Migrating shorebirds strategize when departing from their last stopover before their destination. This is according to a research paper by Kun Tan, Chi-Yeung Choi, Hebo Peng, David S. Melville and Zhijun Ma. The research titled “Migration departure strategies of shorebirds at a final pre-breeding stopover site” studied four shorebird species namely bar-tailed godwit, the grey plover, the great knot and the Eurasian curlew. The birds were observed at their final pre-breeding stopover at Yalujiang Estuary Wetlands in China. This was between 2011 and 2014. In their observations, the researchers took note of the sizes of the flocks, when they departed and the direction they took. They also measured wind speed and direction. Data on precipitation and tide in the area was acquired from the relevant bodies. This data was used to study how external conditions like wind and tide affected departures compared to internal factors. The results found that flock size, time and direction of departure were carefully considered. It speaks of strategy.
According to the research observations, the species departed in flocks of different sizes and at different times. Eurasian curlew, the largest of the four, had the smallest flock. The grey plovers departed in large flocks. Bar tailed godwit which flew in v-shaped formation also departed in large numbers. Time of departure was observed to be mainly in the afternoon. The Eurasian curlews varied their departure with others leaving in the morning and midday on top of the afternoon. Although not seen, it was thought that the great knots were doing night time departures too because they seemed fewer in the mornings. The great knots also varied their departure direction when ascending while the others maintained a consistent departure approach. All the four species did not depart when there was precipitation.
Different factors are thought to be at play in these strategies. The factors are both internal and external in nature. Some of these factors are predators, flight distance, navigation, breeding grounds, size of the window available for breeding and tail winds. Smaller species are wary of predators so they tend to travel in huge numbers. Larger species travel in smaller flocks. Those that travel long distances also tend to go in big numbers and utilize tail winds to conserve energy.
In determining the time of day to depart, safety, wind, forage time, and re-calibration are considered. Departing at night avoids predators and maximizes foraging time. By waiting until the high tide, shorebirds that forage on tidal flats can also maximize their forage time. At sunrise and sunset, some birds can use the strong skylight polarization to re-calibrate. To reduce energy, other birds time for the tail wind which they can use to glide.
The factors were not considered in isolation. The birds had to do a delicate balancing act to accommodate most of them, or give one factor more weight as opposed to others. There seemed to be a battle between external and internal factors. This being the final stopover site before reaching their breeding grounds, the size of breeding window, which is an internal factor, seemed to have a greater pull. A narrower breeding window means the species has to reach its breeding destination fast. That species will not have the flexibility of time to accommodate much of the external factors like predators and foliage time.