Abstract
BackgroundDeparture decisions and behaviors of migratory birds at stopover sites are expected to maximize fitness by trade-offs among avoiding predators, optimizing refueling (energy) capacity, and matching other life-history events. We predict that species with different body sizes and migratory destinations will exhibit different behaviors when departing from the same stopover site. We also predict that with strong time constraint at the final pre-breeding stopover site, departure decisions may be less sensitive to exogenous factors, such as wind condition, compared to other stopover or nonbreeding sites.MethodsWe recorded migratory departures of four shorebird species, i.e. Eurasian Curlew (Numenius arquata), Bar-tailed Godwit (Limosa lapponica), Great Knot (Calidris tenuirostris), and Grey Plover (Pluvialis squatarola), at Yalujiang Estuary Wetlands in China, a final pre-breeding stopover site in the northern Yellow Sea, from 2011 to 2014. We compared flock sizes, departure time and departure directions between species, and investigated the effects of tide and weather conditions (rain and ground wind speed and direction) on the departure decision of shorebirds.ResultsWe found that larger species departed in smaller flocks and were more variable in daily departure time. Departure trajectory of all four species appeared to be influenced by coastal topography. With the east–west coastline and intertidal mudflat on the south, birds exhibited westward or eastward deflection from the shortest migratory routes. Bar-tailed Godwit was the only species that deviated to the southeast and did not climb over the land. Birds avoided departure during precipitation, while their departure was not related to ground wind benefit or tidal condition.ConclusionsBody size among species, which influences their vulnerability to predators, might be important in shaping shorebird departure strategies. Diverse departure directions could be the result of different wind use tactics in climbing stage. Narrow optimal time window of breeding might lead to reduced flexibility in departure date at a final pre-breeding site. Both endogenous and exogenous are important in shaping departure behaviors and decisions.
Highlights
Departure decisions and behaviors of migratory birds at stopover sites are expected to maximize fit‐ ness by trade-offs among avoiding predators, optimizing refueling capacity, and matching other life-history events
Endogenous schedules might prevail over exogenous factors in this stage of the migration and birds might show less flexibility in date selection than those departing from non-breeding or other stopover sites (Warnock 2010; Hua et al 2017; Shamoun-Baranes et al 2017)
Our result is contradicted in the flock size-body size relationship reported in former study (Piersma et al 1990), which might be because either most of former records came from post-breeding migration when birds were more dispersed in migration date; or the smaller species, such as Ringed Plovers (Charladies hiaticula), Sanderlings (Calidris alba) were not tend to aggregate at stopover site before their departure
Summary
Departure decisions and behaviors of migratory birds at stopover sites are expected to maximize fit‐ ness by trade-offs among avoiding predators, optimizing refueling (energy) capacity, and matching other life-history events. Many migratory birds need one or more stopover sites to refuel and/or rest en route They are expected to adjust departure decisions and behaviors at stopover sites to avoid predators, optimize energy deposition and match other annual life-history events (e.g. breeding) to maximize fitness (Newton 2010). Endogenous schedules might prevail over exogenous factors in this stage of the migration and birds might show less flexibility in date selection than those departing from non-breeding or other stopover sites (Warnock 2010; Hua et al 2017; Shamoun-Baranes et al 2017). Departure decisions from final stopover sites have been little studied (Greenberg and Marra 2005)
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