Influence of water dynamics and land use on the avifauna of basin wetlands near Riviera in south Texas.
Although south Texas is characterized by flat terrain and low rainfall, there are numerous depression or basin wetlands between Kingsville and Raymondville, and between the Laguna Madre and Highway 281 (Brown et al. 1977; McAdams 1987). These South Texas Plains are formed of Pleistocene river silts and loess deposits with wind-blown sand areas nearer the coast. Most basin wetlands result from wind action ("deflation swales" of Brown et al. 1977), and most undergo dramatic variation in duration of water (hydroperiod), flooding season and water depth. Wetland density varies greatly within the region from year-to-year and averaged 2.2 wetlands/[km.sup.2] (0.39 [mi.sup.2]) during the post-hurricane period of the early 1980s (McAdams 1987). Because mean annual rainfall at Kingsville (just north of the study area) is only 70.1 cm (27.6 inches) and mean evaporation is at least twice that (Bomar 1995:230), many shallow wetlands filled by heavy rains dry quickly. However, tropical storms may bring torrential rains, with records of 42.9 cm (16.9 in.) in 24 h in 1980 and a mean of 134.1 cm (52.7 in.) in 1958. Resultant water in larger and deeper basins may last through several dry years; hence, they are sometimes called "hurricane lakes." Localized but heavy "seabreeze" showers that parallel the coast (Bomar 1995:189) form an additional rainfall influence on water levels, and conditions at individual wetland sites may not reflect regional rainfall data.
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Uplands surrounding the study-area wetlands were heavily grazed or intensively farmed, with some dispersed housing development. Nevertheless, these wetlands are widely known for attracting waterbirds throughout the year. Understanding when and how they are used by birds in light of human activity is essential to the development of a preservation strategy. Objectives of the study were to: (1) determine seasonal bird-species composition and abundance in a representative group of basin wetlands; (2) relate waterbird species diversity and abundance to size and wetland features; (3) and develop recommendations for preserving maximal bird habitat with minimal impact on land use.
Originally, most of the South Texas Plains were brushland dominated in drier upland by mesquite (Prosopsis glandulosa) sites and retama (Parkinsonia aculeata) in lowlands (Correll & Johnson 1970), but coastal areas once had extensive grassland patches (Jones 1982) separated as part of the Gulf Prairies and Marshes (Gould 1975). The study area represented the more open areas, now farmed or grazed, but with some residual brush patches. It lies between two arms of Baffin Bay in the Riviera loess sheet (Brown et al. 1977) east of Riviera, in Kleberg County, Texas (Fig. 1) (Lat. N27[degrees] 29', Long. W97[degrees] 81'). Wetlands were selected within view of the road because of problems of access on private property. Vehicular travel along county roads was light and was a disturbance to birds only when wetland basins flooded into the road ditch. After several exploratory surveys, 16 wetlands were selected along a route with good road access and wetland visibility. Initially, all had some water in a basin with identifiable hydrophytes. These basins seemed typical of other wetlands in the area, ranging from ca 1 to 34 ha when not flooded beyond their normal basins. The longest inter-wetland distance was about 6.7 km, and nearest-neighbor distance averaged 420 m, thus allowing easy inter-wetland flights by birds. Distance of the wetlands from one arm of the hypersaline Baffin Bay was about 2.5 km, a modest flight for coastal species in search of food or fresh water.
Variation in size of individual wetlands was influenced by basin location and watershed size. Several wetlands flooded quickly in response to rainfall and dried almost as rapidly, whereas others were relatively more stable and may have reflected a near-surface aquifer during drier periods (Baker 1971). These dramatic variations in water presence, size, and depth changed them from habitats for water-dependent bird communities to habitats used mainly by meadow birds and livestock. All wetlands were totally dry during several low-rainfall periods except for three with areas deepened for livestock use in dry periods. Although these "dugouts" attracted a few waders in search of fish during drying periods, their influence on bird populations was negligible.
Only two wetlands were neither grazed nor tilled, which was reflected in the presence of persistent herbaceous vegetation like tule (Scirpus californicus) and cattail (Typha domingensis), or areas of less persistent but important annual plants such as smartweeds (Polygonum sp.) and sedges (Carex sp. and Scirpus sp.), grading into perennial sesbania (S. drummondi) shrubs. One "shrub-scrub" wetland (No. 11 in Table 1) was covered with retama 3-6 m (9.8-19.5 ft), and others had only marginal willows (Salix sp.) or snags. Several wetlands had yellow lotus (Nelumbo lutea) beds that persisted despite deep flooding or extreme drought, and many had the small blue water lily (Nymphaea elegans) that bloomed even in heavily grazed meadows after flooding. Other wetlands commonly lacked central vegetation except for periods following germination of annuals on mudflats. Twelve wetlands were grazed at some time during the study, and sedges and smaller bulrushes at their edges were impacted quickly by cattle or horses. Margins of five wetlands were plowed periodically, one directly through the wetland when drying was timed with field activities.
Fifty-two bird censuses were made at about monthly intervals from October 1994 to December 1998. Observation time at each wetland varied with water and vegetation conditions as well as the number and diversity of birds, but typically varied from 5 to 30 min by two observers using one to several vantage points. Numbers of each species were counted when possible or estimated when flocks were large or active, but accuracy was no doubt influenced by water levels in relation to the vegetation. However, total numbers of birds changed so dramatically that errors in estimates of individual species were not a serious problem in comparing use among wetlands, nor in assessing seasonal or annual changes in use.
Despite the visibility from observation sites, most calidrid sandpipers could only occasionally be identified to species, and were analyzed as "peeps." No attempt was made routinely to separate Greater Yellowlegs (Tringa melanoleuca) from Lesser Yellowlegs (Tringa flavipes), Short-billed Dowitchers (Limnodromus griseus) from Long-billed Dowitchers (Limnodromus scolopaceus), or Double-crested Cormorants (Phalacrocorax auritus) from Neotropic Cormorants (Phalacrocorax brasilianus). Species common to wet-meadow edges like cattle egret (Bubulcus ibis), Long-billed Curlew (Numenius americanus), or Blackbellied Plover (Pluvialis squatarola) were tallied only when within the wetland basin. Ubiquitous species like eastern meadowlark (Sturnella magna), Mourning Dove (Zenaida macroura), Great-tailed Grackle (Quiscalus mexicanus) and Red-winged Blackbird (Agelaius phoeniceus) were not tallied.
Water levels in the study wetlands were below basin maxima when the study started in the fall of 1994, but all had significant water coverage due to rains of September 1994. Because of personal experience with the magnitude of change in water volume in these ponds, water-depth gauges were considered impractical. Initially, total basin area of each wetland was determined from aerial photographs and topographic maps by using a dot-grid. Initial water coverage of the basin was recorded in ha as water index 4 (to allow estimates up or down), and that water area used as a basis for approximating wetland area at each observation day. Census-day wetland areas were calculated by multiplying the water index by the area per index value for each wetland. Thus, bird-area relationships reflect the dynamics of these wetlands as based on each survey day rather than by single measures per season or mean for the entire study period (see summary of published data in Weller 1999).
Although depths could not be assessed regularly, fence posts, topography and bird use suggested that basin water depths ranged from totally dry to about 2.43 m (8 ft) at flood stage in the larger wetlands. Due to their original formation and subsequent wind erosion, most basins sloped gradually from pond margins to center. Thus, as wetlands dried, shore areas dried to mudflats and then wet-meadows of grasses and forbs dependant on grazing history and timing. Unvegetated shorelines and flats with relatively steeper slopes often dried so rapidly that they were not attractive to mudflat birds.
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Wetland characteristics. -- Mean water indices for the 16 wetlands varied dramatically by survey over the four years of observation (Fig. 2). Using these indices to calculate area of water on each survey date as described above, size of individual basin water areas in hectares ranged from zero to four times the mean (Table 1, Fig. 3; an example of water dynamics in one wetland over the entire study period). Water area was generally related to regional rainfall, but spotty even on this small scale. Rainfall records for the nearest site with adequate data for this time frame (Corpus Christi) showed no significant correlation with wetland water indices at the study area.
Dominant avian taxa and their ecological characteristics. -- Although 103 bird species were observed, only 44 taxa had frequencies over 15 which had potential for statistical analysis (Table 2). As commonly separated by taxonomy, locomotion and/or feeding guilds, six waterbird groups dominated the avifauna. Waterfowl (family Anatidae) constituted 15 species (34%), of which eight favored surface swim-feeding in shallow water, and seven regularly dived for underwater foods such as invertebrates. Diving picivores of somewhat deeper water numbered five species (11%): two cormorants, Pied-billed Grebe, Least Grebe, Eared Grebe. There was only one diving herbivore (0.5%), the American Coot, although Fulvous Whistling-Duck, Canvasback, Ringnecked Duck and Redhead probably also used these limited resources. Seven surface feeders that fed by swimming or aerial diving on invertebrates, fish, or their remains (16%) included American White Pelican, Wilson's Phalarope, two gulls and four terns. Eight species were wading picivores (18%) that fed regularly along wetland shallows or edges (ibises, egrets, herons), and nine shorebird taxa (20.5%) used mudflats or sheetwater.
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Seasonality of bird use. -- Of the 44 common taxa (Table 2), 23 use the area only part of the year: a few species for breeding (see status codes in Table 2), some only during migration, and some for wintering as well as migration. Twenty-one taxa were permanent residents, but populations were not stable; some were more common in winter (American Coot), and some increased in numbers during the breeding season (Black-necked Stilt). Others were relatively stable throughout the year, but their distribution by wetland seemed related more to water conditions and food than to other breeding requirements such as nest sites (e.g., Great Blue Heron).
Nesting was uncommon even among resident species but was documented (nests and/or young) for Killdeer, Pied-billed Grebe, Least Grebe, Black-necked Stilt and Mottled Duck. Based on older and flying young, nesting was probable in Black-bellied Whistling-Duck, and pair behavior suggested that Fulvous Whistling-Duck, Blue-winged Teal and Northern Shoveler may have attempted to nest. Some coastal summer residents used the area because all ponds were within 3 km of Baffin Bay, but probably nested on the coast and fed in freshwater wetlands (Least Tern, Gull-billed Tern and Forster's Tern). Herons, egrets and cormorants used the area regularly but seemingly did not nest. A few solitary nests of Red-winged Blackbirds were noted in roadside shrubs, and Great-tailed Grackles were seen carrying nest material in one wetland when tules and cattails were flooded, but nests or young were never observed.
Bird species richness. -- After pooling peeps and deleting several passerines mentioned above, taxon richness ranged from 0 to 28 species per pond per census, and 3 to 47 species per census (Table 1). As would be expected, there was a significant difference in richness among the 16 ponds over the 52 surveys (Kruskal-Wallis ANOVA, KW = 263.63, P = 0.0000). The influence of wetland size (water area in ha) on richness for each survey was reflected in a Linear Regression (data transformed Log + 1 to correct for zeros; [R.sup.2] = 0.6001, t = 35.01, df = 818, P = 0.0000) (Fig. 4). Although richness did not differ significantly among months, summer months were lowest, and species composition varied seasonally due to the multiple uses mentioned earlier.
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Bird numbers. -- There was great variation in bird abundance by survey day, season, and cumulative total numbers per wetland basin (Table 1). Resident birds that probably established territories for much of the year when water conditions were suitable (Black-necked Stilt, Mottled Duck, Killdeer) or during spring (American Avocet, Blackbellied Whistling-Duck) typically were seen as pairs or singles, and hence smaller mean numbers per sighting (Table 2). Birds that moved freely from pond to pond to feed socially or in mixed-species flocks on fish (e.g., American White Pelican, cormorants), foliage and seeds (ducks, American Coot), or invertebrates (Northern Shoveler, Ruddy Duck, Lesser Scaup, Wilson's Phalarope, peeps) were in larger flocks (Table 2). Number of birds differed significantly by wetland as would be expected (KW = 249.20, P = 0.0000), and numbers (log transformed) also were related to wetland area ([R.sup.2] = 0.6237, t = 36.8, df = 818, P = 0.0000) (Fig. 5). Despite larger flocks during migration and in winter, mean numbers did not differ significantly by month, due probably to year-to-year irregularity in water regimes.
Habitat influences on bird use at the cluster level. -- This study area constitutes a cluster or selected block from within a larger area of diverse wetlands. Movements of individuals and small groups from wetland to wetland were commonly observed, especially when disturbed by farming activity or potential predators. To examine the influences of the cluster as a whole, mean data for all 16 wetlands for given survey days or for the study period of 52 monthly surveys were used. A Linear Regression of mean species richness on water index was significant ([R.sup.2] = 0.3143, t24.79, df = 51, P = 0.0000) but the nature of this index stresses size over water availability and bird distribution within the cluster. A Linear Regression of mean species richness for survey data and the number of wetlands with water ([R.sup.2] = 0.5579, t = 5.69, df = 51, P = 0.0000) (Fig. 6) better reflects the extremes of wetlands that dried completely (water index 0) and eliminated most wetland bird use. Total numbers by survey date also were related to the number of wetlands containing water ([R.sup.2] = 0.3934, t = 7.54, df = 51, P = 0.0000) (Fig. 7).
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Habitat influences at the individual wetland level. -- Within this cluster, individual wetlands provided different habitats for different species at any given survey, and often for different species at different times due to the water regime, vegetation and bird needs (e.g., migration stopovers). Chi-square analysis of the distribution of the 44 common taxa (Table 2) indicated that some species were found in certain wetlands more commonly than expected. Coots and Pied-billed Grebes were associated with large and open water, and Killdeers with low water levels and smaller areas. Failure of the data to show other observed relationships presumably was due to the fact that basins were shallow and dynamic, so that each wetland provided a diversity of microhabitats as water levels changed in relation to vegetation and exposed substrates, thereby attracting different species at different water stages. For example, the largest wetland had, at different times, the highest frequency of usage (far above expected in a Chi-square analysis) for bird species with as diverse habitat requirements as American White Pelicans, cormorants, Ruddy Ducks, Canvasback, Great Egret, peeps, dowitchers and American Pipit due to different water levels and dryness of mudflats.
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Feeding patterns. -- Observed feeding behavior during the study demonstrated the flexibility of some species or guilds and the rather static behavior of others in food use. Although presence or abundance of potential waterbird foods were not assessed during this study, some foods (e.g., fish) were conspicuous by the presence of users such as American White Pelican, cormorants and herons. Early in the study, these birds frequented several ponds and were seen with large fish in their bills; after the first drying of the larger and deeper wetlands, such fish-feeders were rarely seen or seen only at loafing sites. Reestablishment of fish populations took several years and floods, probably because ponds dried again too quickly for fish reproduction and survival. The presence of small fish was evidenced by flocks of Least Terns and Black Skimmers feeding during low water levels.
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Mixed species feeding flocks of American White Pelicans, cormorants, and grebes were induced when fish were abundant; typically, the former two fed together, whereas grebes fed near or among them opportunistically. When water depths declined in one wetland to a depth of only a few cm, Snowy Egrets concentrated with Least Terns and Black Skimmers to feed. Presumably the fish were small because cormorants and larger egrets and herons were rare.
Pied-billed grebes also fed in association with shovelers, herons and egrets which apparently served as "beaters" by disturbing aquatic foods which were subsequently captured underwater by the grebe. Wilson's Phalaropes sometimes followed Northern Shovelers, and the latter followed still larger birds such as herons to capture invertebrate prey they disturbed. Cattle Egrets typically used cattle or horses as "beaters" in much the same way but in wet meadow as well as in drier sites. Interspecies feeding associations also included coots, American Wigeon and Gadwall, presumably feeding on submergent vegetation. Coots dived for the deep-water food items and subsequently were harassed for food by the non-diving ducks.
Patterns of waterbird use over time are more complex at this latitude than in northern areas where freezing prevents winter use and species and numbers change mainly with migration. Temperature is not limiting at this site, but the dynamics of water dictate suitability of wetland areas for feeding, resting or nesting. Most species can be found year-round, but warm-temperate and subtropical species used these wetlands mainly during summer for nesting or perhaps nonbreeding habitat. During migration, they provide feeding areas for many northern birds that follow coastal flight corridors, and wintering is common.
These south Texas wetlands are similar in size and depth to those of the Prairie Pothole Region of the United States and Canada. Both are in semiarid regions and are characterized by dynamic water regimes, but water data recorded during this study suggested that water levels are even more variable. Irregularity of water during the growing season and high ambient temperatures that dried mudflats reduced the quality of the site for many emergent plant species. Thus, sparsely vegetated
shorelines and open water resulted, with plant heights often further reduced by intensive grazing. There also was a conspicuous absence of submergent plants such as milfoil and pondweeds, but lotus tubers seemed to survive drought in the deeper wetlands.
Extensive mudflats were extremely important to a wide variety of the bird species observed there, and are a product of years of upslope erosion due to wind and water, and increased sedimentation exacerbated by grazing and tillage (see review in Weller 1996). High turbidity also was common to the larger of these wetlands due to wind, water and livestock action. In a short-term view, such siltation probably induced eutrophication and higher invertebrate productivity as evidenced by concentrations of shorebirds on mudflats along larger wetlands and in the bottoms of drying shallow wetlands. But ultimately, sedimentation will fill in these shallow basins, as evidenced by some that never exceeded a half-meter in depth even at flood stage, and dried to nearly flat and uniform substrates. Clearly, upslope management is vital to their long-term survival.
Thus, the role of these wetlands for breeding birds is dependent on seasonality, persistence and predictability of water, nesting and rearing cover, and lasting food supplies or access to adjacent sources. In typical rainfall years, these wetlands remain wet mainly during non-breeding periods, and are suitable for migrant and wintering ducks. The lack of waterbird breeding presumably is related to the historic patterns of wetland dry-down that would have reduced breeding success, and populations shifted to areas where they were more successful. However, studies by McAdams (1987) on waterfowl breeding and by Briggs & Everett (1983) on waterbird use of similar ponds in the region during post-hurricane years demonstrated a positive response to flooding that persisted for several years. American Coot, Common Moorhen, Pied-billed Grebe commonly nest in Texas coastal wetlands farther north where rainfall is greater, seasonal river overflows influence wetland water regimes, or water levels are managed (Cottam & Glazener 1959; Weller et al. 1996), and use of brackish marshes for nesting by Mottled Ducks (Stutzenbaker 1988). Wintering birds of those same well-watered and vegetated ponds also were more characteristic of species favoring submergent as well as emergent vegetation (e.g., White & James 1978; Weller et al. 1996).
Much of the regional species richness observed here was due to use of different wetland types, sizes and depths for different purposes, sometimes by the same species. However, it was a dynamic situation; larger wetlands that were deeper provided more stability on average, but as water levels gradually declined, the gently sloped basins had extensive shallow feeding areas suitable for diverse shorebirds and waders. Whether wet or dry, large open areas that might be interpreted as unattractive often are used as daytime rest and overnight roost areas for birds that feed elsewhere: American White Pelicans, Snow Geese, White-fronted Geese, Sandhill Cranes, Fulvous Whistling-Duck, Black-bellied Whistling-Duck and several diving ducks.
Specific water-index values as used in this study were not statistically correlated with better or worse conditions for most bird usage, probably because such changes influenced only small portions of individual wetlands. Black-bellied Plovers and Killdeer typically used drier areas than did Buff-breasted Sandpipers and peeps, and both used much drier sites than dowitchers that needed deep and soft mud for probing. Variability in water regimes is therefore vitally important in relation to temporal patterns of migration. For example, a basin that dried quickly did not hold shorebirds or other mud probe-feeders as long as flats that remained muddy; for this reason, flat basins were better than steeper basins because the rate of drying was slower and more uniform.
Areas that dried in mid- to late-winter had low species richness and numbers, but they were used intensively by migrants in spring and fall after even modest rainfall. Fish-eating birds used several shallow ponds for loafing, with cormorants usually on snags or mudflats and American White Pelicans in water or on shorelines. Feeding by cormorants occurred in ponds deep enough for diving, and American White Pelicans used only the largest and deepest wetland for feeding when fish were present. Despite general relationships between richness or numbers and size of wetlands (Figs. 4 and 5, respectively), diversity must be considered comparatively. A small drying wetland often is much more attractive to a diversity of species than a large flooded one. However, the larger wetland may have a great abundance of a few species, but at drying, may have parts of it extremely species-rich. These microhabitats often are short-lived, and may occur seasonally at many ponds, thus requiring short-term analyses to compare wetland conditions and their attractiveness to single or multiple species.
IMPLICATIONS FOR CONSERVATION OF BIRD HABITAT
Past drainage ditches have impacted some of the more shallow wetlands in this area by speeding drying, but larger wetlands would have been costly to drain because of the rolling landscape. Moreover, most landowners valued wetlands for livestock watering, as shown by areas deepened to hold water in the dry years ("dugouts"). Use of mobile pumps for small-scale irrigation resulted in water depth changes in some wetlands. More shallow areas adjacent to farm fields often were tilled during dry periods, and wind and water erosion were common, resulting in the potential of significant modification of wetlands (Briggs 1982). However, periodic flooding prevented continuous cropping. Further unregulated housing development would be harmful because of likely pressures to reduce flooding.
Despite lack of breeding habitat for many birds, these wetlands were heavily used by nonbreeding birds all year. Thus, they should be recognized for their contribution to the conservation of diverse species of regional, national, and international importance. Several authors have indexed bird use of wetlands to assess the dominance or relative importance of each species in the bird assemblage (Boyer & Psujek 1977; Briggs & Everett 1983) or identify key wetlands within a complex. Bird use in this study was indexed to compare different wetlands to provide conservation personnel with assessment methodology for wetland acquisition or other protective strategies. Simple ranks were established on the basis of mean data per survey for richness, abundance, maximal or cumulative richness and numbers per wetland. All rankings identified the same six wetlands but ordered them differently. Similar ranking results from Kruskall-Wallace ANOVA of richness or numbers per wetland, and identified generally the larger wetlands with more persistent water regimes because of their actual use over time rather than assessment of habitat suitability. These rankings selected larger (over 5 ha) wetlands that amounted to 68.7 ha or 81 percent of the total 85 ha of wetland basins in the study area (see size data in Table 1).
Because of the geomorphic setting, this cluster of wetlands can be viewed as a unit hydrologically as well as biologically with a need to preserve various types and sizes of wetlands for the diverse functions of many species of mobile birds. Unlike the large land holdings characteristic of most of south Texas, wetlands of this cluster tend to be individually owned. Thus, conservation approaches must appeal to individuals with different economic and aesthetic values. They are vulnerable to loss because of ever-changing regulations concerning small wetlands on agricultural land. There is sufficient interest in this area for birding that a regional or county plan for preservation might be possible, and could be led by county extension information specialists with a local group of cooperators. But because not all wetlands are likely to be protected, evaluation and prioritization for birds is an essential tool in formulating such a conservation strategy.
To provide habitat for breeding as well as resting and feeding, a larger number of these wetlands should be protected from intensive grazing or farming that increases chances of soil erosion and increased turbidity with subsequent sedimentation. Fencing part of each wetland during late fall, winter and early spring would allow the development of emergent vegetation. This is not to imply that uniformity of management for every wetland is good, as a variety of land uses seems to diversify shoreline food resources and minimize invasion by woody shrubs and trees.
While some conservationists encourage preservation of areas with the greatest bird species richness or significant breeding habitats, this small cluster of multi-function wetlands is unique, and has survived without dedicated funds or any legal conservation designation. Perhaps this is because they do serve society in a variety of ways, and in part because they have built-in protection due to their geomorphic setting. They are well worth preserving.
Table 1. Mean and range in size of wetlands (ha), mean bird-species richness and abundance per survey, and cumulative bird-species richness and number observed at each of 16 wetlands (n = 52). Wetland Size Bird Richness Bird Number No. Mean Range Mean Range Mean Range 1 5.4 0-15.2 8.8 0-18 77.9 0-299 2 1.1 0-3.5 3.8 0-19 32.3 0-528 3 1.4 0.3-4.6 2.7 0-11 10.9 0-57 4 0.9 0-4.1 1.2 0-6 12.3 0-250 5 2.1 0-6.1 3.7 0-14 20.1 0-141 6 31.9 0-88.6 8.6 0-25 131.4 0-529 7 6.0 0-17.7 7.4 0-22 99.1 0-516 8 0.7 0-3.5 0.7 0-8 4.9 0-80 9 1.9 0-7.1 2.6 0-19 19.7 0-359 10 0.9 0-3.0 0.6 0-4 1.2 0-13 11 1.3 0-6.1 0.1 0-2 0.9 0-25 12 11.4 0-45.6 6.1 0-3 107.8 0-708 13 6.5 0-23.0 4.5 0-1 83.3 0-837 14 1.7 0-5.1 2.2 0-9 17.0 0-167 15 7.4 1-18.9 8.1 0-2 65.7 0-272 16 4.3 0-11.1 3.2 0-9 19.4 0-91 Wetland Cumulative Cumulative No. Richness Number 1 57 3977 2 45 1646 3 34 570 4 19 641 5 38 1046 6 62 5519 7 61 5055 8 16 254 9 36 1026 10 16 60 11 2 47 12 60 5606 13 48 4332 14 32 885 15 59 3418 16 39 1010 Table 2. Frequency of occurrence, numbers and mean numbers per sighting of 44 taxa of wetland birds observed more than 15 times on 16 study wetlands on 52 dates, arrayed by frequency of sightings. Scientific names from American Ornithologists' Union (1998) and Texas Ornithological Society (1995). Species status codes in parenthesis after name: (1) permanent resident, (1a) resident but more common in summer, (1b) resident but less common in summer, (2) summer only, (3) spring and fall migration periods only, (4) spring and fall migration periods plus winter. An appended list includes wetland birds observed in lesser incidence (number in parenthesis). Species or Taxon Frequency of Total Number Mean Number Status code in () Occurrence Observed /Sighting American Coot (1b) Fulica 215 7,355 34.2 americana Northern Shoveler (4) Anas 210 4,474 21.3 clypeata Mottled Duck (1a) Anas 146 690 4.7 fulvigula Pied-billed Grebe (1b) 146 493 3.4 Podilymbus podiceps Blue-winged Teal (4) Anas 143 2,148 15.0 discors Black-necked Stilt (1a) 139 896 6.5 Himantopus mexicanus Ruddy Duck (4) Oxyura 117 1,298 11.1 jamaicensis Cormorants (1) 113 690 6.1 Double-crested Cormorant Phalacrocorax auritus & Neotropic Phalacrocorax brasilianus Yellowlegs (3) 113 270 2.4 Greater Yellowlegs Tringa melanoleuca and Lesser Yellowlegs Tringa flavipes Killdeer (1a) Charadrius 111 220 1.9 vociferus Gadwall (4) Anas strepera 90 2,401 26.7 Great Blue Heron (1) Ardea 86 103 1.2 herodias Laughing Gull (1a) Larus 86 324 3.8 atricilla Dowitcher (4) 82 1,160 14.2 Short-billed Dowitcher Limnodromus griseus & Long- billed Dowitcher Limnodromus scolopaceus Lesser Scaup (4) Aythya affinis 82 638 7.8 Northern Pintail (4) Anas acuta 79 1,566 19.8 Snowy Egret (1a) Egretta thula 78 554 7.1 Great Egret (1) Ardea alba 78 277 3.6 Black-bellied Whistling-Duck 69 757 10.9 (1a) Dendrocygna autumnalis White-faced Ibis (1a) Plegadis 69 323 4.7 chihi Green-winged Teal (4) Anas 56 448 8.0 crecca American Wigeon (4) Anas 54 864 16.0 americana Peeps (3) Calidris sp. 45 1,062 23.6 Ring-necked Duck (4) Aythya 42 184 4.4 collaris Long-billed Curlew (1a) 42 207 4.9 Numenius americanus White Ibis (1) Eudocimus albus 41 186 4.5 American Avocet (1a) 40 308 7.7 Recurvirostra americana Fulvous Whistling-Duck (2) 37 444 12.0 Dendrocygna bicolor Cattle Egret (1a) Bubulcus ibis 32 403 12.6 Least Grebe (1a) Tachybaptus 31 340 10.9 dominicus Ring-billed Gull (1a) Larus 31 115 3.7 delawarensis Black-bellied Plover (4) 28 63 2.3 Pluvialis squatarola Forster's Tern (1) Sterna 28 93 3.3 forsteri Redhead (4) Aythya americana 26 79 3.0 Black Tern (2) Chlidonias niger 25 200 8.0 Little Blue Heron (1a) Egretta 25 39 1.6 caerulea Tricolored Heron (1) Egretta 23 30 1.3 tricolor Wilson's Phalarope (3) 22 586 26.6 Phalaropus tricolor Least Tern (2) Sterna 22 101 4.6 antillarum Eared Grebe (4) Podiceps 21 47 2.2 nigricollis Bufflehead (4) Bucephala 19 103 5.4 albeola American White Pelican (1) 18 587 32.6 Pelecanus erythrorhynchos Canvasback (4) Aythya 17 84 4.9 valisneria Gull-billed Tern (2) Sterna 16 43 2.7 nilotica Species observed fewer than 16 times out of 52 monthly surveys; listed in order of incidence of sightings (parenthesis); not used in statistical analyses but included in species richness. Belted Kingfisher Ceryle alcyon (13) Roseate Spoonbill Ajaia ajaia (13) Cinnamon Teal Anas cyanoptera (10) Common Moorhen Gallinula chloropus (10) Common Snipe Gallinago gallinago (10) Greater White-fronted Goose Anser albifrons (10) Spotted Sandpiper Actitis macularia (10) Stilt Sandpiper Calidris himatopus (10) Canada Goose Branta canadensis (7) Green Heron Butorides virescens (7) Pectoral Sandpiper Calidris melanotos (7) Black Skimmer Rynchops niger (6) Buff-breasted Sandpiper Tryngites subruficollis (6) Caspian Tern Sterna caspia (6) Northern Harrier Circus cyaneus (6) Willet Catoptrophorus semipalmatus (6) American Pipit Anthus rubescens (5) Solitary Sandpiper Tringa solitaria (5) Horned Grebe Podiceps nigricollis (4) Wood Stork Mycteria americana (4) Dunlin Calidris alpina (3) Snow Goose Chen caerulescens (3) Herring Gull Larus argentatus (2) Hudsonian Godwit Limosa haemastica 2) Marbled Godwit Limosa fedoa (2) Sanderling Calidris alba (2) Sandhill Crane Grus canadensis (2) Whimbrel Numenius phaeopus (2) Anhinga Anhinga anhinga (1) Common Goldeneye Bucephala clangula (1) Black-crowned Night-Heron Nycticorax nycticorax (1) Least Bittern Ixobrychus exilis (1) Masked Duck Nomonyx dominica (1) Northern Waterthrush Seiurus noveboracensis (1) Osprey Pandion haliaetus (1) Peregrine Falcon Falco peregrinus (1) Reddish Egret Egretta rufescens (1) Sora Porzana carolina (1)
We are indebted to William Kiel of Kingsville who introduced us to this area, to the staff of the Kingsville office of the Natural Resource Conservation Service for use of aerial photos to improve acreage estimation, to James M. Hinson of Texas Parks and Wildlife Department for satellite imagery of the area that facilitated demarcation of wetlands, and to Drs. Marc Woodin and James Dinsmore for helpful comments on the manuscript.
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Boyer, R. L. & M. J. Psujek. 1977. A comparison of wetland bird aggregations and macrobenthos in temporary spring ponds. Transactions of the Ill. State Acad. Science, 70:332-340.
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McAdams, M. S. 1987. Classification and waterfowl use of ponds in South Texas. Unpublished M.S. thesis, Texas A & M Univ., College Station, 112 pp.
Stutzenbaker, C. D. 1988. The mottled duck, its life history, ecology and management. Texas Parks & Wildlife Dept., Austin, 209 pp.
Texas Ornithological Society. 1995. Checklist of the birds of Texas, 3rd edition. Texas Ornith. Soc., Austin, 166 pp.
Weller, M. W. 1996. Birds of rangeland wetlands. Pp 71-82 in Rangeland Wildlife (P.R. Krausman, ed.). Soc. Range Manage., Tucson, 440 pp.
Weller, M. W. 1999. Wetland birds; habitat resources and conservation implications. Cambridge University Press, Cambridge, UK and New York, 271 pp.
Weller, M. W., E. H. Smith & R. M. Taylor. 1996. Waterbird utilization of a freshwater impoundment on a coastal Texas wildlife refuge. Texas J. Science, 48(4):319-328.
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Milton W. Weller and Doris L. Weller
Department of Wildlife & Fisheries Sciences
Texas A & M University, College Station, Texas 77843
P. O. Box 280, Sanibel, Florida 33957-0280
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|Author:||Weller, Milton W.; Weller, Doris L.|
|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 2000|
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