The relationship between the agricultural landscape and the seasonal changes in the foraging habitats of the grey heron (Ardea cinerea) on the Noto Peninsula

In order to evaluate the importance of the agricultural landscape for grey herons, we investigated the seasonal changes in the foraging habitats of the Grey Heron (Ardea cinerea) in two river basins. These were located in the Wakayama River and the Kumaki River on the Noto Peninsula (Ishikawa, Japan). The peninsula’s fertile landscape is well-known for its traditional agricultural practices and fields. We conducted 15 censuses from March until November in 2011 in the entire waterside area within each basin. Sampled habitats included paddy fields, rivers, ditches, irrigation ponds, coasts, and fields. Seasonal fluctuations in the number of herons were mostly synchronized between the two river basins during the breeding season, and a large increase in the population size in June was due to an increase in the number of fledglings. During the non-breeding season, only the Kumaki River basin was a major stopover during migration for grey herons, because a large increase in the population size equivalent to that of June occurred in September. The difference in the seasonality of the population size between the two river basins may reflect the latitudinal difference relating to the fall-winter migration patterns of grey herons. The population size of the paddy fields correlated with the proportion of irrigated paddy fields, suggesting that the patterns of habitat use for foraging by grey herons were dependent on the degree of irrigation in the paddy fields. Furthermore, the number and proportion of herons in the other habitats became larger when those of paddy fields became smaller. Thus, the complementary relationship between the seasonality in population size of paddy fields and of the other habitats suggests that those such as rivers and ditches may play a role as substitutional habitats. However, the combined population size in the rivers and ditches did not amount to the maximum population size of the paddy fields. It may indicate that food organisms in the rivers and ditches in the two river basins are poorer in quality and quantity than those in the paddy fields.