Cavity dynamics and breeding success of the Lanyu Scops Owl (Otus elegans)

Abstract

The Lanyu Scops Owl (Otus elegans) breeds in natural tree cavities on Lanyu (Orchid Island), a subtropical island off the southeastern coast of Taiwan in the western Pacific. There are no cavity-making birds or mammals on the island. This study attempted to understand the relationship between this owl’s cavity use and breeding success. In a 10-ha study site, all cavities on all trees with a diameter breast height larger than 18 cm were documented. There were 257 cavities in 119 trees, and 94 of them were used by owls at least once during the study period. The detailed location and size of these 94 cavities in 74 trees were recorded and monitored. From 1999 to 2006, more than 80% of breeding owls were color-marked and their cavity use and breeding success were tracked. Nest trees were taller than other trees, and cavities used by owls were higher above the ground, and larger than unused cavities. Unused cavities were mostly unsuitable due to flooding or decay, or were on trees already containing a nest. Regardless of how many cavities there were on a tree, a single tree never had more than one Lanyu Scops Owl nest at a given time. More than 60% of the 94 cavities were used multiple times. Cavity occupancy frequently showed gaps, usually following breeding failure. Cavity quality (the proportion of years a cavity was used during the study) and female quality (the proportion of years an owl bred out of the total number of years it competed for cavities) significantly contributed to the average fledging success of owls, but male quality did not.