Spatial and temporal dynamics of foraging habitat availability for reddish egrets in the Laguna Madre, Texas

Abstract

The reddish egret (Egretta rufescens) is the rarest heron in North America and much remains to be learned about in the ecology of the species. The reddish egret is a foraging habitat specialist and relies on shallow coastal ecosystems. There is a paucity of information on foraging habitat requirements and the availability of foraging habitat throughout the annual cycle. Characteristics of foraging habitat at locations within the Laguna Madre, Texas where reddish egrets were observed foraging were measured. These characteristics were used to conduct a geospatial analysis that estimated the spatial and temporal distributions of foraging habitat in the Laguna Madre across 120 weeks from 2012 to 2014. Reddish egrets (n = 372) foraged in an average water depth of 10.1 ± 0.68 cm and in areas with average seagrass cover of 12.3 ± 2.74%. Approximately, 75 000 ha of foraging habitat were delineated to be available in the Laguna Madre across the study period; of this, 4 003 ha were available ≥ 50% of the time. The amount of available foraging habitat was relatively high during the spring and summer, and decreased by ~50% during winter. This model-based approach can be used throughout the species' range to examine foraging habitat availability which is a current conservation need according to the Reddish Egret Conservation Action Plan.

Breeding ecology and habitat use of North America’s rarest Ardeidae: the Reddish Egret Egretta rufescens

“…Reddish Egrets selected foraging habitats during the breeding period with little to no seagrass coverage. This is a characteristic of foraging habitat that has been reported during other portions of their annual cycle as well (Bates et al 2016, Koczur 2017. A recent study on the foraging dynamics of Reddish Egrets in the Laguna Madre found that, on average, egrets foraged in areas with < 12% seagrass coverage throughout the annual cycle (Bates et al 2016).…”

“…Last, we examined the relationships between the amount of available foraging habitat and the density of competitors around each colony on breeding pair abundance of Reddish Egrets for 22 colonies in the Laguna Madre. Available foraging habitat was previously modeled for the Laguna Madre based on bathymetry, tide gauge readings, and benthic habitat (Bates et al 2016) for the years 2010-2013. We created buffers around each colony at a 15km radius and used the spatial models of Bates et al (2016) to calculate the area of foraging habitat within each buffer.…”

“…Available foraging habitat was previously modeled for the Laguna Madre based on bathymetry, tide gauge readings, and benthic habitat (Bates et al 2016) for the years 2010-2013. We created buffers around each colony at a 15km radius and used the spatial models of Bates et al (2016) to calculate the area of foraging habitat within each buffer. We chose 15 km to be the radius of our landscapes around colonies because it was the mean foraging flight distance of transmittered Reddish Egrets from colonies in this study.…”

“…Foraging habitat may be an important component of the breeding ecology of Reddish Egrets; egrets nesting in Florida appear to select nesting islands in close proximity to foraging sites (Hill and Green 2011). The spatial and temporal availability of foraging habitat of Reddish Egrets were recently examined in the Laguna Madre of Texas (Bates et al 2016), permitting investigation into the relationship between colony use and proximity to foraging habitat, and fidelity to these areas. In order to better understand the breeding ecology of Reddish Egrets in Texas, our research objectives were to use locations of adult Reddish Egrets marked with GPS-equipped platform terminal transmitters to examine foraging habitat use during the breeding season, and to determine site fidelity to foraging areas and breeding colonies.…”

Survival of adult reddish egrets Egretta rufescens marked with satellite transmitters

“…Recent research on this cohort of reddish egrets marked with satellite transmitters found that they exhibited a partial migration strategy, with 40% of the marked individuals migrating out of the Laguna Madre during winter months (Koczur 2017). A previous study on the spatial and temporal availability of reddish egret foraging habitat found available habitat to decrease during winter months because of changes in water level during that time (Bates et al 2016). Reddish egrets require shallow water (< 20 cm) for foraging, and winter weather conditions that lead to fluctuations in water levels reduce the amount of available shallow water habitat.…”