Nesting associations of least bitterns and boat-tailed grackles

Interspecific nesting assemblages may result from independent settlement by individuals or groups of each species in the same limited habitat, or association with other species may be actively sought. Although Least Bitterns (Zxobrychus exilis) often nest solitarily, in a South Carolina impoundment they frequently associated with colonies of Boattailed grackles (Quiscnlus major). Bittern nests inside and outside colonies occupied the same microhabitats. However, more bitterns nested in grackle colonies than in equal-sized areas of equivalent habitat in the same marsh. In one of two years, colonial bitterns had higher reproductive success than bitterns nesting outside colonies. We conclude that Least Bitterns actively associate with grackles. Possible reasons for the association of bitterns with grackles are as follows (1) Critical density effect: at high population densities, the advantages of dispersed (cryptic) nesting are lost. Bitterns switching to nest in groups may experience little reduction in reproductive success, relative to those nesting solitarily. (2) Vigilance and mobbing: grackles provide early warning, and actively exclude predatory birds from colonies. Bitterns nesting among grackles appeared to incur few costs, such as may result from competition for food or from intracolony predation.