MIGRATORY SONGBIRDS EXHIBIT SEASONAL MODULATION OF THE OXYGEN CASCADE
Data for associated paper. Migratory flight is an intensive exercise that requires birds to maintain aerobic exercise for many hours or days. Maintaining O2 supply to flight muscles is therefore important during migration, especially since recent research has documented migratory songbirds and shorebirds flying at altitudes greater than 5,000 m above sea level, where O2 is limited. Whether songbirds exhibit seasonal plasticity of the O2 cascade that are important to maintaining O2 uptake and transport during migratory flight is not well understood. Here, we investigated changes in the hypoxic ventilatory response, haematology, and pectoralis (flight) muscle phenotype of 6 North American songbird species (3 families) during photoperiod-induced migration and wintering conditions. Songbirds were captured during southbound migration in southern Ontario, Canada. Half of the birds were assessed during migration, and the rest were transitioned onto a winter photoperiod to induce a non-migratory phenotype and measured. We found that all songbirds exhibit seasonal plasticity at various stages along the O2 cascade, but not all species or families exhibit the same plastic responses. Broadly, fall migration resulted in songbirds having a more effective breathing pattern (slower, deeper breaths), increased hypoxia tolerance, altered haemoglobin-O2 binding affinity, and increased abundance of muscle fibers with smaller transverse surface areas, all of which would enhance O2 uptake and transport to the flight muscle. Our findings demonstrate that in the O2 cascade there is no ideal migratory phenotype for all songbirds, and that more comparative studies investigating aspects of the O2 cascade during migratory and non-migratory seasons are needed.