when habitat type was not pooled. when the 75% foraging zone was considered. There were significant clusterings of foraging The in habitat use among the differences points based on palustrine habitat separated three colonies best be explained by may by any distance between 0.5-3.5 km (Fig. 3). their relative to coastal waters and position Moreover, there was the tendency for estuatheir association with different-sized river by rine foraging points to be surrounded by othbasins and related wetland habidrainage er estuarine foraging points (separated by tats. storks from all colonies However, any distance between 7.5-8.0 km) more often showed in similarities their use of estuarine than expected by random chance (Fig. 3). and palustrine habitat. Habitat use was dependent upon tidal stage, with estuarine St. Simons (tidal creeks) use linked to lower tide levels Twenty-four storks were followed from and palustrine use linked to higher tide levthe St. Simons colony to foraging sites, in- els. Single storks (N = 4) observed flying

Table 3. Chi-square tests to compare Wood Stork foraging habitats with that expected as a function of wetland availability. Test the colony (100% of points) and the areas in which 75% (approximately) of the closest foraging points to the colony occurred. St. Simons

Black Hammock

Wetland Classification Type Tidal creek and associated saltmarsh Palustrine Forested Palustrine Emergent Palustrine Unconsolidated Bottom Other

75% of Foraging Points

100% of Foraging Points

75% of Foraging Points

100% of Foragin Points

% Area

Ea/Ob

% Area

E/O

% Area

E/O

% Area

E/O

91% 6% 2%