Mean measurable corticosterone in House Sparrow (Passer domesticus) primary feathers varies little across life-history stages

The relatively new technique of measuring corticosterone (cort) levels extracted from feathers provides a less invasive, more integrated method of assessing a bird's stress physiology. Current understanding is that cort is deposited into the feather via blood when the feather is replaced during molt. The blood supply in the feather is cut off when the new feather completes growth, thereby ending the influx of cort. Previous studies assumed that cort deposited during feather growth remained constant throughout the feather's attachment to the bird, allowing an accurate retrospective index of circulating cort at the time of feather growth. We tested this assumption by measuring cort levels in feathers collected from different wild House Sparrows (Passer domesticus) across 6 important life history stages spanning a single year, thus representing feathers of different ages but grown during similar environmental conditions. If the common assumption is correct, we predicted that feather cort would not vary across the year. We further simulated substantial feather wear by removing the top 20% length of duplicate samples from the same birds across the same time span. We found significant differences in cort levels across life history stages, but no effect of sex or 20% feather removal. After excluding feather wear and sex and re-running the analysis, the effect of life history stage was no longer significant. Furthermore, despite uncontrolled individual and environmental variation, there was not a sustained decrease in feather cort over time. These data support the assumption that feather cort levels are stable while feathers are on the bird, regardless of feather age or typical wear.