Present condition of the avifauna of Mukojima, Ogasawara archipelago, following the eradication of feral goats

Abstract

Mukojima is an uninhabited island in the northern part of the Ogasawara archipelago. The island's, vegetation was drastically disturbed by feral goats Capra aegagrus hircus after 1945. The change in the avifauna resulting from this habitat disturbance has previously been described. Japanese Bush Warbler Cettia diphone and Bonin Honeyeater Apalopteron familiare became locally extinct. In recent years, the goats have been exterminated and the vegetation is now recovering. To clarify the current stituaion of the avifauna on Mukojima, we counted bird numbers along two routes from March to June, 2007. We also recorded the vegetation along the routes. Many Japanese White-eye Zosterops japonicus (an introduced species) were observed in the forest, and several Blue Rock Thrush Monticola solitarius were observed in open areas. These were the only two species observed throughout our study period; both were confirmed to be breeding. Other native land birds, Common Buzzard Buteo buteo, Brown-eared Bulbul Hypsipetes amaurotis, White's Trush Zoothera dauma, Japanese Bush Warbler Cettia diphone were less abundant, with only a few individuals of each species observed for short periods. In the future, such vistors to Mukojima may increase as the vegetation recovers. Japanese White-eye will contribute to the recovery of the vegetation as they are both seed dispersers and pollinators, although they may compete with indigenous species for food.

Monitoring of Avifauna to Estimate the Effect of Ecological Restoration in Mukojima, Bonin Islands

“…Under appropriate weather conditions (no strong wind and/or no rain), four to eight counts per month were made from March to May, between 10:00 h and 16:00 h (Japan Standard Time). Over 60 of these censuses were started between 14:00 h and 16:00 h, similar to our study in 2007 (Emura & Deguchi 2009). The total number of censuses was 72 for Transect A and 64 for Transect B.…”

“…Since its evacuation during World War II Mukojima has been uninhabited (Okutomi et al 1983), after when the goat population has increased dramatically (300 individuals·km 2 tallied in 1991; Tokida 1992), leading to the degradation of the native vegetation (Shimizu 1993). The avifauna of Mukojima has also been negatively affected by such anthropogenic habitat changes: of the seven resident terrestrial bird species (including the endemic Bonin White-eye Apalopteron familiare familiare) present in the 1930s, only one species, the Blue Rock Thrush Monticola solitarius (Emura & Deguchi 2009 ; Table 1), remains. In addition, the Japanese White-eye Zosterops japonicus was intentionally introduced to Mukojima in the 1960s, where it is now common (Hasuo 1969, Emura & Deguchi 2009 ; Table 1).…”

“…For the short-term comparison we used 2007 terrestrial bird survey data from Emura & Deguchi (2009) and compared it with our own 2011 2012 data taken along the same transects. This study was conducted in the two remnant bush patches of Mukojima.…”

“…The grassland with sparse woods was dominated by the herbaceous species Chrysopogon aciculatus and by shrub Vitex rotundifolia. Each of the four observers involved in this study walked these transects at a pace of approximately 2 km·h 1 , counting the number of individual terrestrial birds per species observed within a 25 m radius, using an identical protocol to Emura & Deguchi (2009). Under appropriate weather conditions (no strong wind and/or no rain), four to eight counts per month were made from March to May, between 10:00 h and 16:00 h (Japan Standard Time).…”

“…The total number of censuses was 72 for Transect A and 64 for Transect B. In 2007, Emura & Deguchi (2009) conducted four to 14 counts per month, with a total number of 30 censuses for Transect A and 60 for Transect B. Results are given in Table 1.…”