Relationship between non-breeding migratory movements and stable isotopes of nitrogen and carbon from primary feathers in black-tailed gull Larus crassirostris

Various seabird foraging strategies during the non-breeding season have recently been revealed by combining the use of bio-logging devices and the study of stable isotopic signatures (δ15N and δ13C) from various tissues. In this study, we used these combined methods to determine the relationships between stable isotopic signatures in Black-tailed Gulls Larus crassirostris primary feathers and the areas in which the feathers are presumed to have been grown. The fifth primary (P5) feathers are replaced during late August, and although the migratory movements for seven of the eight gulls studied during this replacement period differed, the isotopic δ15N and δ13C values were similar. These values indicated that the seven gulls fed on a wide range of prey from krill to demersal fish species. The isotopic values from P5 for the individual gull that moved southward after breeding, were much higher than for the seven other birds. In contrast, all eight gulls showed a relatively narrow distribution during the replacement of their outermost primaries (P10), which were replaced during mid October and November. However, the isotopic values from P10 of the individual that moved southward during replacement of P5 were also much higher. The unique isotopic values of this gull might indicate specialization in anthropogenic food resources or high trophic level resources through the migration period, regardless of location. Contrary to previous studies, our research did not detect links between migratory movements and stable isotopic signatures from feathers in Larus gulls migrating through a relatively narrow range and having considerable individual variation in diet.

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