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Changes in blood stable isotopes (δ13C, δ15N), plasma corticosterone and body mass in exercising birds using a wind tunnel

Blood stable isotope compositions in birds reflect the dietary isotopic signature at the time of the cellular blood synthesis. Several studies suggest that stable isotope ratios of some elements such as nitrogen (δ15N) can change in response to individual metabolic and physiological conditions. Using a wind tunnel experiment we tested if endurance flight in birds alters the metabolic state and thereby induces changes in blood δ13C and δ15N values. We trained European starlings (Sturnus vulgaris), that were held under similar diet conditions, to fly in a wind tunnel for up to six hours. While there was a substantial post-flight increase in plasma corticosterone concentrations and a decrease in body weight, we found no significant difference in blood δ13C and δ15N values before and after flight. These findings suggest that blood δ13C and δ15N values from birds caught in the wild most likely reflect the dietary isotopic sources at the time of blood tissue synthesis even after a previous/recent long-distance migration or endurance exercise. Introduction In avian sciences, stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) measurements has been widely used to study diet, trophic structure, and nutritional stress. Recently, some laboratory experiments have been conducted to determine how variation in metabolic activity affects stable isotope composition (e.g. Carleton et al. 2008; Bauchinger & McWilliams 2009; Bauchinger et al. 2010). For instance, using stable-isotope-labeled diets Bauchinger et al. (2010) quantified the rate of carbon turnover in blood cells of zebra finches (Taeniopygia guttata) hold in three groups: cold-exposed birds, exercised birds that were flown for 2 hours per day in a flight sector and birds that were not exercised. The results indicate that increases in metabolism associated with coldexposure or exercise did not produced significant increases in carbon turnover rate in blood. However, the effect of long-distance flight and its associated physiological conditions on stable isotope values in avian tissues have not yet been explored in detail. Therefore, several authors have been calling for more laboratory experiments (e.g. Gannes et al. 1997, Phillips et al. 2005, Kempster et al. 2007). In a previous experiment, applying an isotopically labeled diet-shift in birds trained to fly in a wind tunnel, we tested the assumption that measurements of δ13C turnover are likely to represent minimal estimates since wild migratory birds undergo increased metabolism and exercise during migratory flights (Hobson and Yohannes 2005). We found no difference between experimental and control groups in the rate of δ13C turnover. For both groups, diet was switched from a primarily C-3 content to a C-4 content and blood samples were taken throughout a period of 53 days. Our results supported the contention that blood carbon isotope turnover rates were not different between exercised and non-exercised birds (Hobson and Yohannes 2005). However, in the mentioned previous experiment, we were not able to hold birds at high levels of exercise and we did not analyse Yohannes, E. et al. 2012. BOU Proceedings – The Ecology & Conservation of Migratory Birds http://www.bou.org.uk/bouproc-net/migratory-birds/yohannes-etal.pdf