Multi-tissue delta H-2 analysis reveals altitudinal migration and tissue-specific discrimination patterns in Cinclodes

Abstract

One of the fastest growing uses of stable isotope analysis in ecology is using hydrogen isotope (delta H-2) values to characterize animal movement and migration strategies. Most studies measure delta H-2 values in metabolically inert tissues such as feathers, which are typically grown during or just after the summer breeding season and provide a limited snapshot of an individual's annual life history. In contrast, isotopic analysis of metabolically active tissues can provide ecological information integrated over weeks to months prior to sampling. Here we characterize delta H-2 patterns among multiple metabolically inert and active tissues in Cinclodes, a genus of South American songbirds noted for variation in altitudinal movement and foraging strategies. We also coupled delta H-2 with carbon (delta C-13) and nitrogen (delta N-15) isotope data to combine information on movement with marine versus terrestrial resource use at the individual level. We find that a combination of physiological and ecological factors control delta H-2 patterns among tissues, which mirrors results of feeding experiments on captive birds. For example, in the coastal resident C. nigrofumosus, metabolically active muscle collected during the winter has higher delta H-2 values than feathers grown the previous summer, a tissue-specific discrimination pattern previously observed in captive birds. This pattern is reversed to various degrees for altitudinal migrants such as C. fuscus and C. oustaleti that spend winters foraging in marine intertidal habitats but migrate to high elevation and forage in stream habitats during the summer. We also find that among altitudinal migrants, individuals that forage sympatrically in intertidal habitats during the winter appeared to summer at a wide range of elevations, as evidenced by large differences of >50 parts per thousand in delta H-2(muscle-feather) offsets. Lastly, a positive correlation between feather delta H-2 and delta N-15 values in Cinclodes that consume a mixed marine-freshwater diet confirms that delta H-2 is a useful proxy for quantifying marine resource use. We anticipate that comparison of delta H-2 values in metabolically active and inert tissues may allow for the reconstruction of animal movement and foraging strategies within the annual life cycle; however, more work is required to better understand the physiological mechanisms responsible for the observed delta H-2 patterns among tissues.