Effect of Nestbox Provisioning on Breeding Density of Dollarbirds (Eurystomus orientalis)

摘要

The Dollarbird (Eurystomus orientalis) is a secondary cavity-nesting bird that is distributed quite locally in Japan. We carried out extensive surveys across Okayama Prefecture to estimate the number of pairs and found that the major breeding site was holes excavated by woodpeckers in wooden electricity and telegraph poles. Wooden poles were abundant until the 1970s, but most were replaced by concrete poles in the 1980s. Removal of wooden poles containing cavities seriously threatened the breeding population over the first half of the 1990s. In an effort to preserve the Dollarbird population in Japan, beginning in 1991 we provided nestboxes in the village of Kibi (Okayama Prefecture). This resulted in only a small increase in population size over the following 5 yr, but between 1996 and 2001 there was a large annual increase in the breeding population. Increase then leveled off after 2002. Population increase showed a time lag following increase in number of available boxes. Finding new nestboxes may take some time, initially resulting in a low occupation rate for some years after start of box provisioning. Finding of nestboxes by pairs that had bred near Kibi and their subsequent reproduction may have led to a sharp population increase between 1995 and 2002. On the other hand, leveling off of increase in Dollarbird breeding pairs after 2003 may have been directly derived from leveling off of increase in available nestboxes. This suggests a maximum nestbox occupancy rate of 70-80 in Kibi.