Environmental conditions on wintering grounds and during migration influence spring nutritional condition and arrival phenology of Neotropical migrants at a northern stopover site

Assessing effects of winter habitat quality and environmental conditions on fitness of migratory birds is important to understand factors that regulate migratory bird populations throughout the annual cycle. We evaluated effects of winter habitat occupancy, as inferred from tissue stable-carbon (δ13C) and -nitrogen (δ15N) isotope values, on nutritional condition and arrival date of seven long-distance Nearctic–Neotropical migratory species sampled at Delta Marsh Bird Observatory (DMBO, Manitoba, Canada) during their spring migration to more northerly breeding areas. Additionally, we used a long-term dataset of DMBO to assess the effect of May minimum temperature, May minimum daily temperature, and El Niño–Southern Oscillation (ENSO) index on body condition and spring arrival date. A positive effect of assumed mesic winter habitat on nutritional condition and arrival phenology was detected primarily in species overwintering in the Caribbean (Palm Warbler Setophaga palmarum, Northern Waterthrush Parkesianovaboracensis). We caution, then, against generalizing the results of previous isotopic studies inferring winter habitat use applied to Neotropical migrants wintering in the Caribbean (e.g., American Redstart Setophaga ruticilla) to those species or populations wintering elsewhere and especially away from dichotomous mangrove versus scrub habitats. In general, birds arrived later and in lower nutritional condition during colder springs, and early migrants did not adjust their timing of migration to spring temperature. Early arrival was associated with poorer nutritional condition, and most species had their lowest nutritional condition during the coldest periods of migration. ENSO, probably through its influence on weather and food availability during winter, carried over to northern latitudes and affected both spring arrival date and nutritional condition.