The declining abundance of Great Bittern during the breeding season at Watarase Marsh

<p>I surveyed the abundance of Great Bitterns Botaurus stellaris annually during the breeding season (late March to June) from 2008 to 2017 in Watarase Marsh, central Japan, and recorded the number of vocalizing (booming) males. The survey was conducted for three hours (two hours before and an hour after sunrise) along a 15 km long course through the three regulated reservoirs at Watarase Marsh. This study stands on the assumption that every booming individual is male, and that their booming indicates an indirect proof of their breeding in the study site. One to five males were recorded per day along the census course. A maximum number of four males was recorded in the first reservoir, which had the highest occurrence rate among the three reservoirs. The abundance of males, however, declined after 2013, after which only one or two males were recorded in the study course. Most of the locations where booming was recorded were less than two hectares in area, and the water depth was less than 20 cm. The highest occurrence rate was recorded in an artificial pond, covered with cattails Typha angustata, on a golf course. Although Watarase Marsh is a relatively large reedbed of about 1500ha dominated by common reed Phragmites australis and Japanese silver reed Miscanthus sacchariflorus, it is relatively dry with very few areas of reed swamp. It is suggested, therefore, that Watarase Marsh does not currently provide the preferred breeding habitat for the bittern. This situation could be improved, as evidenced by a booming male, recorded in the breeding seasons of 2016 and in 2017, which was located on a new pond with reedbed created by a wetland restoration project. Creation of a new reed swamp, by a wetland restoration project implemented at the second regulated reservoir, is considered to be an important for expanding the suitable breeding habitat of this species.</p>