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Nest association between two predators as a behavioral response to the low density of rodents. Factors influencing fall departure phenology in migratory birds that bred in northeastern North America. Does vegetation change over 28 years affect habitat use and reproductive success? Ratio of individuals circling in the same direction as a function of height difference. For each GPS burst that contained at least 30 s of circling, we characterized for every pair whether they circle in the same or in opposite directions.
Thin coloured lines correspond to the average ratio of birds circling in the same directions. Dashed line depicts a random ratio of 0. Thick black line shows the mean of all bursts. Figure 4. Repeatability of synchronization between individuals. Black circles show values averaged over the first and second day.
Coloured markers correspond to five randomly chosen pairs. The bottom plot with grey markers shows the mean of the entire flock. Error bars represent standard deviation. We identified those birds that successfully initiated a directional switch and were followed by at least one other birds by measuring, for each SSCD, the time delay between the directional switches of the different subgroup members. If a follower copies the directional switch of an initiator, time delays are positive and are also typically much smaller than the time to complete a full circle in the thermal.
We examined whether the altitude of the bird, with respect to others, influences its propensity to initiate a switch in circling direction i. We found no correlation between the difference in height and the time delay between shifts electronic supplementary material, figure S2 meaning that directional switches can be initiated from any position within the subgroup. Next, we asked whether there exist consistent inter-individual differences among group members in their propensity to initiate switches in circling direction.
For each SSCD, we defined the initiator as the first individual to perform the directional switch e. Because of the temporal resolution of the GPS tracks 1 s sampling rate , we observed incidents when several individuals switched their circling direction at the same time point. In such cases, we consider multiple individuals as initiator, but weight their initiation by dividing it by the number of initiators e. Figure 5. Initiation and adoption of synchronized switches in circling direction. We defined the initiator as the individual that switched its circling direction first.
Edges are non-directed; their widths represent the total number of times that a pair switched in synchrony the sum of initiations and adoptions. Only values greater than or equal to 1 are shown. Colour coding of the nodes is identical to a. To understand the functional role of these directional switches, we had to explore whether initiators and followers shifted the centre of rotation at specific, non-random locations.
Because climb rates are higher at higher altitudes electronic supplementary material, figure S3 , we cannot simply compare the climb rates before and after a directional change.
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Updrafts vary strongly between, and even within, thermals electronic supplementary material, figure S4 , but birds may still shift their rotation centre at the same locations to avoid losing the thermal or even move into sinking air. By examining the orientation of the birds at the two time points when each performed the switch i. As a final test, we repeated the analysis after removing the effect of horizontal winds using the method of Weinzierl et al.