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Behavior and habitat use by shorebirds in an urban wetland complex along the central Gulf Coast of Texas.

Behavioral observations are important when assessing populations of shorebirds because these observations can be used to determine function of habitat. Many studies have focused on behavior of shorebirds within single habitats such as beaches, mudflats, or rocky intertidal zones (e.g., Goss-Custard, 1980; Burger, 1984), but few have compared behavior among various habitat types during an entire year (Lee, 1995; Burger et al., 1997; Brusati et al., 2001). To better understand ecology of migratory species, it is necessary to observe behavior throughout the year and in various types of habitats (Davis and Smith, 1998; DeLeon and Smith, 1999; Brusati et al., 2001). Many migratory staging, stopover, and wintering sites are located on temperate and tropical coastlines that are threatened by humans and other causes of habitat destruction. Determining behavior of shorebirds within various habitats provides evidence of their reactions to environmental changes, such as habitat loss that lead, in turn, to competition for space and prey resources (Goss-Custard and dit Durrell, 1990). If changes in behavior can be linked to environmental changes, earlier detection of decline of species and the factors contributing to it may be possible. Documenting behavior of shorebirds in a mosaic of habitats provides a better understanding of why different habitat types are selected and may also provide evidence for the need to protect what are perceived as lower-quality or marginal habitats.

Habitat requirements for different species and guilds of shorebirds vary in time and space, and this can be detected only by behavioral studies (De Leon and Smith, 1999). For example, black-bellied plovers (Pluvialis squatarola) exhibit a stereotypical foraging strategy (Baker, 1974), whereas willets (Catoptrohorus semipalmatus) are more versatile. Black-bellied plovers are visual foragers that tend to stand and scan the substrate for movement and peck, or walk and run while periodically stopping to peck. This type of foraging strategy limits both choice of prey and habitat type. Alternately, willets employ a variety of foraging methods and are able to exploit numerous prey items. In California, Stenzel et al. (1976) documented 30 prey items from fecal pellets of willets. Willets also use a wider variety of microhabitat and macrohabitat types when compared to black-bellied plovers. Therefore, assessing behavior of multiple species of shorebirds is necessary to assess the value of wetland complexes to shorebirds.

In this study, we determined behavior and use of habitat by shorebirds in five wetland habitat types within an urban wetland complex along the central Texas Gulf Coast. Other studies have focused on only a few species (Burger et al., 1997; De Leon and Smith, 1999; Brusati et al., 2001), whereas we observed behaviors of 22 species of shorebirds in six guilds. The purpose of this study was to determine behavior of shorebirds in, and use of, five specific habitat types within an urban coastal-wetland complex along the central Texas Gulf Coast.

MATERIALS AND METHODs -- Study Area -- Indian Point and Sunset Lake parks are located in Nueces and San Patricio counties along the Texas coastline of the northwestern Gulf of Mexico (27[degrees]51'N, 97[degrees]21'W, Fig. 1). Climate in this region is humid subtropical with an average temperature of ca. 22[degrees]C and average annual precipitation of 72.4 cm (Brown et al., 1976). Both parks are altered coastal habitats currently managed for both wildlife and recreational use. The study area encompasses 46.4 ha and is located within the Nueces Estuary.

[FIGURE 1 OMITTED]

This site was selected because it offered a mosaic of habitats with potential for differing functions and values to birds. The landscape consisted of undeveloped bay beach frontage, irregularly flooded tidal flats, uplands, an excavated estuarine lake with an unvegetated shoreline, and numerous depressional brackish-water ponds. The area provides wintering, breeding, and migratory stopover habitats for numerous species of birds, including shorebirds, waders, gulls, terns, waterfowl, and songbirds. The federally listed piping plover (Charadrius melodus) also has been documented along the shoreline of Sunset Lake and the bayside beach of Corpus Christi Bay (Hicks and Company, in litt.). The site was separated into five wetland habitat types: beach adjacent to Corpus Christi Bay, tidal and isolated depressional ponds within the park, a section of shoreline bordering Sunset Lake, and a salt marsh.

Census -- Censuses were conducted within 1 h of sunrise with 10 by 40 binoculars in each habitat at least twice monthly during 15 August 2002-25 July 2003. Sampling was not performed when winds were >40.2 km/h or during heavy rains. To determine general behavior (foraging or other), instantaneous scans were performed in concert with censuses within the five habitats (Altmann, 1974), excluding flyovers. Classification of birds followed the AOU checklist (American Ornithologists' Union, 2000); however, six species were difficult to identify during winter. Therefore, long-billed and short-billed dowitchers (Limnodromus scolopaceus and L. griseus) were identified as dowitchers. Western and semipalmated sandpipers (Calidris mauri and C. pusilla) were identified as peeps, and greater and lesser yellowlegs (Tringa flavipes and T. melanoleuca) were classified as yellowlegs.

Statistical Analysis -- Prior to analysis of data, shorebirds were separated into guilds based on taxonomic family, foraging strategy, and size. Guilds were as follows: large scolopacids-long-billed curlew (Numenius americanus), marbled godwit (Limosa fedoa), whimbrel (Numenius phaeopus); medium scolopacids-willet, yellowlegs; sandpipers-peeps, least sandpipers (Calidris minutilla), dunlin (C. alpina), sanderlings (C. alba); dowitchers were placed alone; recurvirostrids-American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus); large charadriids-black-bellied plovers, killdeer (Charadrius vociferous), and ruddy turnstones (Scolopacidae, Arenaria interpres) because they occupy a niche similar to black-bellied plovers; small charadriids-piping plovers, snowy plovers (Charadrius alexandrinus), Wilson's plovers (C. wilsonia), semipalmated plovers (C. semipalmatus); and piping plovers also were analyzed alone due to their endangered status. Seasons were defined with regard to migratory lifecycle of shorebirds, with the designation of each season corresponding with annual movements into this region: autumn, August-October; winter, November January; spring, February April; and summer, May-July.

Prior to analysis, all census data were transformed with logarithmic transformation ([log.sub.10] +1) to normalize distribution (Sokal and Rohlf, 1995). One-way ANOVA (P < 0.05) and Tukey's HSD mean-separation procedure were used to determine differences between mean number of birds foraging or exhibiting another behavior among habitat types. One-way ANOVA (P < 0.05) and Tukey's HSD mean-separation procedure also were used to determine differences between mean number of birds foraging or exhibiting another behavior by season. All transformations and analyses were performed using SPSS version 10.0 (SPSS, Inc., 1998). Differences in overall mean numbers of shorebirds engaged in foraging or other activities were determined using Student's t-test. Bray-Curds cluster analysis was used to explore similarities in behaviors among habitats.

RESULTS -- A total of 3,854 shorebirds representing 22 species and 6 guilds were observed during the 28 censuses-instantaneous behavioral scans completed during the study. These species represented ca. 60% of documented taxa known to occur along the South Texas coastline (Muehl, 1994). Overall, the greatest numbers of shorebirds were in tidally influenced ponds (44.5%) followed by isolated ponds (19.3%). Number of shorebirds in bayshore and saltmarsh habitats were almost equal (ca. 13.6%), with few birds present in lakeshore habitat (8.9%). At least one member of each guild, with the exception of large scolopacids in the isolated-pond habitat, used one of the five habitats in the study area (Fig. 2).

Instantaneous Behavioral Scans -- Overall, 90% of shorebirds were foraging during instantaneous scans (Table 1). Number of foraging shorebirds was significantly greater during spring than summer (P = 0.001, df = 3, F = 6.11). Throughout the study, total number of shorebirds were foraging significantly more at tidal ponds than in the other four habitats (P = 0.002, df = 4, F = 5.84; Fig. 3). However, there was no significant difference among habitats when it came to activities other than foraging. Behavior of shorebirds was most similar in the saltmarsh, isolated ponds, and bayshore (Fig. 4a). However, when each guild was examined individually, behavior within habitats varied (i.e., some guilds foraged significantly more in one habitat; whereas others performed other activities; Table 2).

Instantaneous Behavioral Scans By Guild -- Numher of foraging large scolopacids was significantly greater in saltmarsh habitat (P = 0.001, df = 4, F = 9.94), with no significant difference among habitats for other behaviors. There was no significant difference in foraging or other behaviors among seasons. Behaviors of large scolopacids in bayshore and tidal ponds were dissimilar to that in lakeshore and saltmarsh (Fig. 4b). This dissimilarity between habitats was driven by the fact that large scolopacids only used tidal ponds and bayshore for foraging (Table 2).

Number of foraging medium scolopacids was significantly less in lakeshore and bayshore habitats (P = 0.001, df = 4, F = 23.52). This group engaged in other behaviors significantly more in saltmarsh in comparison to lakeshore and bayshore (P = 0.004, df = 4, F = 4.03). Medium scolopacids foraged significantly more in winter and spring than in summer (P = 0.004, df = 3, F = 4.685). Overall, number of medium scolopacids engaged in foraging was significantly greater than those engaged in other activities (P = 0.001, df= 278, t= 7.11). Behavior of medium scolopacids was more similar in saltmarsh, isolated ponds, and tidal ponds than in bayshore and lakeshore (Fig. 4c). This dissimilarity in use among habitats was driven by the fact that medium scolopacids used saltmarsh, isolated ponds, and tidal ponds for foraging and other behaviors, whereas bayshore and lakeshore were used only for foraging (Table 2).

There was no significant difference among habitats for foraging or other activities of sandpipers. However, number of foraging sandpipers was significantly greater during spring (P = 0.001, df = 3, F = 8.16; P = 0.001). Overall, number of sandpipers engaged in foraging was significantly greater than those engaged in other activities (P = 0.001; df = 278; t = 7.11). Behavior among habitats by members of the sandpiper guild was relatively similar between isolated ponds and bayshore, and between tidal ponds and lakeshore, while behavior in the saltmarsh differed (Fig. 4d). This dissimilarity between isolated ponds and bayshore in comparison to other habitats may be driven by the fact that these habitats were used only for foraging, whereas the others were used for foraging and other behaviors (Table 2).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Number of dowitchers observed foraging and engaged in other behaviors was significantly greater in tidal-pond habitat (P < 0.001; df = 4; F = 7.326; P = 0.003; df = 4; F = 4.29, respectively). There was no significant difference among seasons for foraging or other behaviors; however, numbers of dowitchers engaged in foraging were significantly greater than those engaged in other behaviors (P = 0.006; df = 278; t = 2.793). Behavior of dowitchers within tidal ponds was unique and, overall, behaviors exhibited in bayshore and isolated ponds were not similar to behaviors in lakeshore and saltmarsh (Fig. 4c). This dissimilarity in tidal ponds is driven by the fact that tidal ponds were used almost exclusively by dowitchers (Table 2).

Number of foraging recurvirostrids was significantly greater in isolated and tidal ponds compared to lakeshore and bayshore habitats (P = 0.001, df = 4, F = 1.13). There was no significant difference among seasons for foraging or other behaviors; however, numbers of recurvirostrids engaged in foraging were significantly greater than those engaged in other activities (P = 0.001; df = 278, t = 4.38). Recurvirostrids exhibited similar behaviors within tidal and isolated ponds and saltmarsh, and these behaviors had virtually no similarity to that in the other two habitats (Fig. 4f). This dissimilarity between habitats is driven by the fact that recurvirostrids rarely used bayshore and lakeshore, and when they did, they either only foraged or exhibited other behaviors (Table 2).

Number of foraging large charadriids was significantly greater in bayshore habitat in comparison to lakeshore (P = 0.001, df = 4, F = 1.29). Large charadrids foraged significantly less in summer (P = 0.001, df = 3, F = 8.15). Overall, numbers of large charadriids engaged in foraging was significantly greater than those engaged in other activities (P = 0.001; df = 278; t = 7.88). Behavior of large charadriids in lakeshore habitat was dissimilar to that in the other habitats (Fig. 4g). This dissimilarity is due to the fact that low numbers of large charadriids used the lakeshore, and when they were present, they only foraged (Table 2).

Number of foraging small charadriids was significantly greater in bayshore habitat (P = 0.001, df = 4, F = 1.00) and significantly greater during spring than autumn (P = 0.035, df = 3, F = 2.94). Overall, the number of small charadriids engaged in foraging was significantly greater than those engaged in other behaviors (P = 0.001; df = 278; t = 4.92). Behavior of small charadriids within bayshore was dissimilar to that exhibited in other habitats (Fig. 4h). This dissimilarity is due to the fact that bayshore was used for both foraging and other behaviors, whereas tidal ponds and lakeshore were used exclusively for foraging (Table 2).

Throughout the study, piping plovers were only observed foraging (Table 2). Number of foraging piping plovers was significantly greater in bayshore habitat than in saltmarsh, isolated ponds, and tidal ponds (P = 0.001, df = 4, F = 1.00); they were never documented in lakeshore habitat. There was no significant difference among seasons for foraging.

DISCUSSION -- At Indian Point and Sunset Lake parks, foraging was the dominant activity of shorebirds throughout the year, with a few exceptions. Studies of stopover sites have shown that foraging is a predominant behavior exhibited by shorebirds during autumn and spring migration (Goss-Custard, 1977; Wishart and Scaly, 1980; Davis and Smith, 1998). In our study, foraging was greatest in spring and least in summer when the majority of shorebirds already had traveled north. This area serves as an important stop for spring migrants who are building fat reserves before continuing north. For example, sanderlings (intermediate-distance migrants) need to replenish fat reserves during their long migration (Skagen et al., 1999). In our study, sanderlings, a member of the sandpiper guild, spent 99% of their time foraging. For many shorebirds using the Central Flyway, such as sanderlings, the Texas coastline is the first suitable habitat encountered after crossing the Gulf of Mexico from South America (Withers, 2002).

[FIGURE 4 OMITTED]

As previously reported along the Texas coast (Lee, 1995; Brusati et al., 2001), willets, members of the medium scolopacids guild, foraged the majority of time during our study. In barrierisland habitats, there was no significant difference in behavior among seasons (Lee, 1995). However, in our study, medium scolopacids foraged more during winter and spring than in summer. During winter and spring, both resident and migratory willets are present along the Texas coast, which may explain higher numbers observed foraging during these seasons. Another possibility is the colder temperatures experienced during winter. During summer, they may be allocating more time to breeding and nest care.

In our study, members of the sandpiper guild foraged the majority of time within all habitats when suitable shoreline was exposed. This is similar to observations of Brusati et al. (2001) who observed peeps foraging the majority of time, regardless of habitat type. Numbers foraging were greatest during spring, corresponding to migration and the need to build necessary fat reserves to successfully complete migration to breeding grounds.

In our study, foraging was the dominant behavior exhibited by shorebirds regardless of habitat. Behaviors of shorebirds within the five habitats seemed to be related to morphological differences among species. For example, medium scolopacids and recurvirostrids foraged significantly more in habitats that had greater amounts of vegetation, less bare shoreline, and deeper water, whereas small charadriids and large charadriids used areas with large expanses of shoreline and shallower water. This difference may be due to larger size and longer legs of medium scolopacids and recurvirostrids, which allow them to exploit a more diverse array of habitats. In addition, these two guilds have members that make use of both tactile and visual foraging strategies (Hamilton, 1975; Stenzel et al., 1976; Petersen, 2001). Conversely, small and large charadriids are limited to shallow or exposed habitat by length of leg and bill, making some prey inaccessible. This supports the need to protect and develop wetland complexes that contain a mosaic of habitat types, because they are more likely to support numerous guilds of shorebirds.

Small-bodied shorebirds from the sandpiper and small-charaddid guilds spend more time foraging overall (Goss-Custard et al., 1977), perhaps because metabolic rates of shorebirds increase as body sizes decrease (Pienkowski and Evans, 1984). Small charadriids and sandpipers spent the majority of time foraging during our study. Difference in time spent foraging between large and small shorebirds may be due to different metabolic requirements.

Black-bellied plovers, a member of the large charadrids, did not forage the majority of time. They spent equal time foraging and in motion unrelated to foraging. This combination of foraging and motion has been documented in other studies along the Texas Coast (Lee, 1995; Brusati et al., 2001). Brusati et al. (2001) observed black-bellied plovers standing and in motion mainly at natural sites along the coast, whereas foraging was observed more often in created wetlands. Foraging by black-bellied plovers may have been underestimated in our study, as well as by Brusati et al. (2001), due to their stereotypical visual-foraging strategy (Baker, 1974). They were observed foraging more frequently during spring in our study and that of Lee (1995), corresponding to spring migration and the need to replenish fat reserves. They are mid-distant migrants and the coastline of the Gulf of Mexico is an important stop on their migration route (Skagen and Knopf, 1993; Skagen et al., 1999).

Piping plovers foraged the majority of time in our study, and in others, along the Texas Coast (e.g., Lee, 1995). There was no significant difference in foraging by piping plovers among seasons; however, Lee (1995) observed piping plovers foraging more during spring and in evening. Overall, small charadriids foraged significantly more during spring. Our study focused on early morning, so no comparison can be made in regards to time of day and behavior. In our study, piping plovers spent significantly more time foraging in bayshore habitat. This habitat is similar to gulf-shore beaches and may be an important foraging habitat during times of high tides and rainfall.

In conclusion, behavioral observations at Indian Point and Sunset Lake parks helped determine overall use of this urban wetland complex. This area provides a valuable resource to foraging migrants and residents. Besides providing prey resources, it was a resting site for a few species and provided protection and shelter. As was seen in our study, behavioral observations provided important insight into function of habitat, underscoring the need to incorporate behavioral studies into strategies to monitor shorebirds. Increased diversity of wetland complexes or landscapes allows shorebirds to exploit many resource patches while limiting energetic expenditure or cost of searching for food (Farmer and Parent, 1997). Connectivity of patches of habitat at Indian Point and Sunset Lake parks is important because birds use both prime habitat (tidal ponds) and marginal habitat (lakeshore). Availability of lower-quality, but still usable, habitats within a wetland complex provides birds with alternatives when prime habitats are crowded or unavailable. Data presented here support the need for multiple, linked, habitat types within wetland complexes. For healthy populations of shorebirds to continue along the Texas Coast and other North America coastlines, we must protect these remaining coastal habitats. When both prime and marginal coastal-wetland complexes are in proximity, they offer an important resource for numerous species of shorebirds.

We thank the United States Fish and Wildlife Service, Ecological Services, Corpus Christi, Texas, for assistance in the field and for financial support. We also thank the city of Portland, Texas, for allowing us to conduct research in the two parks, J. M. Garvon, M. R. Kolar, and J. J. Pearce for assistance and support in the field, and the two anonymous reviewers for comments and corrections on the original manuscript; their insight was helpful and worthwhile.

Submitted 23 March 2008. Accepted 8 June 2008. Associate Editor was Michael S. Husah.

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SHANNON R. ROWELL-GARVON * AND KIM WITHERS

United States Fish and Wildlife Service, Ecological Services Field Office, 6300 Ocean Drive, LESS 338, Corpus Christi, TX 78412 (SRG)

Center for Coastal Studies, Texas A&M University-Corpus Christi, 6300 Ocean Drive, NRC 3200, Corpus Christi, TX 78412 (KW)

Present Address of SRG. Department of Biology and Chemistry, Algoma University, 1520 Queen Street East, Sault Ste. Marie, OAT P6A 2G4, Canada

*Correspondent. shannon.rowellgarvon@algomau.ca
TABLE 1--Abundance (means [+ or -] SE) and behavior of shorebirds
recorded during instantaneous scans at Indian Point and
Sunset Lake parks, Nueces and San Patricio counties,
Texas, August 2002 July 2003.

Season Month Number of Mean
 censuses number of
 birds

Autumn August 2002 2 114 [+ or -] 6.8
 September 3 45 [+ or -] 3.9
 October 3 93 [+ or -] 1.9
Winter November 3 131 [+ or -] 8.7
 December 2 188 [+ or -] 10.3
 January 2003 3 199 [+ or -] 2.3
Spring February 2 267 [+ or -] 6.4
 March 2 287 [+ or -] 6.1
 April 2 240 [+ or -] 3.2
Summer May 2 22 [+ or -] 0.2
 June 2 23 [+ or -] 1.0
 July 2 60 [+ or -] 8.2
Total 28 3,854

Season Percentage Percentage
 foraging performing
 other
 activities

Autumn 94.3 5.7
 99.3 0.7
 78.8 21.2
Winter 77.2 22.8
 88.8 11.2
 95.6 4.4
Spring 95.7 4.3
 82.4 17.6
 96.5 3.5
Summer 86.0 14.0
 100.0 0.0
 97.5 2.5
Total 90.0 10.0

TABLE 2--Use of habitat by shorebirds based on total
number and percentage of guild members engaged in foraging
or other activities during instantaneous scans performed
at Indian Point and Sunset Lake parks, Nueces and San
Patricio counties, Texas, August 2002 July 2003.

 Total number of shorebirds

Shorebird Saltmarsh Isolated Tidal
 ponds ponds

Large scolopacids
 Foraging 40 0 7
 Other activities 4 0 0
Medium scolopacids
 Foraging 200 256 264
 Other activities 29 17 10
Sandpipers
 Foraging 59 170 245
 Other activities 0 2 0
Dowitchers
 Foraging 28 10 659
 Other activities 5 28 179
Recurvirostrids
 Foraging 52 140 253
 Other activities 10 9 2
Large charadriids
 Foraging 85 69 51
 Other activities 7 4 4
Small charadriids
 Foraging 11 21 20
 Other activities 1 7 0
Piping plover
 Foraging 1 0 2
 Other activities 0 0 0
Total
 Foraging 476 666 1,501
 Other activities 56 67 196

 Total number of shorebirds

Shorebird Lakeshore Bayshore

Large scolopacids
 Foraging 0 4
 Other activities 11 0
Medium scolopacids
 Foraging 15 45
 Other activities 1 0
Sandpipers
 Foraging 191 172
 Other activities 0 4
Dowitchers
 Foraging 102 6
 Other activities 1 0
Recurvirostrids
 Foraging 0 2
 Other activities 4 0
Large charadriids
 Foraging 4 125
 Other activities 0 10
Small charadriids
 Foraging 9 115
 Other activities 0 31
Piping plover
 Foraging 0 41
 Other activities 0 0
Total
 Foraging 321 510
 Other activities 17 45

 Percentage

Shorebird Foraging Other
 activities

Large scolopacids 77.3 22.7
 Foraging
 Other activities
Medium scolopacids 93.2 6.8
 Foraging
 Other activities
Sandpipers 99.3 0.7
 Foraging
 Other activities
Dowitchers 79.1 20.9
 Foraging
 Other activities
Recurvirostrids 94.7 5.3
 Foraging
 Other activities
Large charadriids 93.0 7.0
 Foraging
 Other activities
Small charadriids 81.9 18.1
 Foraging
 Other activities
Piping plover 100.0 0.0
 Foraging
 Other activities
Total 90.1 9.9
 Foraging
 Other activities
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Author:Rowell-Garvon, Shannon R.; Withers, Kim
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1U7TX
Date:Jun 1, 2009
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