Fault bars and stable isotope signatures reveal effects of severe weather stressors on fledgling sparrows

In August 2013, we observed a high incidence (44%) of synchronous fault bars across the rectrices of juvenile Grasshopper Sparrows (Ammodrammus savannarum) captured near El Reno, Oklahoma. Earlier that year, on May 31, the site was struck by a severe storm which rained hailstones exceeding 5.5 cm diameter and spawned an historic 4.2 km-wide tornado <8 km to the south of the site. An assessment of Grasshopper Sparrow nesting phenology indicated that a large number of nestlings were likely growing tail feathers when the storm hit. To assess the nature of the fault bars, we measured δ¹⁵N and δ¹³C stable isotope ratios within four 0.25-0.40 mg feather sections taken from the distal end of a tail feather from 18 juveniles captured at the site in August. The fault bar, if present, was contained within only one section. Fault bars were located at various positions on the distal half of feathers, and fault bar width significantly increased as a function of distance from the tip (i.e., age at formation). After correcting for consistent, natural δ¹⁵N variation across sections, we found that feather sections containing or located immediately proximal to fault bars showed significantly higher δ¹⁵N than sections from the same feathers located distal to or further from the fault bar region. We also observed significantly higher δ¹³C but lower δ¹⁵N in the feathers of juveniles with fault bars compared to normal appearing juveniles. Our findings support the hypothesis that an abrupt environmental stressor affected Grasshopper Sparrow chicks of various ages, and caused short term catabolism of muscle tissue that resulted in a spike in δ¹⁵N deposited into growing feathers. Furthermore, higher δ¹⁵N and lower δ¹³C among juveniles lacking fault bars suggested a seasonal change in diet consistent with these individuals hatching after the May 31 storm. Severe weather events may represent major stressors to ground-nesting birds, and we recommend exploiting opportunities to study the effects of severe weather as part of ongoing research efforts.