Spatiotemporal and Phenological Pattens of Bird Migration and the Influence of Climate and Disturbance in the Madrean Sky Island Archipelago and North American Southwest

Abstract

Distributional and ecological dynamics of Neotropical migratory birds at stopover sites where they maintain critical fat reserves during migration remain poorly understood in North American aridlands. I examined spatiotemporal abundance and timing of migrants relative to 1) upland and riparian habitats, 2) post-fire landscape mosaics, and 3) phenological synchrony and overlap of migration with tree flowering in southeastern Arizona's Madrean Archipelago (2009-2011), and 4) abundance, habitat breadth, and foraging substrates relative to tree flowering along the Colorado River in southwestern Arizona and northwestern Sonora, Mexico (2000-2003). I explored these dynamics relative to local weather conditions and El Niño Southern Oscillation (ENSO) climate phenomena. In Madrean habitats, migrants showed three non-exclusive responses to high precipitation, snowfall, and low minimum temperatures associated with El Niño in 2010; migration timing adjustments, habitat shifts, and reduced abundances suggesting migration route shifts. Foliage-gleaning insectivores were most abundant in high severity burns, disproportionate to their availability, and decreased with time since fire (TSF); flycatchers were most abundant in low-moderate severity and increased with TSF. Migrant abundance increased with tree flowering. Phenological overlap declined with increasing difference in timing of these events. Overlap was lowest in 2011 in riparian habitat due to low willow (Salix goodinggii) flowering, despite high migrant abundance, but lowest in 2010 in montane conifer, despite high pollen cone production by Douglas-fir (Pseudotsuga meziesii), suggesting temperature limitation of insect abundance at high elevations, but water limitation of plant phenology at lower elevations. Along the Colorado River, migrant abundance and habitat breadth had inverse positive and negative quadratic relationships, respectively. Abundance increased with tree flowering, but only in 2003 during severe drought. Habitat breadth increased with monsoon precipitation. Foraging substrate use tracked flowering, shifting from willow to mesquite (Prosopis sp.); the overlap coincided with peak abundance and narrowest habitat breadth. Maintenance of diverse vegetation and post-fire landscape mosaics in the Madrean Archipelago should benefit migratory bird diversity. Flowering phenology likely provides large-scale cues of local-scale stopover habitat condition associated with interannual climatic variation. Management and restoration of upland habitats and large riparian woody perennials will be critical for migratory bird conservation in aridlands.