Seasonal change in habitat use by Grey Herons in a rural area of western Hokkaido

Abstract

To evaluate the relative importance of rice fields for Grey Herons Ardea cinerea, habitat use by the herons in rural areas, including farmlands, streams, ponds, and lakes, was investigated in Ishikari district, western Hokkaido. To clarify habitat use by herons, censuses along 81 km long routes in rural areas and point counts at four sites (ponds and lakes) were conducted. Grey Herons used rice fields from mid June to mid August, but did not use this habitat either in April or after late September. Some herons used pond, lake, and river habitats in early April, but the number of herons in these habitats decreased from early June onwards and increased again from mid August until mid September. Heron numbers declined in late September, with few herons using ponds, lakes, and rivers until mid November. Herons used natural wetland habitats in spring, but when rice fields were flooded, they switched to this habitat. When water was drained from rice fields in late August, herons returned to natural wetlands. Observations revealed that herons feed mainly on tadpoles in rice fields and on small fish in ponds and lakes. Foraging success was higher, and time to capture one prey item was shorter, in rice fields compared with that in lakes and ponds. Intensive use of rice fields by Grey Herons coincided with the occurrence of Japanese Tree Frog Hyla japonica tadpoles in this habitat type. Grey Herons therefore use rice fields to feed on tadpoles. This suggests that tadpoles are important prey item for Grey Herons in this region.