The adaptive significance of off-lek sociality in birds: A synthetic review, with evidence for the reproductive benefits hypothesis in Long-wattled Umbrellabirds

Abstract

ABSTRACT Anecdotal evidence suggests that lekking birds exhibit considerable variation in form and degree of sociality away from the lek, yet this phenomenon has received very little theoretical or empirical research attention. Here, we provide the first synthetic literature review of off-lek sociality in birds and develop a conceptual framework for the potential adaptive function of off-lek sociality across lekking taxa. We then present a case study of the Long-wattled Umbrellabird (Cephalopterus penduliger), where we find support for the hypothesis that off-lek sociality is primarily driven by male reproductive incentives for coordinating lek attendance during the breeding season. During periods of high lekking activity, male umbrellabirds depart the lek in highly coordinated groups and maintain larger off-lek social groups relative to periods of low lekking activity. These seasonal differences in off-lek sociality do not occur in females, are not explained by patterns of foraging behavior, and are expected to confer individual-level benefits for participating males. Both the literature review and empirical study of umbrellabirds suggest that off-lek interactions and behavioral strategies may shape sexual selection processes at leks in important ways. Further research into this historically understudied area of lekking species' behavioral ecology will likely deepen our understanding of the evolutionary dynamics of lek mating. LAY SUMMARY Social behavior away from the lek may influence and interact with sexual selection processes in important ways, yet this component of lekking species' behavioral ecology remains very poorly known. A synthetic literature review suggests considerable diversity in the form and degree of off-lek sociality among lekking birds, both within and between species. Potential adaptive explanations for the function of off-lek sociality in a given species include foraging enhancement, predation avoidance, and mating benefits. In Long-wattled Umbrellabirds, we find that the size and coordination of male off-lek groups increases during the mating season. This pattern is not observed in females and is not related to the proportion of fruit in the diet. We suggest that off-lek sociality in umbrellabirds enables males to synchronize foraging and display periods, which is expected to confer individual-level reproductive benefits. RESUMEN Evidencia anecdótica sugiere que las aves que forman leks exhiben variación considerable en la forma y el grado de sociabilidad fuera del lek, pero este fenómeno ha recibido poca atención teórica o empírica. Aquí, presentamos la primera revisión sintetizada de la literatura sobre la sociabilidad fuera del lek en aves y desarrollamos una estructura conceptual para la función adaptativa potencial de la sociabilidad fuera del lek en taxones que forman leks. Luego, presentamos un estudio de caso del Pájaro Paraguas Longipéndulo (Cephalopterus penduliger), en donde encontramos sustento para la hipótesis que la sociabilidad fuera del lek es impulsada principalmente por incentivos reproductivos de los machos para coordinar la asistencia al lek durante la temporada de reproducción. Durante períodos de alta actividad en los leks, los machos se alejan del lek en grupos altamente coordinados y mantienen grupos sociales más grandes fuera del lek en relación con los períodos de baja actividad en los leks. Estas diferencias estacionales en la sociabilidad fuera del lek no ocurren en las hembras, no se explican por patrones de comportamiento de forrajeo, y se espera que otorguen beneficios a nivel individual para los machos participantes. Tanto la revisión de la literatura como los estudios empíricos del Pájaro Paraguas Longipéndulo sugieren que las interacciones fuera del lek y las estrategias de comportamiento pueden influir de manera importante en los procesos de selección sexual en los leks. Investigaciones adicionales en esta área históricamente poco estudiada de la ecología del comportamiento de las especies que forman leks probablemente profundizarán nuestra compresión de las dinámicas evolutivas de los sistemas de apareamiento en lek.