Kentaro Kazama

The relationship between pink salmon biomass and the body condition of short-tailed shearwaters in the Bering Sea: can fish compete with seabirds?

Seabirds and large fishes are important top predators in marine ecosystems, but few studies have explored the potential for competition between these groups. This study investigates the relationship between an observed biennial change of pink salmon (Oncorhynchus gorbuscha) biomass in the central Bering Sea (23 times greater in odd-numbered than in even-numbered years) and the body condition and diet of the short-tailed shearwater (Puffinus tenuirostris) that spends the post-breeding season there. Samples were collected with research gill nets over seven summers. Both species feed on krill, small fishes and squid. Although the mean pink salmon catch per unit effort (in mass) over the study region was not related significantly with shearwaters stomach content mass or prey composition, the pink salmon biomass showed a negative and significant relationship with the shearwaters body mass and liver mass (proxies of energy reserve). We interpret these results as evidence that fishes can negatively affect mean prey intake of seabirds if they feed on a shared prey in the pelagic ecosystem.

Consistent individual variations in aggressiveness and a behavioral syndrome across breeding contexts in different environments in the Black-tailed Gull

Individual behaviors of animals do not evolve separately; they do so in association with other behaviors caused by single shared genetic or physiological constraints and/or favored by selection. Thus, measuring behavioral syndromes—suites of correlated behaviors across different contexts—leads to a better understanding of the adaptive significance of variations in behaviors. However, relatively few studies have examined behavioral syndromes in wild animal populations in changing environments. We investigated a potential behavioral syndrome across antipredator nest defense, territorial defense, chick provisioning, and mating behaviors of male Black-tailed Gulls Larus crassirostris in two successive years under different conspecific territorial intrusion risks and food conditions. Males that presented high levels of antipredator nest defense (aggressive antipredator defenders) against a crow decoy (crows are egg predators) defended their territories against conspecific intruders more frequently than did other males (nonaggressive antipredator defenders), independent of the risk of intrusion. Aggressive antipredator defenders also fed their chicks more frequently than nonaggressive males, but only in a year of low food availability. Taken together, this indicates that males show consistent aggressiveness regardless of breeding context (antipredator and territorial defense), but can regulate food provisioning according to food availability.

Experimental Study of the Effect of Clutch Size on Nest Defense Intensity in Black-Tailed Gulls

Abstract Since parent birds are hypothesized to adjust the level of their nest defense against predators so as to enhance the survival of their offspring, parental nest defense is expected to increase in intensity in relation to increasing clutch size. Empirical studies of bird species, in which clutch sizes have been manipulated artificially, however, have produced results contradicting such expectations, partly, it is thought, because of various errors in experimental design, such as a lack of nesting habitat control, or providing too short a time for parents to assess the value of the manipulated clutch, Hence, further evidence needed to be gathered to clarify whether the basic hypothesis is adequately supported. We manipulated the clutch size of Blacktailed Gulls Larus crassirostris and observed the responses of male parents to a crow decoy in controlled nesting habitats. The intensity of defense was not affected by the clutch size. Opportunities for future reproduction, or constant individual levels of aggressiveness of seem to best explain the observed intensity of nest defense in these long-lived Black-tailed Gulls, rather than the value of the current clutch.

Prey Resources used for Producing Egg Yolks in Four Species of Seabirds: Insight from Stable-Isotope Ratios

Abstract Carbon and nitrogen stable-isotope ratios were measured in the egg yolks of four species of seabirds in Teuri Island and Rishiri Island, Japan. Delta 15N of the yolks of Rhinoceros Auklets Cerorhinca monocerata (11.2– 12.3‰) and Black-tailed Gulls Larus crassirostris (11.3 –12.5‰) were lower than those of Slaty-backed Gulls Larus schistisagus (13.4–14.2‰) and Japanese Cormorants Phalacrocorax capillatus (14.0–16.2‰); suggesting that the former two species foraged mainly on low trophic level small fishes and sometimes on krill, while the latter two species fed on higher trophic level prey and larger fishes.

Testosterone and breeding behavior in male Black-tailed Gulls: an implant experiment

Abstract Testosterone affects male sexual-, aggressive-, and parental-behaviors in bird species. To understand the breadth of the proximate contribution of testosterone to breeding behaviors in male Black-tailed Gulls Larus crassirostris, sexual behaviors, aggressive behaviors against egg-predators and conspecifics, and chick-provisioning behavior of five testosterone-implanted males (T-males) were observed and compared with those of three control males (placebo-implanted; C-males). T-males showed significantly higher levels of courtship and copulation behaviors than C-males. The levels of aggressiveness against egg-predators and against conspecifics, and the rate of feeding of chicks did not differ between T- and C-males. These results suggest that sexual and mating behaviors in male Black-tailed Gulls may be affected by testosterone, while aggressive- and feeding-behaviors are affected by certain ecological factors, such as individual age, or a necessity for high levels of feeding by males, rather than by testosterone.

Within- and among-clutch variation in maternal yolk testosterone level in the Black-tailed Gulls Larus crassirostris

Abstract Mother birds can enhance growth and/or survival of chicks by supplying energetic and hormonal resources to the egg yolk (i.e. maternal effect). In several bird species, mothers supply higher level of testosterone to the eggs laid later within the clutch and enhance the growth of the chick to compensate for the disadvantage of the later hatching. As same as this within-clutch mechanism, mothers breeding later in the season can be expected to supply higher level of testosterone to the clutch. However, among-clutch seasonal variation in yolk testosterone levels has been rarely described. Here, we investigated among-clutch seasonal variation in the yolk testosterone level in Black-tailed Gulls Larus crassirostris, as well as within-clutch variation. Mean yolk testosterone level of eggs laid later within the clutch (“b-eggs”) was significantly higher than that of eggs laid earlier (“a-eggs”). The yolk testosterone levels of both a- and b-eggs of later breeders were significantly higher than those of earlier ones, while this trend was not observed in mean egg mass. Our results indicate that mothers establishing the clutch later in the season would invest more maternal testosterone into the clutch.

Intraspecific Kleptoparasitism, Attacks on Chicks and Chick Adoption in Black-tailed Gulls (Larus crassirostris)

Abstract. To understand the adaptive significance of colonial breeding, evaluating the costs and benefits that colonial breeding provides to individuals is necessary. To study some of these costs and benefits of colonial breeding, over 24,000 nest-hour observations of kleptoparasitism (stealing of food being fed to young), intraspecific attacks on and killing of chicks, and chick adoption were conducted in colonial breeding Black-tailed Gulls (Larus crassirostris) during two breeding seasons. Although kleptoparasitism was rarely observed (under 0.001 events/ nest/observational hour), the occurrence of chick-attacks was 5–10 times higher (0.005–0.01 events/nest/observational hour) than that of kleptoparasitism, with over 10% of the attacks resulting in the death or disappearance of chicks. More than 60% of all attacks on chicks were by failed breeders and non-breeding adult floaters. The survival of attacked chicks reached 98–100% if they escaped into other nests and were accepted by “foster” adults, or if they were defended by non-parental adults. Chick adoption and non-parental defense were more likely to be observed among breeding neighbors (52–60% of chick adoptions and almost all examples of non-parental defense) than among non-neighbors. An increased chick survival rate, resulting from nesting in close to proximity to conspecific neighbors, was considered as a factor promoting colonial breeding among the Laridae.

Annual variation in breeding numbers of two gull species in response to regional stock size and local availability of Japanese Sand Lance on Rishiri Island, northern Japan

Abstract Breeding population abundance such as colony size of seabirds is not generally considered to be particularly sensitive to the annual dynamics of the food conditions because of the long life-span and high adult survival rate. However, in seabird species in which adults decide to breed or not depending on the food conditions, population abundance can respond sensitively to the annual variation in the food conditions. Here, we examine the effects of the regional annual stock abundance of Japanese Sand Lance Ammodytes personatus, and their local temporal availability during the egg-laying period on the size of a Black-tailed Gull Larus crassirostris and Slaty-backed Gull L. schistisagus, breeding colony over 12 years on Rishiri Island, northern Japan. The total number of nests of both gull species increased significantly with the regional annual stock abundance, but not with the local temporal availability of the sand lance. The number of Black-tailed Gull nests without eggs was significantly higher in the year with lower local temporal availability indicating that more Black-tailed Gull parents gave up egg-laying after nest building. Colony size in these species can be a useful indicator reflecting local food conditions.

Effects of Tsunami on Ornithogenic Nitrogen in Soils at a Black-Tailed Gull Colony

Abstract 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, &dgr;15N 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.