Long-term Breeding Success of the Cattle Egret in Texas

Abstract

A 28-year (1974-2002) study of the Cattle Egret (Bubulcus ibis) was conducted on Bird Island and Telfair Island at Cedar Creek Reservoir in northeast Texas to determine trends and dynamics of clutch/brood sizes, hatching success, chick mortality, and nest-site vegetation. For the breeding colonies on these islands, annual mean clutch size ranged from 2.71-3.73 (overall mean = 2.98) and from 2.56-4.18 (overall mean = 3.33); annual mean brood sizes ranged from 2.20-3.08 (overall mean = 2.60) and from 2.38-3.67 (overall mean = 2.87). Annual mean hatching success on the islands was high throughout the study (78%-94% [overall mean = 88%] and 71%-96% [overall mean = 86%]). Annual mean chick mortality was low (0.5%-13% [overall mean = 4.5%] and 0%-10% [overall mean = 2.1%]). Among marked broods on both islands, only 6.5% lost chicks, for which the mean loss was 1.16 chicks/brood. Generally, within these broods, there was higher mortality of the younger chicks. In broods of more than two chicks, the mortality ratio between 1st- and 2nd-hatched chicks was lower than other combinations of sibling rank. Decline of nest density coincided with the cumulative adverse effects of guanotrophication on nest-site vegetation. The original native nest-site vegetation was replaced by Chinaberry (Melia azedarach), an exotic tree that provided optimal nest sites until it too became intolerant to guanotrophy, which resulted in a continued decline in nest-site vegetation. During the time of declining availability of nest-site vegetation, the egrets began ground nesting; by 2002, 48% of nests were on the ground.