Acoustic Characteristics of Songs in a Recently Established Population of the Japanese Bush Warbler on an Oceanic Island.

The acoustic structure of birdsongs is determined by ecological and social factors. Moreover, the founder effect can occur when a few colonizers bring a small subset of the song diversity from a source population to a newly established population, generating the acoustic features of its songs. Around 2000, the Japanese bush warbler (Cettia diphone) naturally colonized Minami-Daito, an oceanic island in the northwest Pacific. This raises the question of whether the songs in this population have changed through adaptation to the insular environment or maintained the features of songs in the mainland population. In this study, the acoustic characteristics of Japanese bush warbler songs on Minami-Daito Island at present (i.e., approximately 20 years after colonization) were compared with the songs of conspecifics on the mainland and another island. The acoustic structure of one of two basic song types on this island did not differ from that on the mainland. The other song type had a simpler structure on the island than on the mainland. Analyses of intonation structure showed that a certain pattern of frequency increase and decrease among sound elements was rare (< 10%) on the mainland but dominant on the island. The song characteristics substantially overlapped between the island and the mainland, and have not changed on the island since its colonization. These results suggest that the song characteristics on Minami-Daito Island can be explained by the founder effect. The songs on this island may change adaptively over a long period. Continuous investigation to follow the changes is required.

Risk-taking behavior of bull-headed shrikes that recently colonized islands.

Individuals which have invaded urbanized environments are reported to engage in riskier behaviors, possibly influenced by the scarcity of predators in urbanized areas. Here, we studied the risk-taking behavior of birds which had invaded a new natural environment, rather than an artificial urban environment, using recently established populations of the bull-headed shrike Lanius bucephalus, which naturally colonized three subtropical islands in Japan. We compared flight initiation distance (FID), the distance at which an individual approached by a human initiates flight, between the islands and the temperate mainland. FID was longer for the insular shrikes compared with the mainland shrikes after controlling for other factors, indicating that the individuals which had invaded a new natural environment had a lower propensity for risk-taking. A possible explanation for these results is that low risk-taking behavior might be adaptive on the islands due to predation by the black rat Rattus rattus, an unfamiliar predator not found in shrike habitats on the temperate mainland. Further studies are needed to examine the nest predation rate, predator species, and nest site selection of these insular shrike populations.

Host-feeding habits of Culex pipiens and Aedes albopictus (Diptera: Culicidae) collected at the urban and suburban residential areas of Japan.

To evaluate the vectorial capacity of mosquitoes for viruses in Japan, the host-feeding habits of the mosquitoes were analyzed by sequencing polymerase chain reaction-amplified fragments of the cytochrome b and 16S ribosomal RNA regions of the mitochondrial DNA of 516 mosquitoes of 15 species from seven genera that were collected from residential areas during 2003-2006. Culex pipiens L. and Aedes albopictus Skuse were the most commonly collected species in urban and suburban residential areas. Anautogenous Culex pipiens pallens Coquillett was distinguished from the autogenous Cx. pipiens form molestus Forskal using a polymerase chain reaction-based identification method. Both Cx. p. pallens and Cx. p. form molestus exhibited similar host-feeding habits, broadly preferring avian (50.0 and 42.5% of avian, respectively) and mammalian (38.6 and 45.0% of avian, respectively) hosts, such as tree sparrows, ducks, and humans. Conversely, Ae. albopictus exhibited a highly mammalophilic and anthropophilic feeding pattern, with 84.2% feeding on mammalian hosts and 68.5% of these on humans. We concluded that in Japan, Cx. pipiens might play a significant role in the avian-to-mammal transmission of viruses, such as West Nile virus, whereas Ae. albopictus might play a role in the human-human transmission of dengue and Chikungunya viruses.