Clutch Size and Breeding Success of the Japanese Wagtail Motacilla grandis, with a Special Reference to Its Habitat and Mating System

Abstract

The reproduction of the Japanese Wagtail, Motacilla grandis, was studied from view points of habitats and breeding systems in Mito, Ibaraki Pref., central Japan. The start and end of egg laying may depend on food supply, predation pressure and weather. The seasonal trend of cultch size was of peak type. This clutch size variation may be positively correlated with the food availability for young. The difference in breeding success between riverside and suburban habitats was mainly caused by the predation and river swell, both hatching success and fledging success being higher in the suburban habitat than in the river side habitat. These higher successes in the suburban habitat appear to cause the recent increasing utilization of the habitat by Japanese Wagtail. The Japanese Wagtail occasionally took the bigynous breeding, in spite of disadvantages due to increases in nestling starvation and unfertilized eggs in the bigyny. The present study revealed that the good territory with less predations and less damages due to the external forces such as river swell could enable to establish the bigyny by canceling the disadvantages attached to the bigyny.