Data from: Faster migration in autumn than in spring: seasonal migration patterns and non-breeding distribution of Icelandic Whimbrels Numenius phaeopus islandicus

Migration is fundamental in the life of many birds and entails significant
energetic and time investments. Given the importance of arrival time in
the breeding area and the relatively short period available to reproduce
(particularly at high latitudes), it is expected that birds reduce spring
migration duration to a greater extent than autumn migration, assuming
that pressure to arrive into the wintering area might be relaxed. This has
previously been shown for several avian groups, but recent evidence from
four tracked Icelandic Whimbrels (Numenius phaeopus islandicus), a long
distance migratory wader, suggests that this subspecies tends to migrate
faster in autumn than in spring. Here, we (1) investigate differences in
seasonal migration duration, migration speed and ground speed of Whimbrels
using 56 migrations from 19 individuals tracked with geolocators and (2)
map the migration routes, wintering and stopover areas for this
population. Tracking methods only provide temporal information on the
migration period between departure and arrival. However, migration starts
with the fuelling that takes place ahead of departure. Here we estimate
the period of first fuelling using published fuel deposition rates and
thus explore migration speed using tracking data. We found that migration
duration was shorter in autumn than in spring. Migration speed was higher
in autumn, with all individuals undertaking a direct flight to the
wintering areas, while in spring most made a stopover. Wind patterns could
drive Whimbrels to stop in spring, but be more favourable during autumn
migration and allow a direct flight. Additionally, the stopover might
allow the appraisal of weather conditions closer to the breeding areas
and/or improve body condition in order to arrive at the breeding sites
with reserves.