| Summary: | Aquatic environments in the temperate, boreal and arctic climate zones are highly seasonal in terms of biological productivity. Many poikilothermic aquatic organisms survive the winter, when food availability is low, by reducing their activity levels and/or performing vertical migrations to deep cold waters where energy consumption is low. This causes potential problems for homoeothermic predators, which require constant access to nutritious prey to survive or large body lipid reserves. Diving birds are particularly vulnerable to periods of low prey availability and many species therefore resolve this by migrating to warmer waters, where locating food is energetically less challenging. Migration by flight, however, is extremely energetically costly for birds with wings specialized for underwater swimming. Little is known of how they manage to obtain sufficient energy during migration. In this study we used time-depth recorders (recording pressure, temperature, wet/dry and light every 10th sec. for ~365 days) to investigate the activity budget and locations of thick-billed murres during the non-breeding season. Fall migration by male parent and chick was conducted solely by swimming and covered distances of ~3000 km. Females and non-breeding males conducted the first part of their fall migration by flight (~800 km) and the rest of the migration (~2000 km) by swimming. Movement by flight is the most costly form of locomotion for thick-billed murres and we found thick-billed murres to spend remarkably little time flying (between 0-5.3 % on a 24h cycle) during the period between the fall (Aug/Oct) and spring migration (Apr/May). The longest period of continuous flight during the non-migration months was 1.6 hours. The murres dived the deepest during the months of Dec- Feb, with a maximum recorded dive depth of 191 m. Diving effort increased toward the end of the non-breeding period, most likely in preparation for the energy expensive spring migration conducted by flight.
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