Ecological drivers of coloniality in <i>Oreotrochilus chimborazo chimborazo</i> (Chimborazo Hillstar), a hummingbird endemic to the High Andes of Ecuador

ABSTRACT The adaptive significance of group-living has promoted considerable debate about the potential role of environmental drivers in the evolution of coloniality. Nest aggregations may result as a plastic response of reproductive individuals to limited substrate availability and favorable environmental conditions for nesting (limited substrate availability hypothesis). However, few studies have demonstrated the direct role of environmental factors in promoting social aggregations. A population survey of Oreotrochilus chimborazo chimborazo (Chimborazo Hillstar), a hummingbird subspecies endemic to the Ecuadorian High Andes, resulted in the identification of 7 nesting aggregations and 13 solitary nests. Of a total of 74 active nests detected, most (82%) were found in reproductive colonies. Furthermore, nesting aggregations were consistently associated year-round with roosting groups of males and non-reproductive females, a unique case among birds. These findings challenge the widespread assumption that hummingbirds are highly competitive solitary nesters. The subsequent characterization of O. c. chimborazo nesting habitat revealed that wind speed, temperature, humidity, and landscape features associated with substrate availability, ground cover vegetation, and water represent environmental factors associated with nesting, consistent with the limited substrate availability hypothesis. However, a semi-natural “experiment” comparing occupancy of solitary nesting locations vs. nesting aggregations revealed that only 45.5% of the sites available for solitary nesting showed individual active nests, significantly lower than the 95% expected by random simulations. In contrast, nesting aggregations consistently had multiple active nests at significantly higher frequencies than expected, producing broods across multiple reproductive seasons. These results suggest that other factors beyond those associated with the limited substrate availability hypothesis may play a role in the development of nesting/roosting aggregations in O. c. chimborazo, emphasizing the importance of the interaction of environmental and social factors in driving coloniality in birds.