Faster migration in autumn than in spring: seasonal migration patterns and non‐breeding distribution of Icelandic whimbrelsNumenius phaeopus islandicus

Abstract

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.

Why Are Whimbrels Not Advancing Their Arrival Dates Into Iceland? Exploring Seasonal and Sex-Specific Variation in Consistency of Individual Timing During the Annual Cycle

“…We consider stopover any stop during travel between breeding and wintering locations, irrespective of length of stay, site quality or previous or future flight distance and duration (i.e., we do not discriminate from staging; Warnock, 2010). Although stops of few hours may be undetected with geolocator data, stopovers of Icelandic whimbrels are usually of several days (Carneiro et al, 2019), with the minimum stopover duration recorded during this study being 6 days.…”

“…Icelandic whimbrels show two migratory strategies in spring: a direct non-stop flight or two flights with a stopover in between (henceforward: "direct" and "stopover"; Carneiro et al, 2019). However, and adding to previous information, during this study one individual was recorded undertaking a stopover during autumn migration.…”

“…During autumn migration, a lower consistency in timing was observed compared to spring (Tables 2, 3 and Figures 2, 3) and the values at arrival mirror the ones at departure because in nearly all occasions individuals flew directly from Iceland to the wintering sites ( Table 1; Carneiro et al, 2019). On the other side of the Atlantic, Hudsonian whimbrels showed the same general pattern, as inter-individual variation was greater at autumn departure and arrival dates, than at spring departure and arrival dates (Johnson et al, 2016).…”

“…Sixty five individuals were sexed using biometrics following Katrínardóttir et al (2013), 22 molecularly (as in Katrínardóttir et al, 2013), eight through behavioral observation (copulating position, assuming males on top) and two remained undetermined. Geolocator data analysis and determination of individual departure and arrival timings were performed using light, and temperature, conductivity and wet contacts as described in Carneiro et al (2019). Due to the accuracy of geolocators (Phillips et al, 2004), we considered arrival and departures to/from the general area.…”

“…The Icelandic subspecies breeds predominantly in Iceland, with an estimated population of 256,000 pairs (Skarphéðinsson et al, 2016), and winters in West Africa (Gunnarsson and Guðmundsson, 2016;Carneiro et al, 2019). In spring, their departure from the wintering sites occurs on the second fortnight of April and arrival into Iceland from late April to early May, with individuals performing one of two migratory strategies: a non-stop flight or two flights divided by a relatively short stopover; in autumn, migratory movements occur from late June to late August and only the direct flight strategy has been recorded in this season (Alves et al, 2016, Carneiro et al, 2019. Icelandic whimbrels are site faithful to the breeding territory, monogamous, and most reproduce with the same partner from year to year (BWPi, 2006), with males arriving earlier and departing later from breeding sites than females (Carneiro et al, 2019).…”