Stable hydrogen isotope measurements of songbird feathers: effects of intra-feather variability and sample processing

Abstract

Deuterium composition of feathers has been widely applied to establish provenance of birds. Surprisingly, little attention has been paid to the potential sources of unwanted hydrogen isotopic variability that may affect the comparability and replicability of deuterium measurements. Feathers may exhibit hydrogen isotopic differences between distal and proximal sections, as well as between rachis and vane. For this reason, it is recommended to always analyse ground vane from the same section of the feather. However, this protocol may be seriously limiting when working with songbird feathers, which are usually small and light. In this paper, intrafeather hydrogen isotopic variability was studied in four species of songbirds by comparing four sections of the same feather. No difference in hydrogen isotopic composition was found between distal and proximal sections of the vane. This was expected as songbird feathers grow rapidly and individuals avoid geographical movements during moult, favouring a homogeneous source of hydrogen during keratin synthesis. Conversely, as previously demonstrated for other avian species, rachis showed systematically c. 11 ‰  2 H more negative values than vane. This is probably a result of their biochemical and ultrastructural differences, which seem strongly conserved across taxa. Such differences imply that we need to describe which feather part was used in the methods of our reports to ensure full comparability and replicability among studies. Nevertheless, vane and rachis  2 H were strongly correlated, demonstrating that rachis can be also used in isotope analyses because its  2 H can be accurately rescaled. This is useful since rachis represents an important fraction of feather mass in songbirds. Values of  2 H from ground and non-ground samples from the same feather were also compared. There was no effect of the processing protocol, suggesting that grinding is unnecessary when working with passerine feathers.

Stable isotope (C, N, O, and H) study of a comprehensive set of feathers from two Setophaga citrina

“…We believe our bulk analysis of the upper segment of feathers is appropriate for the purposes of this study. Gordo (2020) [ 18 ] showed no isotopic gradient between the bottom and tip for passerine feathers, which allows for sampling to occur from anywhere along the feather. Bontempo et al (2014) [ 17 ] found homogeneous isotopic values for the rachis and vane when the calamus was excluded from analysis.…”

“…However, isotopic fractionation between the vane and the rachis has been observed. Gordo (2020) [ 18 ] showed that the rachis and vane were slightly different in δD values, but their isotopic compositions were highly correlated. In our study, we bulk sampled the feathers, resulting in relatively constant proportions of rachis to vane.…”

“…In our study, we bulk sampled the feathers, resulting in relatively constant proportions of rachis to vane. This systematic sampling approach, combined with the fact that the rachis dominates the total feather mass, ensures that any isotopic variability between feathers in our study is not due to artifacts of biased sampling of vane or rachis material [ 13 , 14 , 18 , 19 ].…”