Status : Fairly common
Distribution : Breeds on Islands off Hokkaido to Okinawa Pref (appears at their breeding sites in late Feb to early Mar, laying in late June, hatching in mid Aug, fledging in late Oct to early Nov) See below about breeding sites.
Hokkaido: 6 nests on Oshima-o-shima (環境省 2021)
Iwate Pref: Presumed about 2,000 nests on Hide-jima (環境省 2023), 632 nests on Sangan-jima (環境省 2013), Funakoshi-oshima (the number of nests are unknown)
Miyagi Pref: Presumed about 15,000 nests on Ashi-jima (環境省 2020), Burrows have been found in which this species may be breeding on Kasakai-jima (環境省 2005).
Tokyo: About 24,700 nests on Tadanae-jima, 28 nests on Hachijo-ko-jima Kojine, presumed 2,548,125 nests (about 23.1% were used) on Mikura-jima, Toshima (the number of nests are unknown), Udone-jima (the number of nests are unknown), Aoga-shima (the number of nests are unknown), Izu Islands (岡 2004, 環境省 2021, 環境省 2023).
Niigata Pref: 84,000 birds were found on Awa-shima (山本 2011).
Ishikawa Pref: 40,000 birds were found on Nanatsu-jima (いであ株式会社 2019).
Kyoto Pref: Presumed 140,189 nests (about 43.3% were used) on Kanmuri-jima (環境省 2023), 13 nests on Kutsu-jima (環境省 2020).
Mie Pref: Presumed 457 nests on Oshima (須川 2010).
Shimane Pref: 141 nests on Hoshigami-jima, 44 nests on Okino-shima, 369 nests on Ohaka-jima, 33 nests on Shira-shima, 92 nests on Matsu-shima, 84 nests on Futamata-jima, 80 nests on Omori-jima (環境省 2023).
Yamaguchi Pref: Saba-shima (the number of nests are unknown), 75 nests on Uwa-jima (飯田&山本2012).
Kochi Pref: Futanarabi-jima and Kou-jima (the number of nests are unknown) (環境省 2018).
Fukuoka Pref: 250 nests on Chikuzen-okino-shima (環境省 2021).
Nagasaki Pref: 56 nests on O-shima, 25 nests on Me-shima, Danjo Islands (環境省 2021). Kamiajika-jima and Shimoajika-jima (the number of nests are unknown).
Oita Pref: 122 nests on Taka-shima (Bird Research 2021), presumed several hundreds birds were breeding on Okiguro-jima (大分県 2011).
Miyazaki Pref: Biro-jima (the number of nests are unknown) (環境省 2018).
Kagoshima Pref: Presumed 24,000 nests on Kaminone-jima (環境省 2022), 49 nests on Ko-jima (環境省 2018), Tokara Islands. 22 nests on Hamiya-to (環境省 2022).
Okinawa Pref: 507 nests on Nakanougan-jima (環境省 2022). Kubashima, Senkaku Islands (the number of nests are unknown).
Common around breeding islands in late Feb to Nov. Rare around Tori-shima, Izu Islands to Ogasawara Islands.
Tips : Juveniles that leave the nest at Awa-shima, a breeding site on the Sea of Japan side, pass through the mountains of the Tohoku and Chubu regions to the Pacific Ocean side (Yoda et al. 2017).
Although information on the moulting of this species is very limited, adult birds generally begin moulting around Oct to Nov. Some individuals, such as those that failed to breed, begin molting in September. The exact timing of the molt from First Basic to Second Prebasic is unknown, but we have observed several individuals that began moulting around the end of July so it is assumed that these individuals are getting Second Prebasic.
The breeding population on Nakanougan-jima, Okinawa Pref., is smaller in body size than other breeding populations, suggesting that it followed Bergmann’s/James’s rule, and possibly a different subspecies or species. (Yamamoto et. al. 2016; Yamamoto et. al. 2019).