Stable hydrogen isotope variability within and among plumage tracts (δ2HF) of a migratory wood warbler

Abstract

Hydrogen isotope analysis of feather keratin (δ2HF) has become an essential tool for tracking movements between breeding and wintering populations of migratory birds. In particular, δ2HF has been used to create δ2HF isoscapes that can be used to assign the geographic origins of molt. The majority of past studies have sampled a portion of a single feather as an isotopic proxy for the entire plumage although surprisingly little is known about variation of stable isotopes within and between feather tracts of individuals in local populations. Here we examine δ2HF variation in 24 pterylographic variables (9 primaries, 6 secondaries, 6 rectrices, and 3 patches of ventral contour feathers) in individual specimens of black-throated blue warbler (Setophaga caerulescens) breeding in the Big Santeetlah Creek watershed (5350 ha), southern Appalachian Mountains. By restricting our study to territorial ASY males (after second year) inhabiting a small watershed, we could focus on δ2HF variation generated during the complete prebasic annual molt in a circumscribed population while factoring out age and sexual differences in foraging behavior, isotopic incorporation, and post-breeding dispersal. Summed within-individual variation (δ2HF) across 24 pterylographic variables ranged from 12 to 60‰ (= 21.8 ± 9.4‰), with 81% of the individuals exhibiting variation ≥ 16‰ (reproducibility of analyses was ≤ 4 ‰). Adjacent feathers in feather tracts tend to have more similar δ2HF values than feathers grown weeks apart, consistent with the stepwise replacement of flight feathers. The pooled population sample exhibited significant δ2HF variation in primaries (-78 to -21‰), secondaries (-80 to -17‰), rectrices (-78 to -23‰), and ventral contour feathers (-92 to -32‰). Strong year effects in δ2HF variation were observed in each of the 24 pterylographic variables. Altitudinal effects were observed only in ventral contour feathers. The current findings demonstrate that within-individual variation (δ2HF) may be much greater than previously thought in migratory species that molt on or near breeding territories. Our study also highlights the need for greater pterylographic precision in research design of isotope-based studies of avian movement. Within-individual and within-population δ2HF variation should be incorporated in geographic assignment models. In a broader context, the staggered Staffelmauser pattern of molt in wood warblers provides an exceptional view of the seasonal variation of hydrogen isotopes circulating in blood plasma during the six-week period of annual molt.

Fig 1. Schematic of the major flight feathers of black-throated blue warbler.

Primaries (P1-P9) and secondaries (S1-S6) are numbered from the center of the wing. Rectrices (R1-R6) are numbered from the center of the tail outward. Illustration modified from an original drawing by Jacqueline Mask.

Fig 2. Variation of δ²H_F in plumage of black-throated blue warblers in the Big Santeetlah Creek watershed.

Box plots depict minimum values, first quartile, median, third quartile, and maximum values observed among population samples for primaries (P1-P9), secondaries (S1-S6), rectrices (R1-R6), and ventral contour feathers (V1-V3). Data are presented by year (2013 and 2014).

Fig 3. Histograms depicting the range of within-individual variation in δ²H_F in primaries (P1-P9), secondaries (S1-S6), rectrices (R1-R6), and ventral contour feathers (V1-V3) in black-throated blue warblers collected in 2013 and 2014.

Individuals that exhibited a range of δ²H_F values > 8‰ (twice the maximum range of analytical variability observed within runs for laboratory reference materials) within the selected feather tracts are shown in red. Upper values for each bin are shown on the x-axis.

Fig 4. Histogram depicting the range (δ²H_{F maximum} - δ²H_{F minimum}) of within-individual δ²H_F observed across all 24 pterylographic variables in black-throated blue warblers.

Individuals that exhibited a range of δ²H_F variation > 8 ‰ are shown in red. Upper values for each bin are shown on the x-axis.