Parkes, Michael Lawrence (2007-08). Residential cattle egret colonies in Texas: geography, reproductive success and management. Master's Thesis.
A phenomenon of large, upland breeding colonies of cattle egrets in residential
areas of Central Texas has been observed since the early 1960s. These large
concentrations of breeding birds can be a nuisance to nearby residents and their
management has been difficult. To help understand why cattle egrets choose upland,
residential breeding sites, and predict where these might occur, the geographic extent of
the phenomenon was bounded within Texas, a habitat suitability model constructed, and
reproductive success compared by breeding habitat type to evaluate if residential nesting
confers an adaptive advantage..
Records of upland cattle egret colonies were found only in Central Texas, not
other parts of the state. The habitat suitability model was constructed using total edge of
three land use classes: water, forest, and developed classes. The model classified 78.6 %
of upland colonies in very high or high suitability classes and 7.1% of colonies in low or
very low suitability classes. This distribution was significantly different than expected
considering the overall ratio of suitability scores in the entire raster model (p = 0.036).
Nineteen active colonies were found in or bordering the Post Oak Savannah and
Blackland Prairie ecoregions. Colonies were in residential, urban, island, and flooded tree and shrub habitat. Nests were found in 12 different tree and shrub species.
Residential colonies had more breeding pairs, greater nest survival, and were less
productive than non-residential colonies on average, but these differences were not
statistically significant. Colonies where nest substrate was removed were not reused and
no breeding was initiated nearby the next year. Propane cannons discouraged reuse of
colony after prolonged application.
Herons and egrets likely use residential sites when wetland habitats are limited.
Their overall breeding distribution reflects state wide rainfall and wetland availability
patterns with upland nesting in Central Texas, wetland nesting in eastern and coastal
regions, and little large scale nesting in western Texas. Egrets and herons may use edges
of development as breeding sites to limit predation by ground predators when flooded
tree and shrub or island habitats are absent, but this hypothesis needs more testing.