The Ecology of Colonial Nesting Green Herons (Butorides virescens) in Texas
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Green Herons (Butorides virescens) are small herons found throughout the eastern United States, the west coast of the United States and throughout most of the state of Texas. While this bird can be found along the Texas Coast year round, they occur in greater densities during the breeding season. Green Herons are solitary foragers and often nest singly, with a breeding pair defending a breeding territory. Green Herons sometimes form loose breeding aggregations or colonies presumably as a function of habitat availability and/or predator pressure. A colony of at least 35 breeding pairs of Green Herons annually breed along a tidal creek in Port Lavaca, Texas. This study sought to determine a relationship between nest density and nest success and to use observational data to examine factors of this poorly understood behavior in Green Herons. A secondary goal of the study was to examine juvenile dispersal by banding chicks and monitoring adults in subsequent years to test whether juveniles return to their natal colony to breed. Nearest neighbor spacing varied from < 1 m to 42.5 m apart (mean=9.57m). All nests occurred in low shrubs Marsh Elder (Iva frutescens) along the water’s edge. Nesting began in early April and ceased in late July/early August. Clutch size ranged from 1-5 eggs for both years with a mean of 3.09 (SE=0.106) and 3.43 (SE=0.163) for 2014 and 2015 respectively. Nest success varied between years (2014, 53.57% nest success; 2015, 12.25% nest success); high nest mortality in 2015 was likely due to extreme weather events and human disturbance. AIC model selection favored models containing the quadratic effect of nearest neighbor estimate, Julian lay date, and year suggesting the possibility of an optimum nearest neighbor distance of around 16m for Green Herons at this location, though more years of data will be needed to reveal a strong trend given the high amount of density independent mortality in 2015. No chicks banded in 2014 were re-sighted in 2015.