ABSTRACT
Abstract. Small-bodied songbirds replenish fat reserves during migration at stopover sites where they continually encounter novel and often unpredictable environmental conditions. The ability to select and utilize high-quality habitats is critical to survival and fitness. Vegetation phenology is closely linked with emergence of insect prey and may provide valid cues of food availability for stopover habitat selection. Climate change is disrupting phenological synchrony across trophic levels with negative impacts on bird populations. However, whether synchrony or mismatch indicates historic or disrupted systems remains unclear. Many Neotropical migratory songbirds of western North America must cross arid regions where drought conditions related to climate change and human water use are expected to increase. We studied migrant abundance and the diversity (niche breadth) and proportional use of vegetation species as foraging substrates and their synchrony with vegetation flowering during spring migration along the lower Colorado River in the Sonoran Desert of the United States and Mexico. Peak migrant abundance in late Marchearly April coincided with the period of narrowest
niche breadth. Abundance of migrants increased with willow and mesquite flowering, but significantly only in 2003, a severe drought year. Annual niche breadth was negatively correlated with flowering and with total monsoon precipitation. Birds proportionally foraged most in willow and mesquite, shifting from one species to the next, temporally synchronous with their flowering. The period of greatest overlap in willow and mesquite flowering and use also coincided with peak migrant abundance and narrowest niche breadth. Our results show that migration timing and foraging habitat selection is highly synchronized with flowering of large woody perennials, particularly in dry years. However, if food availability declines in dry years, increased selection of fewer species for foraging could result in greater competition and reduced fitness. Therefore, increased synchrony rather than mismatch could indicate negative climate-driven impacts on some systems.
