Habitat use by wintering shorebirds along the lower Laguna Madre of south Texas.
The lower Laguna Madre, which is located in subtropical southern Texas, is characterized by extensive unvegetated mudflats and sandflats near mean sea level as well as large areas covered by seagrasses below mean sea level (Britton & Morton 1989). Mild winters and extensive intertidal flats support the existence of many shorebirds in the Laguna Madre region of both Texas and Tamaulipas (Mitchell & Boyd 1992; Morrison et al. 1993; Withers & Chapman 1993). Some of the largest known wintering populations of the threatened Piping Plover and the declining Snowy Plover also occur in the Laguna Madre of south Texas (Haig & Plissner 1993). This study was undertaken to determine patterns of habitat use by shorebirds and their responses to changing habitat conditions during winter months along the lower Laguna Madre of south Texas.
All study areas were located along the western shore of the lower Laguna Madre, Cameron Co., Texas, 26[degrees]04'-26[degrees]22'N and 97[degrees]10'-97[degrees]20' W. Habitats were classified based on frequency of exposure and presence of pools, algal flats, or seagrass. Large areas of intertidal habitat are alternately exposed and covered by wind shifts associated with fronts during the winter (Mitchell 1992), whereas the effects of astronomic tides are restricted to passes (Breuer 1962).
High Mudflat/Pool. -- This area is comprised of a large complex of enclosed shallow water and exposed, firm mudflat just west of Horse Island in Unit 5, Laguna Atascosa National Wildlife Refuge (LANWR). This is the only plot to have a relatively permanent, shallow pool, which covered the deepest portion of the study area. Strong south and southeast winds exposed extensive mudflats [greater than or equal to]2000m wide, whereas north and northwest winds exposed more limited areas of mudflats. Blue-green algae formed isolated mats over the mud in areas periodically exposed, but such algal mats were poorly developed and temporary. Although nearly devoid of vegetation in November 1992, some of the mudflats within 50 m of the usual water's edge became partially covered with glasswort (Salicornia bigelovii) during December-February 1992-1993. By November 1993, glasswort covered large areas of secondary coves and other regularly-inundated sites throughout the study area.
High Algal Flat. -- This area is located immediately south and east of Horse Island and is characterized by wind-tidal flats covered with a well-developed algal mat. The algal mat provided a firm, tough surface in much of this study area. High Algal Flat was covered with water during strong southeast winds or seasonal high water, and was exposed by strong north and northwest winds. Limited Salicornia covered this flat by February 1993, but coverage never became dense in this area. There was no increase in glasswort density during the second winter, but dead glasswort stalks remained on the flats during the second winter. New germination was not noticed until late February 1994.
Low Mudflat. -- This area (the shoreline section of Bayside Tour Loop, Unit 7, LANWR) includes the shoreline and associated mudflat and seagrass areas visible from the shore. These mudflats are frequently covered with water and are exposed mainly after cold fronts with associated north and northwest winds (Mitchell & Boyd 1992). Seagrasses (primarily Halodule beaudettei, with some Thalassia testudinum, Syringodium filiforme and Halophila engelmannii) covered a limited area exposed offshore during the period of lowest water levels.
Low Seagrass Flat. -- This area includes the intertidal area between Port Isabel and Laguna Heights, north of Texas Route 100. Turtle-grass (Thalassia testudinum) covered about half of these low mudflats, which were inundated and exposed on a daily basis by tides throughout most of the winter months.
Each area was censused every two weeks, as accessibility allowed. Usually, all areas were censused during the same day, although occasionally two days were needed. During some rainy periods, not all areas were censused, due to road conditions. Each study area was censused 15-17 times. The distance to the water's edge was estimated at reference points during each census. Species were identified based on plumage, silhouette, behavior, and calls (Hayman et al. 1986). Because of frequent identification difficulties, only 4% of all dowitchers were identified to the species level. Therefore, Short-billed and Long-billed Dowitchers were treated as "dowitchers" in this paper. At times, mainly on the low mudflat plot, small sandpipers were lumped as Calidris spp. ("peeps") due to distance. This category does not include the Stilt Sandpiper (C. himantopus), a much larger species. 10 X 40 binoculars were used to scan for shorebirds, and a 15-45 X zoom telescope was used to identify most individuals. During each census, microhabitat use was recorded for each species, as shallow water only, shallow water and exposed mud, or exposed mud only.
General patterns. -- A total of 23 shorebird species (Table 1) were observed on the study plots, with 19 species recorded in numbers averaging at least one individual per census on at least one study plot (Table 1). The highest number of shorebirds present at a single time on a study area was 20,978 birds on High Mudflat/Pool, on 11 January 1994. That plot averaged the highest total numbers of shorebirds, and contained the largest single species total: 16,930 Western Sandpipers, also on 11 January 1994. Eleven species were recorded on all four plots, but three species were restricted to [less than or equal to]2 study plots (American Avocet, Marbled Godwit, and Stilt Sandpiper). Peak total numbers of shorebirds occurred at different times on each of the four plots (Fig. 1).
High Mudflat/Pool. -- The largest numbers of many shorebird species were recorded here. Several species, including American Avocet (peak 1200 on 23 January 1993) and Stilt Sandpiper (peak 480 on 17 December 1992), were essentially restricted to this habitat. Total monthly shorebird numbers peaked in December 1993 and January 1994 (Figure 1), and Western Sandpipers averaged 48% of the shorebird community. Total shorebird numbers peaked during low water (Fig. 2). Least Sandpipers reached peak abundance here (Table 1) and foraged both on wet and dry mudflats and among Salicornia. Species in the water-foraging group regularly occurred in large numbers here (Table 2). During each winter, such pool-users peaked in December and January, but monthly averages fluctuated relatively little, compared to numbers of the shorebird community as a whole (Figures 1 & 3). Large pool-using species such as American Avocets often foraged in or near ardeids, such as Reddish Egret (Egretta rufescens) and Snowy Egret (Egretta thula), which often occurred in large numbers.
Small plovers, including Piping Plover, Snowy Plover, and Semipalmated Plover, occurred mainly on High Mudflat/Pool and on High Algal Mudflat. They were observed in small, loose flocks away from or at the edge of larger groups of other mudflat foragers. Plover numbers varied considerably, with lowest numbers in January 1993 and February 1994 (Figure 4). Piping Plovers averaged [greater than or equal to]10/census only in November 1992 and December 1993. Snowy Plovers peaked during November-December of each winter, but they remained fairly common until February 1994. In contrast, Semipalmated Plovers were uncommon during most of the first winter but became fairly common during December 1993 and January 1994. Snowy Plovers foraged both near and far from the water's edge, while Piping and Semipalmated Plovers foraged mainly on the wet flats near the water's edge (below the glasswort zone, once it became established).
High Algal Mudflat. -- This area supported lower numbers of shorebirds, and was used sporadically (Figure 1). Peak numbers occurred during intermediate water levels, with more limited use during peak low or high water (Figure 2). Least Sandpipers were the most common Calidris sandpiper identified here and often foraged near small plovers. Flocks of dowitchers occurred erratically. Mixed flocks of small plovers consistently occurred here during only November-December 1992. Peak plover numbers were 95 Semipalmated Plovers on 27 November 1992, and 69 Piping Plovers on 14 November 1992. However, small plovers were nearly absent during the second winter and were never seen foraging among the glasswort.
[FIGURE 1 OMITTED]
Low Mudflat. -- This area, along Bayside Drive, was used heavily by shorebirds during extreme low water conditions in November 1993 and January 1994 (Figures 1 & 2). Peak numbers for the following widespread shorebirds were recorded on this plot: Calidris spp.: 7700 on 5 January 1994; dowitchers: 2290 on 5 January 1994; Black-bellied Plover: 1450 birds, on 27 November 1993; and Willet: 1100 on 11 January 1994. Western Sandpipers were common to abundant, while Dunlin were occasionally common. Least Sandpipers were uncommon (max. 40 individuals). Small plover flocks were limited to extreme low water conditions, during which they foraged on moist offshore bars with Calidris sandpipers and Black-bellied Plovers.
[FIGURE 2 OMITTED]
The largest shorebird species, Long-billed Curlews and Marbled Godwits were essentially or entirely restricted to Low Mudflat. Long-billed Curlews peaked at 205 on 26 November 1993, while Marbled Godwits peaked at 150 on 17 December 1992. Both these species were often seen in relatively deep water, near Reddish Egrets, Great Egrets (Casmerodius albus) and White Ibis (Eudocimus albus). Curlews foraged in loose flocks of 10-50 or in small groups of 2-3 birds, and were often observed flying between the intertidal flats and inland areas of LANWR. They foraged by probing into the exposed mud and into mud or seagrass below shallow water. Godwits usually occurred in more compact, larger flocks, when present. They foraged on exposed wet mud or in shallow water, and were never observed flying inland.
[FIGURE 3 OMITTED]
Low Seagrass Flat. -- This long, linear study area was utilized by a limited number of shorebirds, mainly during low water (Figures 1 & 2). Only Willets occurred here in large numbers (peak 400 on 16 December 1993), but dowitchers, Black-bellied Plovers, and Western Sandpipers occurred in modest numbers during low water. Willets foraged frequently in turtle-grass when its tops were exposed, along with some Long-billed Curlews, Reddish Egrets and Tricolored Herons (Egretta tricolor). The other shorebirds foraged on the bare, moist mudflats exposed during lowest water conditions. Peak monthly averages of all shorebirds exceeded 1500 only during January 1994, while shorebirds were absent during the two November 1993 censuses (Figure 1).
[FIGURE 4 OMITTED]
Discussion and Conclusions
Many shorebirds of several species wintered commonly on the mudflats of the lower Laguna Madre. Although densities could not be determined in an unbiased fashion, specific patterns of habitat use were primarily based on the foraging requirements of each species. Poolusing birds preferred the large enclosed pool on one study area (High Mudflat/Pool) and were relatively consistent in numbers compared to mudflat foragers. The substantial wintering Stilt Sandpiper population on High Mudflat/Pool during the first winter was unexpected, given its winter status in the United States as irregularly uncommon (Oberholser & Kincaid 1974) or casual (AOU 1983). The large, enclosed pool which the Stilt Sandpipers used is typical Stilt Sandpiper habitat (Hayman et al. 1986) but probably represents a rare habitat in the Lower Laguna Madre.
Mudflat and mudflat/pool foragers, dominated by Western Sandpipers and other Calidris spp., fluctuated in response to changing water levels in all study areas. Highest numbers were recorded when extensive flats in the High Mudflat/Pool and Low Mudflat study areas were newly exposed and still moist. When mudflats dried out due to prolonged exposure, such as in High Algal Mudflat, shorebird use declined. Since ideal conditions are partially dependent on wind shifts due to passage of periodic fronts, such use by shorebirds appears to be very opportunistic.
Relative abundances of particular species in the Low Mudflat area were generally similar to those found by Mitchell & Boyd (1992) in the same area (Bayside Drive). Assuming that most of the Calidris spp. sandpipers in Low Mudflat were Western Sandpipers, then only Black-bellied Plovers, both yellowlegs, and Marbled Godwits were disproportionately more common during my censuses than Mitchell & Boyd's (1992) in Low Mudflat habitat. The peak of >1400 Black-bellied Plovers appears high, as it represents more than three times the total number seen along the Laguna Madre of Tamaulipas (Morrison et al. 1993). Marbled Godwits, uncommon during this study, may winter in large flocks in the Laguna Madre of Tamaulipas, a similar hypersaline lagoon in northeastern Mexico (Morrison et al. 1993). The nearness of coastal saline prairie to extensive intertidal feeding areas may explain the higher numbers of Long-billed Curlews on Low Mudflat than in my other areas or in Oso Bay (Withers & Chapman 1993). Snowy Plovers and Dunlin were somewhat less common during my censuses than in Mitchell and Boyd (1992), particularly given the generally higher numbers of shorebirds found in this study. The total absence of Black-necked Stilts during this study is inexplicable, given their winter occurrence in the Corpus Christi area (Withers & Chapman 1993).
The modest peak numbers of small Charadrius plovers were expected, but the failure of many small plovers to winter in any plots was somewhat unexpected. Open algal flats normally support Snowy and Piping Plovers and Least Sandpipers in the Laguna Madre, but the spread of glasswort onto both algal flats and high mudflats may have made those habitats unsuitable for small plovers. This would be particularly true for Snowy Plovers and Piping Plovers, which forage on somewhat higher flats--the area covered by Salicornia. None of the three small plovers was recorded foraging in Salicornia-covered areas, even in very open stands. Relatively large numbers of plovers only occurred when damp or wet flats were available above or below the Salicornia-covered areas.
Shorebird use of the lower seagrass meadows and lower, bay-margin mudflats by large numbers of Calidris sandpipers, Willets, and Black-bellied Plovers was linked with cold fronts (northers) and seasonal low water in winter (Breuer 1962). Such low mudflats probably supply abundant food resources for shorebirds when they are exposed. These flats remain moist during short-duration exposures during the cool temperatures of winter. In contrast, the frequently exposed higher mudflats often dry out and are apparently unsuitable for most shorebirds in the absence of rainfall (Skagen & Knopf 1993). Shorebirds have seldom been observed previously foraging in seagrass, except in intertidal areas (Dann 1987).
There is variability in all wetland/mudflat ecosystems, but the climatic variability, large size, shallow water, and flatness of the lower Laguna Madre ecosystem appear to make it particularly variable. Areas heavily used one day, month, or season may be little used at another time due to slight changes in water level. During times when neither the High Mudflat/Pool area nor the Low Mudflat area would be suitable for large numbers of mudflat-foraging shorebirds, other nearby areas may be suitable. For example, on the 1993 Laguna Atascosa Christmas Bird Count, >20,000 shorebirds (mainly Western Sandpipers and dowitchers) were recorded on the Buena Vista Ranch, located adjacent to LANWR. Mitchell (1992) noted similar shifts in Redheads (Aythya americana) in the Lower Laguna Madre, due to changing water levels.
Evidence to date suggests that (1) extensive areas along an elevational gradient are necessary to support large numbers of wintering shorebirds, and (2) that high mudflats free of Salicornia are needed to support wintering flocks of small plovers, including the declining Snowy and Piping Plovers.
Table 1. Mean and maximum (in parenthesis) numbers of shorebirds for species averaging at least 1/census on at least one study plot (1), Lower Laguna Madre, November-February 1992-1993 and 1993-1994. Asterisk (*) indicates highest mean or maximum for a given species. Habitat Type Avian Species High Mudflat Low Mudflat Black-bellied Plover, 77.4 (320) *246.9 (1450) (Pluvialis squatarola) Snowy Plover *17.9 (51) 1.2 (21) (Charadrius alexandrinus) Semipalmated Plover 13.1 (91) 2.5 (23) (Charadrius semipalmatus) Piping Plover 4.9 (24) 2.1 (13) (Charadrius melodus) Killdeer *5.8 (23) 0.9 (5) (Charadrius vociferus) American Avocet *412.9 (1200) 0.7 (12) (Recurvirostra americana) Greater Yellowlegs *52.4 (198) 16.2 (110) (Tringa melanoleuca) Lesser Yellowlegs *69.8 (360) 22.9 (270) (Tringa flavipes) Willet 127.7 (590) *261.5 (1100) (Catoptrophorus semipalmatus) Long-billed Curlew 0.1 (1) *71.2 (205) (Numenius americanus) Marbled Godwit 0.0 (0) *37.7 (55) (Limosa fedoa) Ruddy Turnstone 0.1 (2) *3.8 (12) (Arenaria interpres) Sanderling *1.3 (7) 1.0 (8) (Calidris alba) Western Sandpiper *2259.1 (16930) 390.9 (4370) (Calidris mauri) Least Sandpiper *281.7 (1320) 4.1 (40) (Calidris minutilla) Peep spp. 330.0 (2000) *1156.5 (7700) (Calidris spp.) Dunlin *367.3 (1960) 85.9 (640) (Calidris alpina) Stilt Sandpiper *126.0 (480) 0.0 (1) (Calidris himantopus) Dowitcher spp. 556.0 (2000) *660.4 (2290) (Limnodromus spp.) (2) Total (3) (Maximum (4)) *4704.8 (20978) 2966.5 (11980) Habitat Type Avian Species Algal Mudflat Seagrass Flat Black-bellied Plover, 1.1 (5) 17.5 (75) (Pluvialis squatarola) Snowy Plover 8.0 (36) 0.0 (0) (Charadrius alexandrinus) Semipalmated Plover *14.2 (95) 0.6 (3) (Charadrius semipalmatus) Piping Plover *5.1 (69) 0.1 (1) (Charadrius melodus) Killdeer 0.8 (4) 0.4 (6) (Charadrius vociferus) American Avocet 0.0 (0) 0.0 (0) (Recurvirostra americana) Greater Yellowlegs 3.6 (12) 1.7 (10) (Tringa melanoleuca) Lesser Yellowlegs 3.2 (15) 0.2 (3) (Tringa flavipes) Willet 1.5 (6) 99.7 (400) (Catoptrophorus semipalmatus) Long-billed Curlew 0.0 (0) 4.8 (25) (Numenius americanus) Marbled Godwit 0.0 (0) 0.0 (0) (Limosa fedoa) Ruddy Turnstone 0.3 (4) 0.9 (6) (Arenaria interpres) Sanderling 0.5 (6) 0.0 (0) (Calidris alba) Western Sandpiper 36.4 (208) 58.5 (800) (Calidris mauri) Least Sandpiper 61.0 (500) 0.0 (0) (Calidris minutilla) Peep spp. 33.3 (500) 10.0 (150) (Calidris spp.) Dunlin 3.5 (30) 11.9 (124) (Calidris alpina) Stilt Sandpiper 2.4 (35) 0.0 (0) (Calidris himantopus) Dowitcher spp. 191.6 (1340) 69.7 (600) (Limnodromus spp.) (2) Total (3) (Maximum (4)) 366.6 (1932) 276.8 (1528) (1) Other species, detected in lower numbers, include Wilson's Plover (Charadrius wilsonia), American Oystercatcher (Haematopus palliatus), Red Knot (Calidris canutus), and Semipalmated Sandpiper (Calidris pusilla). (2) Of the 4% of all dowitchers identified by call, 77% were Long-billed Dowitchers, Limnodromus scolopaceus, while 23% were Short-billed Dowitcher, Limnodromus griseus. (3) Average number of shorebirds, all species combined, per census. (4) Maximum number of shorebirds, all species combined, present during the same census. Table 2. Winter foraging locations of shorebirds along the Lower Laguna Madre. Species in shallow water and exposed mudflat categories were observed foraging in those habitats on [greater than or equal to] 95% of observations. Asterisk (*) indicates species that were exclusively foraging in shallow water or exposed mudflat. Shallow water Exposed mudflat Water/mudflat American Avocet* Black-bellied Plover Willet Greater Yellowlegs Snowy Plover* Long-billed Curlew Lesser Yellowlegs Semipalmated Plover* Marbled Godwit Stilt Sandpiper* Piping Plover* Western Sandpiper Sanderling Dunlin Killdeer* Dowitchers Ruddy Turnstone* Least Sandpiper
Special thanks go to Steve Thompson and the staff of the Laguna Atascosa National Wildlife Refuge for their crucial logistic support during this study and to the Faculty Research Council of the University of Texas-Pan American for funding this study. Thanks also go to Curt Zonick for information on Piping and Snowy Plovers in South Texas. Appreciation is extended to Chris Onuf for providing information relative to seagrass cover along Bayside Drive, and to Brian R. Chapman and Terry C. Maxwell for their constructive comments on an earlier draft.
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Department of Biology, The University of Texas--Pan American, Edinburg, Texas 78539
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|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 1995|
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