1. The stable isotopes of nitrogen (δ 15 N) and carbon (δ 13 C) were analysed in 22 species of marine birds from coastal waters of the northeast Pacific Ocean. Analyses confirm that stable nitrogen isotopes can predict seabird trophic positions. 2. Based on δ 15 N analyses, seabird trophic-level inferences generally agree with those of conventional dietary studies, but suggest that lower trophic-level organisms are more important to several seabirds than was recognized previously. 3. Stable-carbon isotope analysis may be a good indicator of inshore vs. offshore feeding preference

https://www.jstor.org/stable/pdfplus/5256.pdf

Using stable isotopes to determine seabird trophic relationships

Behavioural Mechanisms of Food Selection examines animals belonging to diverse trophic groups, from carnivores, herbivores, micro-algal grazers, to filter-feeders and detritus-feeders. In the past Optimal Foraging Theory has been applied to all these groups, but in different ways and in disci plines that rarely overlap. Here concepts and developments hitherto scattered in the literature are drawn together. This uniquely broad synthesis captures the state of the art in the study of diet selection and prescribes new objectives in theoretical development and research.

Behavioural mechanisms of food selection.

Summary
Juvenile birds differ from conspecific adults in their diet and methods of prey capture and prey handling. Juvenile-adult differences in foraging result from (1) immaturity of the beak, skeleto-muscular and neurological systems and (2) the time required to learn foraging skills. These conclusions are largely based on field observations. More experimental studies to assess the relative importance of the various constraints are needed.

Juvenile birds appear to be under strong selection to reach adult form and function as rapidly as possible. Remaining differences between juveniles and adults are largely attributable to constraints. In contrast to many other taxa there are few examples in which juvenile-adult differences in foraging have been accentuated by selection on the juvenile behaviours.

Differences in the foraging of juvenile and adult birds: the importance of developmental constraints

Many aquatic animals face a fundamental problem during foraging and migratory movements: while their resources commonly vary at large spatial scales, they can only sample and assess their environment at relatively small, local spatial scales. Thus, they are unable to choose movement directions by directly sampling distant parts of their environment. A common strategy to overcome this problem is taxis, a behaviour in which an animal performs a biased random walk by changing direction more rapidly when local conditions are getting worse. Such an animal spends more time moving in right directions than wrong ones, and eventually gets to a favourable area. Taxis is inefficient, however, when environmental gradients are weak or overlain by ‘noisy’ small-scale fluctuations. In this paper, I show that schooling behaviour can improve the ability of animals performing taxis to climb gradients, even under conditions when asocial taxis would be ineffective. Schooling is a social behaviour incorporating tendencies to remain close to and align with fellow members of a group. It enhances taxis because the alignment tendency produces tight angular distributions within groups, and dampens the stochastic effects of individual sampling errors. As a result, more school members orient up-gradient than in the comparable asocial case. However, overly strong schooling behaviour makes the school slow in responding to changing gradient directions. This trade-off suggests an optimal level of schooling behaviour for given spatio-temporal scales of environmental variations. Social taxis may enhance the selective value of schooling in pelagic grazers such as herrings, anchovies and Antarctic krill. Furthermore, the degree of aggregation in a population of schooling animals may affect directly the rate and direction of migration and foraging movements.

Schooling as a strategy for taxis in a noisy environment

We investigated trophic ecology variation among colonies as well as sex- and age-related differences in the diet of the southern giant petrel Macronectes giganteus, a long-lived seabird that is sexually dimorphic in size. We measured stable isotopes (δ 13 C, δ 15 N) in blood samples collected during breeding at Bird Island (South Georgia, Antarctica) in 1998 and at 2 colonies in the Argentinean area of Patagonia in 2000 and 2001. Individuals from South Georgia showed lower δ 13 C and δ 15 N values than those in Patagonia, as expected from the more pelagic location and the short length of the Antarctic food web. Males and females showed significant differences in the isotopic signatures at both localities. These differences agree with the sexual differences in diet found in previous studies, which showed that both sexes rely mainly on penguin and seal carrion, but females also feed extensively on marine prey, such as fish, squid and crustaceans. However, males from Patagonia showed significantly higher δ 15 N and δ 13 C values than females did, and the reverse trend was observed at South Georgia. This opposite trend is probably related to the different trophic level of carrion between locations: whereas penguins and pinnipeds in Patagonia rely mainly on fish and cephalopods, in South Georgia they rely mainly on krill. Stable isotope values of male and female chicks in Patagonia did not differ; both attained high values, similar to adult males and higher than adult females, suggesting that parents do not provision their single offspring differently in relation to sex; however, they seem to provide offspring with a higher proportion of carrion, probably of higher quality, and more abundant food, than they consume themselves. Stable isotopes at South Georgia were not affected by age of adults. We have provided new information on intraspecific segregation in the diet in a seabird species and have also underlined the importance of considering food web structure when studying intraspecific variability in trophic ecology.

/pdf/stable-isotopes-reveal-trophic-segregation-by-sex-and-age-in-6m307vzag0.pdf

Stable isotopes reveal trophic segregation by sex and age in the southern giant petrel in two different food webs

Multispecies feeding flocks of seabirds were studied in Barkley Sound, British Columbia. The flocks occurred during the post-breeding dispersal of migrant adult and juvenile seabirds. A consideration of comparative foraging success indicated that juvenile seabirds were less skilled at locating and capturing prey. Juveniles did not initiate flocks, were less adept at finding food, and had lower feeding success rates. The juveniles appeared to compensate for these inadequacies by using a different foraging strategy than adults. They increased the amount of time spent feeding and appeared to be more attracted to the presence of other feeding birds. Juveniles were less able to recognize good potential feeding areas and accessible food items. Juveniles had a greater tendency to loaf on the centrally located terrestrial loafing sites, thus increasing their chances of sighting feeding flocks. Our results demonstrated how improved post-fledging survival was achieved for juvenile seabirds feeding in the flocks. Our findings support the interpretations put forth by others to explain delayed maturity, reduced clutches in younger birds and prolonged parental care, all characteristic of seabirds.

Julie M. Porter

Papers

Dynamics of Seabird Multispecies Feeding Flocks: Age-Related Feeding Behaviour

Dynamics of Seabird Multispecies Feeding Flocks: Chronology of Flocking in Barkley Sound, British Columbia, in 1979