The Postbreeding Migration of Eared Grebes

Eared Grebes (Podiceps nigricollis) in autumn make a postbreeding/molt migration from breeding areas in western North America to hypersaline lakes in the Great Basin. We studied their biology in 2001–2006 during this phase of the annual migration near Green River, Wyoming, USA where migrants en route to Great Salt Lake, Utah land on industrial ponds. Most evidently originate in the Prairie Pothole Region of North Dakota. The main movement extends from late July to mid-October. Migrants arrive almost daily with the cumulative percentage of transiting birds increasing by about 1% per day. Adult males and females migrate on the same schedule and precede juveniles by 2–3 weeks. Annual differences in phenology, abundance, age ratio, and wing molt vary with availability of wetland habitats in the main source area. Data on mass, body composition, energetics, and stomach contents indicate a typical flight involves a direct 2–3 day non-feeding migration, which is accomplished at night. Grebes are quiescent during the day and do not resume their migration until 45 min after sunset. We documented two undescribed vocalizations, a short-range contact note and one associated with departure. The possibility that Eared Grebe productivity, as inferred from studies of migrants through Wyoming, can provide insight into the status of waterbirds in the source area is worth further investigation.