To clarify the cost of mate guarding, when males attract additional mates, I studied the mate guarding behaviour of the Black-browed Reed Warbler Acrocephalus bistrigiceps in south-central Honshu, Japan in 1997. The percentage of time spent by males close (<5m) to their mates was significantly higher, and the distance between mates was significantly closer during the fertile than during the post-fertile period. Males followed their mates, whereas females did not follow the males. Territorial intrusions, by conspecific males, occurred more frequently in territories with nesting females than in those of solitary males, and an intruding male was found to court a fertile female when her mate was absent. These observations indicate that there is a real risk of extra-pair fertilization. Three males switched their behaviour from mate guarding to singing and attracted secondary females by resuming singing. Two of them stopped mate guarding on the day the first egg was laid, so their mates were unguarded during the remainder of their fertile period. In contrast, three other males, which maintained monogamous pair-bonds, continued to guard their mates throughout the fertile period. Male warblers who attract secondary mates appear to incur an additional cost in mate guarding.
It has been inferred that the population of Bull-headed Shrikes, Lanius bucephalus, increased rapidly on Minami-daito Island, Japan, beginning in 1973-74. This isolated population on a remote island in the Pacific Ocean is about 500km away from the previous southern limit of the breeding area of this species. The plumage and external morphological characteristics of the Minami-daito Bull-headed Shrikes are described, and the results of an investigation into the distribution, sex ratio, and breeding ecology of the population made from January to February and from April to May 1998, are discussed. As there have been previous reports of hybrids between the Bull-headed Shrike and the L. cristatus lucionensis subspecies of the Brown Shrike (a summer migrant to Japan), the characteristics of the Bull-headed Shrikes currently on Minami-daito were compared with those of both L. c. superciliosus and L. c. lucionensis. The Minami-daito Bull-headed Shrikes were found not to show either plumage or external morphological characteristics typical of L. c. lucionensis. Shrikes were observed in all of the typical habitats on the island, at a rate of 2.1 males per km and 1.6 females per km. Most nests were built in Calophyllum inophyllum, and the mean nest height above ground was 252cm. Egg laying was observed for the first time in late January, and continued until April. It was not possible to determine whether or not shrikes continued to lay after May.
Scanning electron microscopy (SEM) was used to study the morphology of Thelazia (Thelaziella) sp. specimens obtained from a captive Oriental White Stork Ciconia boyciana in Japan. The morphological characteristics were found to be identical to those of T. (T.) aquillina, with some additional new morphological features revealed by SEM.
An American Golden Plover Pluvialis dominica, in non-breeding plumage, was observed feeding with 24 Pacific Golden Plovers Pluvialis fulva at Okubo rice field, Urawa, Saitama Prefecture, central Honshu, on 4 April 1987. It was photographed, and confirmed to be the first record of this species from Honshu, Japan.