Overtaking on migration: does longer distance migration always incur a penalty?

Abstract

For many migratory bird species, the latitudinal range of the winter distribution spans thousands of kilometres, thus encompassing considerable variation in individual migration distances. Pressure to winter near breeding areas is thought to be a strong driver of the evolution of migration patterns, as individuals undertaking a shorter migration are generally considered to benefi t from earlier arrival on the breeding grounds. However, the infl uence of migration distance on timing of arrival is diffi cult to quantify because of the large scales over which individuals must be tracked. Using a unique dataset of individually-marked Icelandic black-tailed godwits Limosa limosa islandica tracked throughout the migratory range by a network of hundreds of volunteer observers, we quantify the consequences of migrating diff erent distances for the use of stop-over sites and timing of arrival in Iceland. Modelling of potential fl ight distances and tracking of individuals from across the winter range shows that individuals wintering further from the breeding grounds must undertake a stop-over during spring migration. However, despite travelling twice the distance and undertaking a stop-over, individuals wintering furthest from the breeding grounds are able to overtake their conspecifi cs on spring migration and arrive earlier in Iceland. Wintering further from the breeding grounds can therefore be advantageous in migratory species, even when this requires the use of stop-over sites which lengthen the migratory journey. As early arrival on breeding sites confers advantages for breeding success, the capacity of longer distance migrants to overtake conspecifi cs is likely to infl uence the fi tness consequences of individual migration strategies. Variation in the quality of wintering and stopover sites throughout the range can therefore outweigh the benefi ts of wintering close to the breeding grounds, and may be a primary driver of the evolution of specifi c migration routes and patterns.

Costs, benefits, and fitness consequences of different migratory strategies

“…The very positive energetic balance available at this location is therefore sufficient to both fuel the longer migration and facilitate early arrival (Alves et al 2012a). Wind assistance can reduce the costs of migration (Shamoun-Baranes et al 2010) and, in other migratory systems, the higher costs of migration for individuals wintering further from the breeding grounds can be offset by wind subsidies encountered en route (Piersma et al 1991).…”

“…3a). During spring migration, the majority of Icelandic godwits wintering in Portugal undertake two flights in order to reach Iceland (Alves et al 2012a), with the majority of them stopping over in the Netherlands (covering a total migratory distance of ;3800 km), whereas most Icelandic godwits wintering in south Ireland and east England are able to reach Iceland in one flight (covering ;1425 km and ;1560 km in total, respectively). The migratory flight to Iceland for a male godwit of average mass departing from south Ireland or east England requires 1290 kJ and 1400 kJ, respectively, while departure from west Portugal requires a total of 2790 kJ, of which 1540 kJ are required for the first migration to the stopover sites (Fig.…”

“…Long-term marking and tracking of individual Icelandic godwits has been carried out throughout the entire migratory range since 1995 (Gill et al 2001a, Gunnarsson et al 2004, Alves et al 2012a). Estimates of annual adult survival rates for godwits marked in east England between 1995 and 2000, the period encompassing energetics measurements in this location, have previously been published (Gill et al 2001a).…”

“…Annual adult survival rates for Icelandic godwits wintering in west Portugal and south Ireland in 2006-2008 were calculated using Cormack-Jolly-Seber models (see Appendix A) in program MARK (White and Burnham 2000), and differences in survival rates between locations were assessed by 95% confidence interval overlap. The energy costs of migratory flights from each winter location to Iceland were estimated using Flight model (Version 1.18; Pennycuick 2008), parameterized for Icelandic godwits (details in Alves et al 2012a).…”

“…This seasonal matching of habitat quality and fitness may be a major driver of population dynamics in migratory systems, and is likely to have shaped the rate and pattern of population expansion throughout this migratory range. As individuals from the furthest winter locations can arrive in Iceland earlier than those from locations closer to the breeding grounds (Alves et al 2012a), something other than migration distance must be influencing the timing of arrival in this system. However, the mechanisms driving seasonal matching and, in particular, the way that variation in migration costs and environmental conditions across the winter range impact upon fitness remain unknown.…”