The reddish egret (Egretta rufescens) is the rarest Ardeidae in North America and is highly restricted geographically (International  Union  for  Conservation  of Nature, 2012). It is a coastal wetland specialist found along the Gulf of Mexico, the Caribbean and Bahamas, the  Atlantic  Coast  of  Florida,  and  the  Pacific Coast of Mexico (Lowther and Paul, 2002). The reddish egret was nearly extirpated early in the 20^th century due to plume hunting, and today there is an estimated 2000 breeding pairs in the United States with nearly half occurring in Texas (Lowther and Paul, 2002; Green, 2006). Because of its narrow habitat requirements and limited distribution, the reddish egret was listed as a species of concern by the U.S. Fish and Wildlife Service. It is also listed as near threatened on the International Union for Conservation of Nature Red List (2012).

Foraging habitat of the reddish egret typically consists of shallow coastal flats and lagoons where it actively forages for small fish such as sheepshead minnow (Cyprinodon variegatus), longnose killifish (Fundulus similis) and pinfish (Lagodon rhomboids) (Lowther and Paul, 2002). Because the reddish egret is restricted to coastal areas, it is sensitive to disturbances and alterations within these habitats. The rapidly growing human population along the Texas Coast has had major impacts on coastal habitats (Moulton et al., 1997). The Gulf Intracoastal Waterway is frequently dredged and new sites are needed for dredge disposal. Placement of dredge material can alter  tidal  flat  habitats  by  covering seagrass meadows that provide habitat for fish, impacting water quality, and changing the hydrology of foraging areas for waterbirds (Farmer, 1991). Therefore, an understanding of the spatial and temporal dynamics of foraging habitat will enable managers to reduce negative impacts to these areas and support foraging requirements for this unique species. In fact, the recently developed Reddish Egret Recovery Plan (Wilson et al., 2014) specifically states the identification of key foraging areas as one of three primary goals. Consequently, the goal of this study was to develop a spatial model that delineates the temporal distribution of foraging habitat for the reddish egret in the Laguna Madre of Texas and to test the performance of the model with an independent sample of locations of foraging reddish egrets marked with GPS equipped satellite transmitters. 

Study area description

The Laguna Madre is located along the lower Texas Coast (Figure 1) and  supports the largest concentration of reddish egrets in North America (Green, 2006). It is a large, hypersaline lagoon stretching ~185 km in length with an average width of 7 km, and relatively shallow water depths, averaging 1 m deep (Tunnell, 2002). Tides in the Laguna Madre are mainly wind-driven and can cause water levels to fluctuate by 0.6 to 0.9 m (Tunnel, 2002). Seagrass meadows and wind tidal flats are the primary habitat types found in the Laguna Madre. Shoalgrass (Halodule wrightii) and manateegrass (Cymodocea syringodium) are the dominant species in the upper and lower Laguna, respectively (Quammen and Onuf, 1993). Islands are abundant and widespread throughout the Laguna Madre, most of which were formed by placement of dredge material during construction and routine maintenance dredging of the Intracoastal Waterway (Tunnell, 2002). Some of these islands have become occupied by colonial nesting waterbirds, including the reddish egret. 

correspond with each day and assess performance of the model at a finer temporal scale. They considered locations ≤ 50 m from available habitat as ‘in’ available habitat to account for GPS accuracy (±18 m) and inherent, unknown error in extrapolation of bathymetry and tide data. ArcGIS 10.1 was used to measure the distance from foraging locations to available foraging habitat for each week.

Foraging habitat of 372 reddish egrets that were observed foraging throughout the Laguna Madre were sampled; 211 were observed foraging solitarily, and 161 were observed foraging in groups (n = 21). Average water depth used by foraging reddish egrets was 10.1 ± 0.68 cm with no differences (P > 0.05) between ages, color morphs or foraging classes (Table 1). Approximately, 91% of reddish egrets foraged in water depths of 1 to 20 cm. Average seagrass coverage along transects was 12.3% (± 2.74), and 67% of egrets foraged in ≤ 10% seagrass coverage. There were no differences (P > 0.05) in percent seagrass coverage between ages, color morphs or foraging classes (Table 1). 

The Laguna Madre encompasses 215 223 ha of which 123 395 ha were considered potential foraging habitat based solely on benthic habitat characterization (prior to adding water depth to the model). Once water depth was added to the model, available foraging habitat was reduced considerably and varied throughout the annual cycle with a total of 74 985 ha of foraging habitat available at some point during the 120-week period. In 2012, the amount of foraging habitat was relatively low and highly variable from January to mid-March; availability became much higher and remained relatively stable from mid-March through late September, then was again relatively low, but highly variable from October to December (Figure 2a).  The  lowest  amount  of  available foraging habitat occurred in March (8 608 ha) and peaked at 27 247 ha in late September. In 2013, the overall pattern was similar with relatively large amounts of foraging habitat during April–October, and lower amounts with greater variability during winter (Figure 2b). Foraging habitat peaked at 27 534 ha in July and reduced to 7 099 ha in February. On average, available foraging habitat throughout winter was about 50% of that available during summer.

The availability of a given foraging area fluctuates; the longest duration specific areas available was 82 weeks (2.52 ha). Only 4 003 ha were available ≥ 50% of the time as compared to 51 208 ha available ≥ 10% of the time. Areas containing the greatest amount of foraging habitat in the upper Laguna Madre (Figure 3a and b) were located east of the mouth of Baffin Bay and south to Nine Mile Hole. Large areas of foraging habitat in the lower Laguna Madre were located on the east side, south of the Mansfield Cut, north and south of the Arroyo Colorado on the east side, and south of the Brownsville Ship Channel in South Bay (Figure 3b and c).

It was found that 90% of the GPS locations of foraging reddish egrets fell within our modeled available foraging habitat during the cumulative 120 week period, providing compelling evidence that our model performed well in delineating cumulative foraging habitat across the annual cycle. Less agreement (51%) was found between GPS locations of foraging reddish egrets as compared to corresponding estimates of foraging habitat during each week of the 120 week period. Locations falling outside the estimated foraging habitat deviated with an average of 407 m (median = 180 m) from the foraging habitat boundary and generally were shoreward. About 9% of locations were greater than 500 m from available habitat and in general were on the western part of the Laguna Madre towards the mainland and west of the land-cut. These  areas  had  a  much  more  patchy  distribution  of foraging habitat relative to other areas.

This study’s foraging habitat model appeared to perform well in estimating the distribution of foraging habitat for reddish egrets in the Laguna Madre, as there was 90% overall agreement with an independent dataset of 6 385 foraging locations from individuals marked with satellite transmitters. The weekly locations (n = 770) were less accurate, with half occurring within available habitat; yet the average distance the locations were from available habitat  was  relatively  small.  Potential  causes    of   the reduction in agreement between GPS locations of foraging reddish egrets and the modeled foraging habitat include birds not actually foraging during the time the location was recorded (e.g., flying between foraging locations), inherent error in geospatial datasets and wind-driven water level changes in shallow flats not being accurately recorded by tide gauge stations. However, the influences of these potential sources of variability on modeled foraging habitat that became apparent at finer time scales appear to be less evident when modeled over longer time periods. This model showed that the availability of foraging habitat in the Laguna Madre varied both temporally and spatially. Spatial and temporal fluctuations  of  foraging  habitat in wetland ecosystems is not uncommon; Arengo and Baldassarre (2002) found that size, quality and duration of available foraging habitat for American flamingos (Phoenicopterus ruber ruber) varied considerably within and among foraging sites in Yucatán, Mexico. Spatial and temporal variability of wading bird foraging habitat is common due to tidal cycles and variable precipitation (Kahl, 1964; Kushlan 1981; Gonzalez, 1997). Tides in the Laguna Madre are primarily wind driven (Tunnell, 2002); thus, any change in wind speed and direction can cause fluctuations in foraging habitat. The amount of available foraging habitat for reddish egrets was much greater during spring and summer, and large peaks of available habitat occurred within this time period. This period coincides with the breeding season, when energy requirements are high relative to other periods of the annual cycle (Lowther and Paul, 2002).

The amount and location of foraging habitat during the breeding season are important factors to consider when managing this species. The location and size of breeding colonies are often directly related to the availability of nearby foraging habitat (Gibbs et al., 1987; Gibbs, 1997). Green Island in the lower Laguna Madre typically contains the largest abundance of breeding reddish egrets, as well as all other herons and egrets, along the Texas Coast (Texas Colonial Waterbird Society, unpublished data). This island is located proximal to large areas of foraging habitat identified by our model and that are known to be used by reddish egrets nesting on Green Island (Farmer, 1991). Protection of this foraging habitat is likely an important factor in the maintenance of the colony. Likewise, spatially linking foraging habitat to other colonies throughout the Laguna Madre could be beneficial in understanding resources that are critical for sustaining breeding colonies and help reduce adverse impacts from anthropogenic influences (e.g., dredge disposal). More information is needed to examine how reddish egrets utilize foraging habitat in relation to breeding colonies and the distances reddish egrets can travel to foraging sites without reproduction being affected.

The timing of reproduction results in both post-breeding adults and young of the year having large amounts of stable foraging habitat during early fall. Bates and Ballard (2014) found groups of foraging adult and juvenile reddish egrets just after the breeding season from August to October. Farmer (1991) observed a similar pattern of group foraging by hatch-year reddish egrets in fall. Hatch-year reddish egrets are often less efficient foragers and may not be as skilled at identifying quality foraging areas (Bates and Ballard, 2014). The abundance and spatial stability of foraging habitat in fall probably results in hatch-year egrets spending less time and energy searching for quality foraging sites than if there was high temporal variation in availability, and allows them to spend more time foraging to meet their energy demands.

The   decline   in   availability   of   foraging   habitat  from December to March corresponds with the time when reddish egrets depart the Laguna Madre during winter and arrive during spring (Farmer, 1991). There is a partial migration of reddish egrets in late autumn, but how far south they migrate is unclear (Paul, 1991). Limited banding data suggest reddish egrets migrate into Mexico and even El Salvador and Guatemala in search of foraging habitat (Paul, 1991). Recently, adult reddish egrets equipped with satellite transmitters were shown to migrate into Mexico during the end of November and beginning of December, returning to the Laguna Madre in March-April (Ballard, unpublished data). Partial migration often occurs when resources become limited and are unable to support the entire population (Ketterson and Nolan, 1976; Lundberg, 1988). The timing of departure from the Laguna Madre during early November by migrant reddish egrets coincides with the decrease in available foraging habitat, indicating that the amount of winter foraging habitat in the Laguna Madre may not be sufficient to maintain the large concentration of reddish egrets in this system.

An initial consideration when prioritizing foraging area for protection should be the temporal extent of its availability. Our habitat model indicated that no foraging habitat pixels (from the final raster image) were available for the entire two-year period and only a small fraction was available more than half of the time. The highly dynamic nature of foraging habitat availability in the Laguna Madre is primarily the result of wind-driven tides (Tunnel, 2002). Because of this high degree of spatial variation in where foraging habitat is located at any time during the annual cycle, it would be required to consider large areas of foraging habitat when devising conservation plans to ensure that enough is available at any given time to sustain the reddish egret population.

Foraging habitat assessment at both smaller spatial and temporal scales within the Laguna Madre may be beneficial in understanding how these habitat dynamics influence reddish egrets. For example, foraging habitat availability during the nestling period influences the provisioning rate by adults, thereby indirectly impacting fledging success. This was the case in the Florida Everglades where nest success of great egrets and white ibises (Eudocimus albus) is lower in years with below average prey availability as compared to years with high prey availability (Herring et al., 2010). Prey availability was not examined in this study; however, sheepshead minnow has been identified as a major prey item of reddish egrets breeding in Texas (Holderby et al., 2014) and the habitat of sheepshead minnow largely coincides with the foraging habitat of reddish egrets, that is, shallow water with minimal vegetation. Therefore, foraging habitat availability is likely related to prey availability.

The authors were able to delineate foraging habitat over a large spatiotemporal scale and to identify important foraging areas for protection and conservation, which is a current goal of the Reddish Egret Conservation

Action Plan (Wilson et al., 2014). This model may also be used in future analyses to examine shifts in foraging habitat and the potential impact of sea level rise on foraging habitat availability. Further, it can be used quite effectively to plan dredging operations in the Laguna Madre, by identifying areas where deposition of dredge material will have minimal impacts on foraging habitat for reddish egrets and other waterbirds.

The authors have not declared any conflict of interest.

This project was funded by the U.S. Fish and Wildlife Service Regions 2 and 4 Migratory Bird Programs, the Walter Fondren III Fellowship in Shorebird and Wading Bird Research at the Caesar Kleberg Wildlife Research Institute, the Rachel and Ben Vaughan Foundation, and the Fort Worth Zoo. The authors thank E. Redeker for providing input for the model development and three anonymous reviewers for their constructive comments. This is manuscript # 16-133 of the Caesar Kleberg Wildlife Research Institute.