185 The Auk 119(1):185–200, 2002 PLASMA AMINO ACID CONCENTRATIONS AS AN INDICATOR OF PROTEIN AVAILABILITY TO BREEDING HERRING GULLS (LARUS ARGENTATUS ) CRAIG E. HEBERT, 1,3 J. LAIRD SHUTT, 1 AND RON O. BALL 2 1 Environment Canada, Canadian Wildlife Service, National Wildlife Research Centre, Hull, Quebec K1A 0H3, Canada; and 2 Department of Agricultural, Food, and Nutritional Sciences, University of Alberta, Edmonton, Alberta T6G 2P5, Canada ABSTRACT.—In a captive-feeding study using Herring Gulls (Larus argentatus), plasma amino-acid concentrations increased in response to an increase in dietary protein. Plasma amino-acid concentrations were also measured in wild Herring Gulls captured during in- cubation at eight Laurentian Great Lakes colonies. Those concentrations were used as an indicator of protein availability at those locations. Significant differences in amino acid con- centrations were observed among colonies. Lower amino acid levels, particularly of the es- sential amino acids, were measured in gulls nesting on Lake Superior, whereas values in gulls captured on Lake Ontario and Lake Erie were greater. Those geographic differences in protein availability likely reflected spatial differences in availability of high quality prey (e.g. fish). Geographic differences in prey availability probably affected diet composition. Com- parison of amino-acid levels in wild birds to reference values obtained through the captive feeding study indicated that gulls nesting on Lake Superior may have been protein limited. Colony-wide estimates of adult female body condition, intraclutch variation in egg size, and productivity were correlated with an index of plasma amino-acid concentrations. Received 30 June 2000, accepted 10 October 2001. RESUMEN.—En un estudio en cautiverio de Larus argentatus, las concentraciones plasma ´- ticas de aminoa ´cidos incrementaron en respuesta a un incremento de proteı ´nas en la dieta. Las concentraciones plasma ´ticas de aminoa ´cidos tambie ´n fueron medidas en individuos sil- vestres de L. argentatus capturados durante perı ´odos de incubacio ´n en ocho colonias de los Grandes Lagos. Estas concentraciones fueron usadas como indicador de la disponibilidad de proteı ´nas en estas localidades. Se observaron diferencias significativas en las concentra- ciones de aminoa ´cidos entre colonias. Los niveles de aminoa ´cidos, particularmente de ami- noa ´cidos escenciales, fueron menores en las gaviotas nidificando en el Lago Superior y ma- yores en las gaviotas capturadas en el Lago Ontario y en el Lago Erie. Estas diferencias geogra ´ficas en la disponibilidad de proteinas probablemente reflejaron diferencias espaciales en la disponibilidad de presas de alta calidad (e.g. peces). Las diferencias geogra ´ficas en la disponibilidad de presas probablemente afectaron la composicio ´n de la dieta. Comparacio- nes de los niveles de aminoa ´cidos de aves silvestres con valores de referencia obtenidos du- rante el estudio de alimentacio ´n en cautiverio indicaron que las gaviotas que nidifican en el Lago Superior pueden haber estado limitadas por la disponibilidad de proteı ´nas. Estima- ciones para colonias enteras de la condicio ´ n corporal de las hembras adultas, de la variacio ´n en el taman ˜o de los huevos de una misma nidada y de la productividad estuvieron corre- lacionadas con un ı ´ndice que mide la concentracio ´ n plasma ´tica de aminoa ´cidos. THE HEALTH AND reproductive success of birds can be affected by many factors (Morrison 1986). In the Laurentian Great Lakes, emphasis has been placed on the utility of fish-eating birds as indicators of effects of anthropogenic contam- inants, particularly research involving Herring 3 E-mail: craig.hebert@ec.gc.ca Gulls (Larus argentatus). During the 1970s, re- productive success of Herring Gulls and other species of fish-eating birds in the Great Lakes was poor because of exposure to environmental contaminants such as DDE, polychlorinated di- benzo-p-dioxins, polychlorinated biphenyls, and hexachlorobenzene (Gilbertson 1974, Gilbertson et al. 1991, Hebert et al. 1999a). By the late 1970s, as contaminant levels declined, reproductive Downloaded from https://academic.oup.com/auk/article/119/1/185/5562120 by guest on 09 March 2022