Wind effects on the migration routes of trans-Saharan soaring raptors： geographical, seasonal, and interspecific variation

Wind is among the most important environmental factors shaping birds＇ migration patterns. Birds must deal with the displacement caused by crosswinds and their behavior can vary according to different factors such as flight mode, migratory season, experience, and distance to goal areas. Here we analyze the relationship between wind and migratory movements of three raptor species which migrate by soaring-gliding flight： Egyptian vulture Neophron percnopterus, booted eagle Aquila pennata, and short-toed snake eagle Circaetus gallicus. We analyzed daily migratory segments （i.e., the path joining consecutive roosting locations） using data recorded by GPS satellite telemetry. Daily movements of Egyptian vultures and booted eagles were significantly affected by tailwinds during both autumn and spring migrations. In contrast, daily movements of short-toed eagles were only significantly affected by tailwinds during autumn migration. The effect of crosswinds was signifi- cant in all cases. Interestingly, Egyptian vultures and booted eagles showed latitudinal differences in their behavior： both species compensated more frequently at the onset of autumn migration and, at the end of the season when reaching their wintering areas, the proportion of drift segments was higher. In contrast, there was a higher drift at the onset of spring migration and a higher compensation at the end. Our results highlight the effect of wind patterns on the migratory routes of soaring raptors, with different outcomes in relation to species, season, and latitude, ultimately shaping the loop migration patterns that current tracking techniques are showing to be widespread in many long distance migrants.

The variability of juvenile dispersal in an opportunistic raptor

The juvenile dispersal of raptors is a crucial stage that stretches from parental independence to the establishment of the first breeding area.Between 2012 and 2020,44 juvenile red kites Milvus milvus from the Spanish breeding population were tagged using GPS telemetry to study their dispersal.Juveniles left the parental breeding area at the end of their first summer and performed wandering movements throughout the Iberian Peninsula,returning to the parental breeding area the following year,repeating the same pattern until they settled in their first breeding area.We analyzed the mean distance from the nest,the maximum reached distances,and the traveled distances(daily and hourly)during the first 2 years of dispersal and compared them.Despite the high individual variability,variables describing the dispersal movements of juveniles showed a decreasing trend during the second dispersal year:80% of individuals reached a shorter maximum distance in the second year,70% decreased their mean distance to the nest,65% decreased their hourly traveled distances,and 50% decreased their daily traveled distances.On the other hand,the red kites usually combined wandering movements with the establishment of temporary settlement areas(TSA).The average duration of settlement in the TSAs was 75±40 days(up to 182 days)and was located at 182±168 km from the nest.In those areas,juveniles used 781.0±1895.0 km^(2)(KDE 95%).Some of the TSAs were used by several individuals,which suggests that these areas might be good targets for conservation in future management plans.