In some avian species, male plasma testosterone levels are maximum during the courtship or copulation periods, but decline abruptly once incubation starts. However, testosterone levels of male Black-tailed Gulls Larus crassirostris during the course of breeding remain unknown. In 2006, blood samples of 23 incubating males were collected at Rishiri Island, Hokkaido, Japan and testosterone was measured by enzyme immunoassay kit. Plasma testosterone level (mean±S.E.: 0.737±0.282 ng/ml) was assumed to decline with elapsed days from the onset of incubation, as shown in other species.
Large amounts of marine-derived nitrogen are carried ashore by seabirds in their feces (ornithogenic N). Various topological, meteorological, and environmental factors can affect microbial transformation of ornithogenic N altering the physical and biochemical conditions of the soils. Tsunami may be a factor affecting ornithgenic soil N dynamics leading to changes in soil conditions. We examined the effects of the tsunami triggered by the 11 March 2011 earthquake off the East Coast of Honshu, Japan, on ornithogenic soil N in the Black-tailed Gull Larus crassirostris breeding colony on Kabushima (Kabu Island) four months after the disaster. There were no differences in the total N content in soils between tsunami splashed (upper half of the island) and flooded areas (lower half), and the visual topography was comparable to that reported before the tsunami. It seems that the tsunami may not have extensively removed accumulated ornithogenic N in the soils, nor did it induce deposition of organic matter derived from marine sediments. However, δ15N values of soil N were lower in the area flooded by the tsunami than in the splashed area, suggesting the possibility that the tsunami caused leaching of ornithogenic N from the soil. Soils flooded by the tsunami contained higher levels of marine salts than those from the splashed area. N isotopic differences between NH4- and NO3-N were smaller in the flooded soils than in the splashed area. This indicates that nitrification of ornithogenic soil N is suppressed in the flooded area because of the remarkable increase in the salt concentration of the soil.
The physiological state of parent birds combined with the value of their clutch may affect the intensity of their nest defense. In colonially breeding birds, nest-defense intensity may also be affected by the behavior of neighbors. We investigated individual variation in the nest-defense intensity among colonial Black-tailed Gulls ( Larus crassirostris Vieillot, 1818) over 2 years. Only 30%–40% of males attacked a decoy of an egg predator (Large-billed Crow ( Corvus macrorhynchos Wagler, 1827)), and the other males and females rarely attacked. Males attacking the decoy had higher levels of plasma testosterone than males that did not attack. Each male’s, but not female’s, nest-defense intensity was consistent throughout the incubation period and also across years. The intensity was not related to egg-laying date, clutch size, or age of offspring. The intensity was likely to be higher when individuals had one or more neighbors, representing higher nest-defense intensity in the year where gulls had larger number of adjacent neighboring nests (5.23 nests), but this trend was not observed in the year where they had smaller number of the neighboring nests (3.73 nests). Thus, in addition to testosterone levels, behavior of neighbors also influences the nest-defense intensity.
