Weather and prey availability affect the timing of fall migration of Eared Grebes (Podiceps nigricollis) from Great Salt Lake

Abstract

Great Salt Lake (GSL) is a critical migratory stopover location for Eared Grebes (Podiceps nigricollis), but the factors influencing the timing of their fall migration have not previously been investigated. We used archived Doppler radar data to visualize the nocturnal departures of Eared Grebes from GSL over 16 years, from 1999–2015. We used generalized linear models (GLMs) to examine interannual variability in the timing of migration of Eared Grebes in relation to prey availability and lake temperature, as well as variation in the nightly departure likelihoods of Eared Grebes in relation to weather. On average, Eared Grebes departed from GSL over a period of 31 days each year, with departures occurring on 17 of those days. We did not find any trend toward earlier departures over the years of the study. Departures typically began earlier in the year when densities of brine shrimp adults (Artemia franciscana) were higher than average, densities of brine shrimp cysts were lower than average, and lake temperatures were warmer than average. The span of migration departures was most strongly related to the day of first departure, with longer spans occurring in years with an earlier migration initiation day. We also found that likelihood of departure from GSL was greater on nights with high barometric pressure and low lake temperature. High barometric pressure was related to low wind speeds and low incidence of precipitation. We concluded that Eared Grebes depart when they have gained sufficient mass to successfully migrate rather than lingering at GSL for as long as possible, and that migration departure is most likely on nights with fair weather.