BLOOD ISOTOPIC ([delta]^sup 13^C AND [delta]^sup 15^N) TURNOVER AND DIET-TISSUE FRACTIONATION FACTORS IN CAPTIVE DUNLIN (CALIDRIS ALPINA PACIFICA)

Auk, The, Jan 2004 by Ogden, Lesley J Evans, Hobson, Keith A, Lank, David B

ABSTRACT.-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 pacifiai) 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 � 0.8 days and 10.0 � 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.

RESUME.-Les etudes portant sur la faune avienne sont souvent interpretees en utilisant des modeles isotopiques doubles (e.g. ^sup 13^C, ^sup 15^N), assumant que le renouvellement des deux isotopes se fait a des taux similaires. Seulement quelques etudes ont quantifie les taux de renouvellement pour plus d'un de ces isotopes simultanement. Pour etudier la generalisation de ce renouvellement, le fractionnement des estimes et l'hypothese de patrons synchrones des taux de renouvellement en C et N, nous avons capture des Calidris alpina pacifica hivernant dans le delta de la riviere Fraser, Colombie Britannique. Nous avons egalement derive les taux de renouvellement isotopiques ainsi que le fractionnement des facteurs diete-tissu en manipulant experimentalement le regime alimentaire. Les oiseaux (n = 15) ont ete tout d'abord nourris avec un regime derive de type terrestre (moyenne [delta]^sup 13^C: -24.7[per thousand], moyenne [delta]^sup 15^N: 3.5[per thousand]) pendant 54 jours. Un groupe traitement (n = 11) a ete ensuite soumis a un regime derive de type marin (moyenne [delta]^sup 13^C:-18.3[per thousand], moyenne [delta]^sup 15^N: 13.7[per thousand]). Un groupe controle (n = 4) a ete maintenu au regime de type terrestre durant 59 jours supplementaires. Un modele exponentiel decrivant les patrons de renouvellement isotopique en ^sup 13^C et ^sup 15^N, a ete correle aux taux de renouvellement et avix demivies des deux isotopes, confirmant l'hypothese de patrons synchrones de renouvellement des ces isotopes. Les demi-vies du ^sup 13^C et du ^sup 15^N dans le sang chez Calidris alpina pacifica etaient de 11.2 � 0.8 jours et 10.0 � 0.6 jours, respectivement, et sont parmi les valeurs les plus basses obtenues jusqu'a aujourd'hui pour des oiseaux sauvages. Les variations inter-individuelles du taux de renouvellement n'etaient pas reliees aux indices de conditions corporelles.

THE USE OF stable-carbon and nitrogen isotope analysis is expanding as a tool for elucidating the relative proportion of diet gleaned from two isotopically distinct (e.g. marine vs. terrestrial) sources (e.g. Hobson 1986, 1990; Mizutani et al. 1990; Hobson and Sealy 1991; Hobson et al. 200Ob). The stable-isotope approach has several advantages over traditional techniques by: (1) providing information for all individuals sampled, including those with empty stomachs; (2) avoiding bias resulting from differential digestion of soft- versus hard-bodied prey items; (3) providing information on foods assimilated into body tissues, not just ingested; and (4) integrating information over a relatively long window of time, rather than just a time "snapshot," as is the case for stomach or fecal analysis (Cree et al. 1999). One important disadvantage of stable-isotope analysis is that different tissues may reflect the isotopic composition of different dietary constituents, and stable isotopes underestimate food used directly for energy, such as carbohydrates (Cannes et al. 1998, Hobson et al. 2000). Also, stable-isotope analysis usually does not provide taxonomic information on prey ingested and thus provides a complementary rather than alternative approach to gut contents analysis (e.g. Hobson et al. 1994).

The period for which tissue isotopic values reflect diet depends upon the isotopic turnover rate in that tissue. Tissues with high turnover rates, such as liver and plasma, reflect recent diet; whereas tissues with slower turnover rates, such as blood cells and muscle, reflect diet over longer terms (Hobson and Clark 1993). Although stable-isotope analysis has been used widely to interpret the diets of free-living bird populations, few controlled studies have been performed to determine isotopic turnover rates and determine how isotopes fractionate or change once they are incorporated into avian tissues (Cannes et al. 1997, Hobson 1999). Avian field studies using stable-isotope analysis have thus far assumed that the results of controlled laboratory studies on captive Japanese Quail (Coturnix japonica), Ring-billed Gulls (Larus delawarensis), and American Crows (Corvus brachyrhynchos) (Hobson and Clark 1992a, b; Hobson and Clark 1993) are representative of a wide taxonomic range of bird species and interpreted their results on the basis of turnover rates and diet-tissue fractionation factors derived from those species. Most recently, Haramis et al. (2001) and Bearhop et al. (2002) provided information on both stable-carbon and nitrogen isotope turnover in whole blood of wintering Canvasback (Ay thy a valisineria) and captive Great Skuas (Stercorarius skua), respectively. Those studies were the first to report isotopic turnover rates for both isotopes simultaneously and suggested such patterns were similar. Haramis et al. (2001) reported different turnover rates depending on the nature of the diet switch in captive birds. Although not the focus of those studies, such results imply an interesting physiological complexity that needs to be considered when applying isotopic models to wild birds (Bearhop et al. 2002).