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Food-supplementation studies on birds during egg laying have generally manipulated macronutrients and energy. Such studies have shown variable effects on egg size in American Coots (Fulica americana). We determined the relative influences of local environment, food quantity, and food quality on egg size in American Coots by supplementing food and carotenoids at three sites in Saskatchewan. Eggs were collected and analyzed for carotenoid content and stable isotopes (6~5N and 613C) in the yolk to determine whether variation in the type of food eaten contributes to egg size. We also report on the isotopic analysis of American Coot tissues and their eggs to assess evidence of endogenous versus exogenous protein and lipid allocations to reproduction. We provide isotopic evidence that American Coots used endogenous lipid, but not protein, reserves for egg formation and that egg size is more dependent on exogenous sources of nutrients. Laying sequence was the only variable across nesting locales that hada significant influence on the size of eggs. There was an effect of provisioning food at only one site; however, this response was driven by a component of diet quality other than carotenoids. Yolk 613C values were positively correlated with egg volume, which further suggests the importance of diet quality in determining egg size in American Coots. Received 6 October 2008, accepted 28 Apri12009. Key words: American Coot, carotenoids, egg size, Fulica americana, laying sequence, stable isotopes, supplemental feeding. Los estudios que han brindado suplementos de alimento a las aves durante la postura de huevos generalmente han manipulado los macronutrientes y la energia. Dichos estudios han mostrado efectos variables sobre el tamano de los huevos en Fulica americana. Determinamos las influencias relativas del ambiente local, la cantidad de alimento y la calidad de alimento sobre el tamano de los huevos en esta especie mediante la provision de alimento suplementario y carotenoides en tres lugares en Saskatchewan. Los huevos fueron recolectados y se analizo su contenido de carotenoides y de isotopos estables (615N y 613C) en el vitelo para determinar si la variacion en el tipo de alimento consumido contribuye al tamano de los huevos. Tambien informamos acerca de un analisis isotopico de tejidos y huevos de E americana con el objetivo de examinar la evidencia de asignacion endogena vs. exogena de proteinas y lipidos a la reproduccion. Presentamos evidencia isotopica de que las aves usan reservas endogenas de lipidos, pero no de proteinas, para la formacion de los huevos. Ademas, el tamano de los huevos depende mas de las fuentes exogenas de nutrientes. La posicion en la secuencia de postura fue la unica variable que tuvo un efecto significativo en el tamano de los huevos a traves de los sitios de anidacion. Existio un efecto del aprovisionamiento de alimento solo en uno de los sitios. Sin embargo, esta respuesta fue causada por un componente de la calidad de la dieta distinto a los carotenoides. Los valores de [[sigma].sup.13]C del vitelo se correlacionaron positivamente con el volumen de los huevos, una sugerencia mas de la importancia de la calidad de la dieta para determinar el tamano de los huevos en E americana. DOI: 10.1525/auk.2009.08197

Stable-isotope analysis of feathers has become a useful tool for examining migration patterns and annual connectivity of migratory songbird populations. We used this approach, combined with molecular sex-identification, to examine expected breeding destinations and migration ecology of Swainson's Thrush (Catharus ustulatus) at a spring stopover site on the Osa Peninsula of Costa Rica. The number of individuals passing through the site increased in late March and peaked in mid-April before declining toward late April. Plumage and expected feather-deuterium ([delta][D.sub.f]) values for North America indicated that most individuals had probably bred or fledged in west-central regions of the boreal forest and western mountains, rather than farther north in the Yukon and Alaska of in eastern regions of Canada and the United States. Average [delta][D.sub.f] values differed little between males and females, indicating similar breeding destinations, though females tended to show greater variability. The [delta][D.sub.f] values of second-year and after-second-year birds differed only slightly, which suggests that most adults molted near the breeding grounds. Timing of migration through the site was not related to breeding latitude as inferred from feather [delta]D. We also found little relation between the energetic reserves carried by individuals and their expected breeding destination, possibly because the reserves carried at this stage have little relation to the total reserves needed by northern breeders for the final stage of migration. Received 28 Apri12006, accepted 14 March 2007. Key words: breeding destinations, Catharus ustulatus, Costa Rica, deuterium, migration ecology, stable isotopes, Swainson's Thrush. Los analisis de isotopos estables de las plumas se han convertido en una herramienta util para examinar los patrones de migracion y la conectividad anual de las poblaciones de aves canoras migrantes. Utilizamos este enfoque, combinado con la identificacion molecular del sexo, para examinar los destinos de cria esperados y la ecologia de migracion de Catharus ustulatus en un sitio de escala de primavera en la Peninsula de Osa, Costa Rica. El numero de individuos que pasan a traves del sitio incrementa a fines de marzo y tiene un pico a mediados de abril, antes de disminuir hacia fines de abril. El plumaje y los valores esperados de deuterio en las plumas ([delta][D.sub.p]) para America del Norte indicaron que la mayoria de los individuos habian probablemente criado o emplumado en las regiones del centro-oeste del bosque boreal y de las montanas del oeste, en lugar de mas al norte en Yukon y Alaska, o en las regiones del este de Canada y los Estados Unidos. Los valores promedio de [delta][D.sub.p] difirieron poco entre machos y hembras, indicando los mismos destinos de cria, aunque las hembras tendieron a mostrar mayor variabilidad. Los valores de [delta][D.sub.p] de las aves del segundo ano y posteriores difirieron muy poco, lo que sugiere que la mayoria de los adultos mudaron cerca de los sitios de cria. La fecha de migracion a traves del sitio no estuvo relacionada con la latitud de cria inferida a partir del [delta]D de las plumas. Tambien encontramos poca relacion entre las reservas energeticas de los individuos y sus destinos esperados de cria, posiblemente porque las reservas que portaban en este periodo tienen poca relacion con las reservas totales necesarias de los migrantes para la fase final de la migracion.

Most tropical passerines feed on insects, fruit, or a combination of the two. The sugary pulps of fruit have lower amounts of protein than insects. We used stable-nitrogen isotope analysis ([delta][sup.15]N) of blood from two tropical rainforest birds that regularly feed on fruit--Red-throated Ant-Tanager (Habia fuscicauda) and Ochre-bellied Flycatcher (Mionectes oleagineus)--to quantify the relative amounts of assimilated protein from animal and plant sources. Because fruit and insect abundances vary seasonally in the tropics, the study was conducted during one year in Los Tuxtlas, Mexico. The study site has one major fruiting peak between April and July and a secondary peak between September and October. Some insects are more abundant from May to August. Red-throated Ant-Tanagers and Ochre-bellied Flycatchers rely heavily on insect protein when fruit is scarce, and then steadily increase their input of fruit protein as fruit abundance increases. Red-throated Ant-Tanagers rely almost entirely on fruit protein during the major fruiting peak, whereas Ochre-bellied Flycatchers have the largest input of fruit protein during the secondary fruit peak. Incubation in both species occurs from June to August, and most incubating individuals rely on a mixture of insects and fruit. In both species, examination of fecal contents showed the ingestion of the largest number of fruit species during the major fruiting peak. Key words: Habia fuscicauda, Mionectes oleagineus, nitrogen-15, nutritional ecology, Ochre-bellied Flycatcher, protein requirements, rainforest, Red-throated Ant-Tanager, stable-isotope analysis, tropical birds.

Stable-isotope analyses of egg components can illuminate the contributions of endogenous and exogenous nutrients to egg formation in cases where birds move from one isotopic biome to another just before egg-laying, as in Red Knots (Calidris canutus islandica) and Ruddy Turnstones (Arenaria interpres interpres) nesting in the north eastern Canadian High Arctic. Those populations use inshore marine habitats during the winter and northward migration periods, but switch to feeding and breeding in terrestrial C-3 habitats in the Arctic. Upon the birds' arrival at Alert, Nunavut, Canada, stable carbon [[delta].sup.13] and nitrogen ([[delta].sup.15]) isotope values for their red blood ceils were consistent with those expected from an inshore marine diet. After the birds switched to a terrestrial diet, those values decayed in a negative exponential fashion until they reached asymptotic values consistent with a local terrestrial C-3 diet. Components of eggs laid later in the season, including lipid-free yolk, albumen, yolk lipid, and albumen lipid, generally showed isotope values consistent with routing from exogenous (i.e. local) nutrient sources, which supports the notion that High Arctic shorebirds are primarily income and not capital breeders. However, eggs in earliest clutches were enriched in [sup.13]C and [sup.15]N, which suggests that some residual marine nutrients were available for reproduction. The extent to which endogenous stores are transferred to eggs in High Arctic shorebirds may depend on year and climatic conditions.

Avian studies are often interpreted using dual (e.g. [sup.13]C, [sup.15]N) isotope models, assuming turnover of both isotopes occur at similar rates, but only a few studies have quantified turnover rates for more than one of those isotopes simultaneously. To test the generality of previous turnover and fractionation estimates and assumption of synchronous C and N patterns of turnover rates, we captured Dunlin (Calidris alpina pacifica) wintering in the Fraser River Delta, British Columbia, and derived isotopic turnover rates and diet-tissue fractionation factors by experimentally manipulating diet. Birds (n = 15) were initially fed a terrestrially derived diet (mean [delta][sup.13]C: -24.7 [per thousand], mean [delta][sup.15]N: 3.5 [per thousand]) for 54 days. A treatment group (n = 11) was then switched to a marine-derived diet (mean [delta][sup.13]C: -18.3 [per thousand], mean [delta][sup.15]N: 13.7 [per thousand]); a control group (n = 4) was maintained on the terrestrial diet for a further 59 days. An exponential model described patterns of isotopic turnover for [sup.13]C and [sup.15]N, and turnover rates and half-lives of the two isotopes were correlated, confirming the assumption of synchronous patterns of turnover for those isotopes. The half-lives for [sup.13]C and [sup.15]N in Dunlin whole blood were 11.2 [+ or -] 0.8 days and 10.0 [+ or -] 0.6 days, respectively, and are among the lowest values obtained to date for wild birds. Variation in turnover rate among individuals was not related to indices of body condition. Received 5 March 2003, accepted 5 October 2003.

The measurement of the turnover of naturally occurring stable isotopes such as carbon-13 in the blood of birds may be a used as a sampling method in avian dietary research. Blood plasma and cell fractions may both be used for such purposes. The technique is ideal when non-destructive diet sampling is desired. Other tissues may be sampled if short-term and long-term dietary information is needed. The use of stable-isotope determinations in the blood of crows to obtain information on the crows' diet is described.