Abstract
Over-abundance of deer population in forest landscapes are thought to have significant impacts on the abundance and diversity of birds by reducing shrub and ground vegetation layers. There is, however, little evidence of deer impacts on bird at species level because previous studies only provided deer impacts on avian guild level. Here, we assessed the impacts of sika deer on breeding site selection of ground-nesting bird, Asian stubtail in the forest landscape with over-abundant deer population, Nakajima Island of Lake Toya, Hokkaido. First, we recorded population trends of forest birds from April to June 2010 by using line-transect distance sampling. Secondary, we used automated sound recording survey for determining songs of Asian stubtail in 10 rectangular quadrats (20 x 20 m) between 5:00 - 5:30 a.m. from mid-April to late-July, 2011. Then, we collected understory vegetation coverage and height from 80 sub-quadrats (8 sub-quadrats in 1 x 1 m for each rectangular quadrat). A number of bird species at forest understory was low, and some major shrub-layer birds such as Japanese bush warbler were absent in this study area. On the other hand, we collected 525 hours of active bird songs with automated sound recording, and were able to detect 11,021 callings of Asian stubtail during breeding seasons. The forest understory was dominated by unpalatable plants for deer, Japanese pachysandra and Aleutian ragwort. Although the stubtails were present at all 10 quadrats during mating season, only 6 quadrats were used for natal and breeding sites. Based on limited numbers of calling observed consecutive breeding seasons, deer overabundance could affect breeding site selection of Asian stubtail; however, even if unpalatable plants cover exceeded 20 % of ground, and if height of ground vegetation exceeded 20 cm in height, we concluded the stubtail could breed successfully in the deer over-browsed forest landscapes.
