Seasonal patterns in δ² H values of multiple tissues from Andean birds provide insights into elevational migration

Elevational migration is a widespread phenomenon in tropical avifauna but it is difficult to identify using traditional approaches. Hydrogen isotope (δ² H) values of precipitation decrease with elevation so δ² H analysis of multiple bird tissues with different isotopic incorporation rates may be a reliable method for characterizing seasonal elevational migration. Here we compare δ² H values in metabolically inert (feathers and claws) and metabolically active (whole blood) tissues to examine whether an upslope migration occurs prior to the breeding season in the Yungas Manakin (Chiroxiphia boliviana). We compare results from C. boliviana with data from a known elevational migrant, the Streak-necked Flycatcher (Mionectes striaticollis). Opposite to our expectations, tissue δ² H values increased over time, largely reflecting seasonal patterns in precipitation δ² H rather than elevational effects; linear mixed-effects models with strongest support included ordinal date, tissue type, and elevation. This seasonal increase in precipitation δ² H is a general phenomenon in both tropical and temperate mountain ranges. We use these data to propose a hypothetical framework that predicts different patterns in tissue δ² H values collected in different seasons from residents and elevational migrants. This framework can serve as a reference for future studies that assess elevational migration in birds and other animals.