Faster migration in autumn than in spring: seasonal migration patterns and non-breeding distribution of Icelandic whimbrels Numenius phaeopus islandicus. Carneiro, C., Gunnarsson, T.G. & Alves, J.A. 2019. Journal of Avian Biology. DOI: 10.1111/jav.01938. VIEW
For migratory birds, arriving early at the breeding sites can be beneficial as it might allow individuals to secure a good quality breeding territory and mate, increase the period spent in the breeding area (and therefore the possibility of a replacement clutch), and ultimately enhance the chances of fledging young (Møller 1994, McNamara et al. 1998, Kokko 1999). The pressure on arrival timing at the breeding site is thus relatively high and, in fact, spring migration tends to occur at a faster pace than autumn migration (Nilsson et al. 2013, Zhao et al. 2017).
Icelandic whimbrels (Numenius phaeopus islandicus) are large, long distance migratory waders that breed mostly in Iceland and winter in West Africa. Intriguingly, early evidence from a few tracked individuals (Alves et al. 2016) suggested that this subspecies migrates faster in autumn than in spring, contrary to the overall trend. This asked for further investigation, and between 2012 and 2016 we considerably increased the sample size by tracking, with geolocators (Figure 1), 19 individuals during 30 autumn and 26 spring migrations.
Figure 1 Geolocators were tied to a flag, placed in the right tibiae of Icelandic whimbrels © Camilo Carneiro
Besides mapping the migration routes and wintering and stopover regions (Figure 2), we assessed migration duration, migration speed and ground speed. Given that tracking methods only provide temporal information on the migration period between departure and arrival, but that migration starts with the fuelling that takes place ahead of the first flight (Alerstam and Lindström 1990), we estimated the first fuelling period using published fuel deposition rates (Lindström 2003).
Figure 2 Icelandic Whimbrel migration routes as tracked by geolocators, in autumn (A–B) and spring (C–D), displayed separately for each sex (male: A and C; females: B and D). Polygons represent kernel densities of 95, 75 and 50%, from pale to dark, respectively, for winter (orange) and stopover areas (blue). The yellow circle represents the breeding site
We found that all birds undertook a direct flight to the winter areas in autumn, but in spring most made a stopover (Figure 2), and that Icelandic whimbrels, indeed, migrate faster in autumn than in spring (migration duration: 59.2 ± 2.6 days in autumn, 65.5 ± 6.2 days in spring; migration speed: 102.6 ± 2.2 km/day in autumn and 98.6 ± 5.3 km/day in spring; Figure 3).
Figure 3 Autumn and spring migration duration (A) and migration speed (B) of Icelandic Whimbrels; boxes show the median and 25 and 75% quartiles, whiskers extend up to 1.5 times the inter quartile range from the hinge and points beyond that are individually marked in black; all data points overlay the boxplots, coloured by strategy (green: direct; pink: stopover) and sex (dots: females; triangles: males)
Wind conditions can differ between seasons and contribute to the stopover during spring. Similarly to red knots (Calidris canutus), that make an additional stopover when facing unfavourable winds on the way to the breeding sites (Shamoun-Baranes et al. 2010), Icelandic whimbrels might also stop during spring, as unfavourable wind conditions can be more prevalent in this season than in autumn. They usually stopover in Ireland, which is relatively close to the breeding areas (ca 1500 km) and this might also allow individuals to appraise weather conditions closer to their destination and fine-tuning their timing, in order to avoid harsh conditions upon arrival. Alternatively, or complementarily, if conditions at stopover allow for a higher rate of resource accumulation than at the breeding areas, whimbrels that stopover might gain an advantage, despite the later arrival, by being in better physiological condition than individuals that fly non-stop (Alerstam 2006). This might allow saving time in the breeding site by starting energy-demanding activities, such as display flights, as quickly as possible. In the case of females, such resource accumulation in the stopover sites could be useful for egg production, but given the period spent at those sites (ca 14 days) and the time between arrival and egg laying in Iceland (ca 22 days), this is an unlikely scenario.
In autumn, the non-stop flights over water could be the most economic option in terms of time and energy and/or provide a route likely free of predators, parasites and pathogens (Gill et al. 2009), and occur if wind conditions are favourable in that season.
A detailed analysis of the wind effects on the migratory flights of Icelandic whimbrels will likely help clarify the causes behind the faster autumn migrations which apparently is uncommon.
Nominate this article for a BOU Science Communication Award.
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Featured image: Icelandic Whimbrel Numenius phaeopus islandicus © Camilo Carneiro