Competition between Cattle Egrets and Native North American Herons, Egrets, and Ibises

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Abstract
The behavior of cattle egrets [Babulcus ibis] in a mixed colony of herons and ibises in New Jersey [USA] where all species laid eggs during the same period, was studied. All the herons competed for stick nests in the bushes from the previous year. Cattle egrets acquired significantly more nests than predicted by their percent occurrence. In 2 similar study areas, the native species situated the height of their nests according to body length. Little blue herons, [Florida caerulea] nested highest in the bushes and glossy ibises [Plegadis falcinellus] nested on the ground. Cattle egrets had a higher mean nest height than predicted by the body length relationship found for the native species. Cattle egrets were twice as aggressive as any other species and their conflicts were more vigorous than those of other species. They fought more with each other and won more of their interactions with other species. Of their interactions 80% ended in pecking at the intruder, compared to 50% for all other species. Cattle egrets supplanted most intruders at distances > 1 m from their nests, whereas, for other species most such actions were at distances < 1 m. For the native species, larger species defending nests chased intruders that landed farther from the nest than did smaller species. Larger intruding species could more closely approach the nests of others before being chased than could smaller species. The mean number of young at hatching and the mean number of chicks surviving to 14 days indicated that cattle egrets had the highest breeding success while snowy egrets [Egretta thula] had the lowest breeding success.