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SUMMARY.-Disturbance and predation risks from terrestrial animals decline the higher up the nest sites selected by birds that nest in wall cavities are located. Terrestrial predators can also negate the protective quality of higher nesting sites by approaching from above in walls. It is unknown how terrestrial predation risks from below and above walls determine nest site selection in cavity-nesting species. In relation to this situation, we describe nest-site selection in common swifts Apus apus in the medieval city walls of Ávila, Spain. We recorded the entry size, hole depth and the horizontal and vertical positions of cavities. Most cavities were empty despite their size being suitable for nesting. Swifts nested in cavities at least 12 cm deep and with an entry between 3.5 cm and 13 cm wide. Nests were 3.5 m above the ground and 1.7 m below the top of the wall, although there were suitable cavities at the lower and higher extremes, respectively. Higher predation risks and disturbances could explain why suitable cavities were empty at lower and higher heights. The distances to the ground and to the top of the wall, as well as the distance to the nearest corner, accounted for about one-tenth of the probability that a cavity was used for nesting. Our data do not indicate a possible reason for nesting near corners, but weather is an obvious candidate.Key words: Apus apus, castle, cavity-nesting, common swift, nest selection.RESUMEN.-Las molestias y el riesgo de depredación por animales terrestres determinan que las aves que emplean paredes para anidar seleccionen cavidades a mayor altura. También podrían seleccionar el lugar del nido a menor altura si los depredadores terrestres se encuentran presentes encima de la pared, pero se desconoce cual es la selección de cavidades para anidar cuando hay riesgo de depredación terrestre en ambos lados de la pared. Para dilucidarlo, estudiamos la selección de cavidades en el vencejo común Apus apus en las murallas medievales de Ávila, España. Registramos el tamaño de la entrada, la profundidad de orificio y las posiciones horizontal y vertical de las cavidades. La mayor parte de las cavidades se encontraron vacías, incluso cuando su tamaño era apropiado para anidar. Los vencejos anidaron en cavidades con una profundidad de al menos 12 cm y orificios de entrada no más pequeños de 3,5 cm ni mayores de 13 cm. Las oquedades con nidos no se encontraron a menos de

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