Ecological drivers of coloniality in Oreotrochilus chimborazo chimborazo (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.

RESÚMEN

La investigación sobre el significado adaptativo de la vida en grupos sociales ha generado debates acerca del papel potencial que los factores ambientales pueden tener en la evolución de la colonialidad. Las agregaciones de nidos pueden ocurrir como resultado de una respuesta plástica de individuos reproductivos a las limitaciones tanto de sustrato como de condiciones ambientales favorables para la anidación (Hipótesis de Limitación en la Disponibilidad de Sustrato). Sin embargo, pocos estudios han demonstrado el rol directo de factores ambientales en el desarrollo de agregaciones sociales. El muestreo poblacional de Oreotrochilus chimborazo chimborazo (Estrellita del Chimborazo), un colibrí endémico de los Altos Andes del Ecuador, resultó en la identificación de 7 agregaciones de nidos y 13 nidos solitarios. De un total de 74 nidos activos detectados, la mayoría (82%) fueron encontrados en colonias reproductivas. Mas aún, estas agregaciones de nidos estuvieron consistentemente asociadas a grupos de machos y de hembras no reproductivas pernoctando juntos, un caso único entre las aves. Estas observaciones cuestionan la idea comunmente aceptada de que los colibríes son nidificadores solitarios altamente competitivos. La subsecuente descripción del hábitat de nidificación de la Estrellita del Chimborazo reveló que el viento, la temperatura, la humedad, y las irregularidades del paisaje asociadas con la disponibilidad de sustrato, la cobertura vegetal y la disponibilidad de agua representan factores ambientales asociados con la presencia de nidos, consistente con la Hipótesis de Limitación en la Disponibilidad de Sustrato. Sin embargo, un “experimento” seminatural comparando la presencia de nidos solitarios vs. nidos en agregaciones, reveló que sólo el 45.5% de los sitios disponibles para nidificación solitaria mostraron nidos activos individuales, lo cual es significativamente más bajo que el 95% estimado por simulaciones aleatorias. Contrariamente, las agregaciones presentaron consistentemente múltiples nidos activos en frequencias más altas que las esperadas por las simulaciones, produciendo crías a lo largo de distintas temporadas reproductivas. Estos resultados sugieren que factores adicionales de aquellos asociados a la Hipótesis de Limitación en la Disponibilidad de Sustrato pueden jugar un papel importante en el desarrollo de agregaciones de nidos junto a grupos de individuos perchando, lo que enfatiza la importancia de la interacción de factores ambientales y sociales en el desarrollo de la colonialidad en aves.

Lay Summary

• We provide the first report of Oreotrochilus chimborazo chimborazo (Chimborazo Hillstar) hummingbirds living in nesting and roosting groups in the Ecuadorian High Andes, challenging the widespread assumption that hummingbirds are highly competitive solitary nesters.

• During the dry season of 2021, we detected 82% of nests in 7 nesting aggregations consistently associated with roosting groups of non-reproductive individuals, a unique case among birds.

• We showed that wind speed, temperature, humidity, and landscape features associated with substrate availability, ground cover, and water represent important environmental factors delimiting nesting sites.

• A semi-natural experiment revealed that nesting aggregations may not be solely explained by environmental factors associated with nesting sites. The presence of roosting groups co-occurring in all nesting aggregations may have promoted social adaptive behaviors favoring the evolution of coloniality.

• Our results provide insights into the role that environmental and social factors may play in the origin and evolution of coloniality in birds.