ИСТИНА

Seabirds are good indicators of the condition for marine ecosystems. However the patterns of colony attendance in nocturnal seabirds are still not clear for many species, thus complicating the monitoring of their populations. We studied seasonal vocal activity on the colony surface in two nocturnal seabirds, Fork-tailed (Oceanodroma furcata) and Leach’s (O. leucorhoa) Storm-petrels, and tested the impact of abiotic factors. The work was conducted from 14 June to 23 August 2012, on Medny Island, the Commander Archipelago, where the both species are common and usually share the breeding grounds. We used two automatic SongMeter acoustic sensors that nightly recorded first 5 minutes of each half-hour period from sunset to sunrise on the two colony sites that differed in storm-petrel’s nest density. We also regularly noted the weather conditions: air temperature, wind speed, cloud cover, amount and form of precipitation, sea state etc. Storm-petrels acoustic activity (defined as total flight call number recorded per night) varies greatly from night to night (on the study site with higher density – 10-180 calls for O. furcata and 4-91 calls for O. leucorhoa). However we didn’t find any significant effect of air temperature and cloud cover on the vocal activity in both species. But vocal activity significantly and negatively correlates with wind speed in both species, and with humidity and sea state in O. leucorhoa but not in O. furcata. Interestingly, we found that nightly peaks of vocal activity of two species on Medny is quite divergent (O. furcata – 164±26 min and O. leucorhoa – 233±36 min after sunset), maybe due to competition for noise-free periods of night. We discuss the possible impact of our study for improvement of methods of acoustic monitoring for storm-petrels populations.