Abstract

Cavity-nesting bird populations are most frequently limited by the number of tree cavities available in second-growth forests. However, this possible limitation of a key resource is less clear in old-growth forests. We compared forest attributes (i.e., basal area, density of larger trees, density of dead trees, and tree cavity density) in second-growth and old-growth stands in Andean temperate rainforests in southern Chile. To examine the role of nest-site availability in limiting the populations of Aphrastura spinicauda (Thorn-Tailed Rayadito), we monitored their populations in both forest types during a 5-year period (2008-2013), while we conducted an experiment in which nest boxes were added and then, after two years, removed by blocking cavity entrances. In old-growth forests, as compared to second-growth forests, we found a more than double basal area (99.6 vs. 43.7 m2 ha-1), a 3 times higher density of larger trees (88.2 vs. 36.4 trees ha-1), and a 1.5 times higher number of small cavities (25.9 vs. 10.3 cavities ha-1). The density of cavities also strongly increased with tree diameter and basal area. In second-growth forests, A. spinicauda showed a strong response to the addition, and later to the removal of nest boxes, with population abundance increasing by 13% and then decreasing by 50%, respectively. In contrast, we found no impact on old-growth stands. Our experiment emphasizes the importance of maintaining large and dead trees in second-growth, disturbed, and managed forests. These trees provide suitable cavities for A. spinicauda, and likely many other secondary cavity nesters, increasing their abundance in a Globally significant Biodiversity Hotspot in southern South America. A Spanish translation of this manuscript is available as Supplementary Material. center dot Cavity availability is considered a limiting factor, potentially regulating the breeding population density of avian cavity nesters.center dot We compared forest attributes in second-growth and old-growth stands and assessed the role of nest-site availability in limiting populations of Aphrastura spinicauda (Thorn-Tailed Rayaditos) in Andean temperate rainforests in southern Chile.center dot We conducted an experiment in which nest boxes were added and removed in both forest types during a 5-year period (2008-2013).center dot In old-growth forests compared to second-growth forests, we found 2.3 times higher basal area, 2.4 times higher density of larger trees, and a 2.5 times higher number of small cavities.center dot In second-growth forests, A. spinicauda populations increased by 13% after the nest box addition, and then decreased by 50% after removal, in contrast to no change in old-growth forests.center dot Our findings highlight the importance of maintaining larger trees that provide suitable cavities for this species in second-growth forests. Las poblaciones de aves que nidifican en cavidades est & aacute;n frecuentemente limitadas por el n & uacute;mero de cavidades disponibles en bosques de crecimiento secundario. Sin embargo, esta posible limitaci & oacute;n de este recurso clave es menos clara en bosques antiguos. Comparamos los atributos del bosque (i.e., & aacute;rea basal, densidad de & aacute;rboles grandes, densidad de & aacute;rboles muertos y densidad de cavidades de & aacute;rboles) en bosques templados andinos de crecimiento secundario y antiguo del sur de Chile. Para examinar el rol de la disponibilidad de sitios de nidificaci & oacute;n en la limitaci & oacute;n de poblaciones de Aphrastura spinicauda, monitoreamos sus poblaciones en ambos tipos de bosques durante un periodo de 5 a & ntilde;os (2008-2013), mientras realizamos un experimento en el que se agregaron cajas nido y, luego de dos a & ntilde;os, se removieron bloqueando la entrada. En los bosques antiguos, en comparaci & oacute;n con los bosques de crecimiento secundario, encontramos un & aacute;rea basal de m & aacute;s del doble (99.6 vs. 43.7 m2 ha-1), una densidad de & aacute;rboles grandes tres veces mayor (88.2 vs. 36.4 & aacute;rboles ha-1) y un n & uacute;mero de cavidades peque & ntilde;as 1.5 veces mayor (25.9 vs. 10.3 cavidades ha-1). La densidad de cavidades tambi & eacute;n aument & oacute; fuertemente con el di & aacute;metro de los & aacute;rboles y el & aacute;rea basal. En los bosques de crecimiento secundario, A. spinicauda mostr & oacute; una fuerte respuesta a la adici & oacute;n y subsecuente eliminaci & oacute;n de cajas nido, con un aumento en la abundancia poblacional del 13% y luego una disminuci & oacute;n del 50%, respectivamente. En contraste, no encontramos impacto en los bosques antiguos. Nuestro experimento enfatiza la importancia de mantener & aacute;rboles grandes y muertos en los bosques de crecimiento secundario, degradados y/o manejados. Estos & aacute;rboles proporcionan cavidades adecuadas para A. spinicauda, y probablemente para muchas otras aves que se reproducen en estas, aumentando sus abundancias en un bosque prioritario para la conservaci & oacute;n de biodiversidad a nivel mundial en el cono sur de Sudam & eacute;rica. Una versi & oacute;n en espa & ntilde;ol de este manuscrito est & aacute; disponible como material suplementario.