Migration departure strategies of shorebirds at a final pre-breeding stopover site

Abstract

Background: Departure 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. Methods:We 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. Results:We 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.Conclusions: Body 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.

Migrating Curlews on Schedule: Departure and Arrival Patterns of a Long-Distance Migrant Depend on Time Rather than on Wind Conditions

“…Although curlews rarely departed during hours with precipitation (in accordance with other bird species) [12,[23][24], this predictor had no signi cant in uence in our nal model. The current ndings were therefore not in agreement with previous studies, which found that the probability of departure of curlews from a pre-breeding staging site was reduced during precipitation [46].…”

“…In contrast, our ndings were in accord with observations of departing curlews in China, where there was also no signi cant in uence of wind force or wind direction [46]. The same holds true for the closely related whimbrel (Numenius phaeopus islandicus), for which there were no correlations with wind conditions for either direct ight or one-stop-over migration [47].…”

“…Our results support these patterns, with more curlews departing during the early evening or early nighttime. However, an earlier study of curlews departing from a nal pre-breeding stopover site in China showed high variability in terms of the time of day for departures [46]. The reason for these different ndings remains unclear.…”

Migrating Curlews on Schedule: Departure and Arrival Patterns of a Long-distance Migrant Depend on time and Breeding Location Rather than on Wind Conditions

“…Although curlews rarely departed during hours with precipitation (in accordance with other bird species), this predictor had no signi cant in uence in our nal model. The current ndings were therefore not in agreement with previous studies, which found that the probability of departure of curlews from a pre-breeding staging site was reduced during precipitation [53]. However, our ndings were in accord with observations of departing curlews in China [53], and of the closely related whimbrel (Numenius phaeopus islandicus), which also demonstrated no signi cant in uence of wind force or wind direction [53][54].…”

“…Our results support these patterns, with more curlews departing during the early evening or early nighttime. However, an earlier study of curlews departing from a nal pre-breeding stopover site in China showed high variability in terms of the time of day for departures [53]. The reason for these different ndings remains unclear.…”