Stable-hydrogen isotope turnover in red blood cells of two migratory thrushes: application to studies of connectivity and carry-over effects

Abstract

ABSTRACT Understanding turnover rates of stable isotopes in metabolically active tissues is critical for making spatial connections for migratory birds because samples provide information about pre-migratory location only until the tissue turns over to reflect local values. We calculated stable-hydrogen isotope (δ2H) turnover rate in the red blood cells of two long-distance migratory songbirds, Bicknell's Thrushes (Catharus bicknelli) and Swainson's Thrushes (Catharus ustulatus), using samples collected at a breeding site in New Brunswick, Canada. Blood from both species captured early in the breeding site was more positive in δ2H than blood sampled later in the summer, but did not match blood values for wintering Bicknell's Thrushes. An asymptotic exponential model was used to estimate turnover of red blood cell δ2H and yielded a half-life estimate of 21 days and 14 days for Bicknell's and Swainson's thrushes, respectively. Red blood cells of both species approached the local breeding site value one month after the first individuals were detected at the site. For Bicknell's Thrushes, estimated δ2H in blood at arrival (−72‰) was closer to blood collected at wintering sites (mean −61‰) than to expected breeding site δ2H (−120‰). Discrimination values calculated for red blood cells collected at the breeding site for both species were greater than expected based on studies using keratin. Turnover during migration currently limits the use of blood sampled early in the breeding season for connectivity/carry-over effect studies. However, direct tracking technology such as geolocators can provide information about migration duration, timing, and stopovers that can be used to improve isotopic turnover equations for metabolically active tissues.