Migratory Timing and Site Use of Marbled Godwits Breeding in Alaska, 2008–2015

We estimated the migratory timing and site use of Marbled Godwits throughout the annual cycle by visualizing the filtered location data (data available; see Source ASC_Godwit_Telemetry). We used Google Earth (Goggle LLC, Mountain View, California, USA) for visualizing the time-stamped tracking data for each individual godwit. We analyzed each godwits' locations in chronological order to document associated movements and site use. Because of the transmitters’ 10-hr-on/48-hr-off reporting cycle, we were often unable to determine the exact timing of a bird’s departure from or arrival to a site. In these instances, we estimated the timing of such movements by applying a series of decisions based on the timing of locations immediately prior to departures and following arrivals. In-flight tracks revealed that godwits did not travel over land other than when traversing the Alaska Peninsula while arriving at or departing from breeding sites. Thus, if transmitters detailed in-flight movements over marine waters within 100 km of either a departure or arrival site, we applied the rate of movement from those in-flight tracks to the first or last in-flight location to estimate the timing of departure or arrival, respectively. In the event that movements occurred such that we received no in-flight tracks, we assumed that the last on-land location prior to movement was the departure time, and applied a rate of 55 km hr-1 to estimate the time of arrival. For the purposes of this study, we defined a ‘site’ as any location visited by a godwit where at least 3 standard locations indicated local or no movement (i.e., rates of movement ≤55 km hr-1). Given the positional accuracy of the transmitters, we did not attempt to assess within-site movements and restricted our investigations to broad-scale movements. Further, we defined a nonbreeding site as the most southerly location that a particular godwit resided after their post-breeding migration, and a stopover site as any site visited outside of the breeding and nonbreeding periods during which a bird’s locations and rates indicated only local movement. In this way, we determined the timing of migratory movements, the sites associated with these movements, the duration of each stage of a migratory movement, and the overall duration of a migratory movement.