Severe en route tropical weather is a predictor of morphological variation and body condition of a Nearctic-Neotropical migratory songbird.

Tropical storms and hurricanes pose a significant impediment to southbound Nearctic-Neotropical migratory songbirds. The effects of severe weather on transient songbirds may be significant but have been difficult to assess. We used two meteorological indices of inclement weather, Accumulated Cyclone Energy (ACE) values from the western Atlantic Basin and the Southern Oscillation Index (SOI), to investigate whether the effects of inclement weather during southward migration are manifest in the population morphometrics of a migratory songbird at a North American breeding site in Delaware and Pennsylvania, USA. Over a 23-year period (2000-2023), both ACE and SOI were predictors of wing chord and body condition of returning Veeries (Catharus fuscescens). Elevated September ACE (more severe en route weather in September) was a predictor of shorter wing chord and lower body condition indices in returning adult females. The effects on SY birds differed. Years with higher September ACE were a predictor of higher body condition indices in returning SY females and elevated September and October ACE values were a predictor of longer wing chord in returning SY birds. Positive SOI values (La Niña type years) were a predictor of shorter wing chord and higher body condition indices in adult Veeries. In general, en route severe weather was a stronger predictor for females than males. Sex- and age-specific differences are likely due to differing spatiotemporal migratory patterns. Our results show that inclement tropical weather is a source of morphological variation in a Nearctic-breeding migratory songbird and are consistent with the premise that severe tropical weather constrains Nearctic-breeding migratory songbird populations. These data contribute to the mounting evidence that the projected increase in the severity of en route tropical storms and hurricanes due to climate change is a significant emerging threat to Nearctic-Neotropical migratory songbirds.