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Saturday, 22 February 2014

Variation of annual migratory timings in Hudsonian Godwits does not accumulate detrimental effects in breeding success or survival

Article Title
An Exception to the Rule: Carry-Over Effects Do Not Accumulate in a Long-Distance Migratory Bird

Nathan R. Senner, 1
Wesley M. Hochachka, 1
James W. Fox, 2
Vsevolod Afanasyev, 2

1. Cornell Lab of Ornithology, Cornell University, Ithaca, New York, United States of America
2. British Antarctic Survey, Cambridge, United Kingdom

PLoS ONE 9(2): e86588. doi:10.1371/journal.pone.0086588

Recent years have seen a growing consensus that events during one part of an animal’s annual cycle can detrimentally affect its future fitness. Notably, migratory species have been shown to commonly display such carry-over effects, facing severe time constraints and physiological stresses that can influence events across seasons. However, to date, no study has examined a full annual cycle to determine when these carry-over effects arise and how long they persist within and across years. Understanding when carry-over effects are created and how they persist is critical to identifying those periods and geographic locations that constrain the annual cycle of a population and determining how selection is acting upon individuals throughout the entire year. Using three consecutive years of migration tracks and four consecutive years of breeding success data, we tested whether carry-over effects in the form of timing deviations during one migratory segment of the annual cycle represent fitness costs that persist or accumulate across the annual cycle for a long-distance migratory bird, the Hudsonian godwit, Limosa haemastica. We found that individual godwits could migrate progressively later than population mean over the course of an entire migration period, especially southbound migration, but that these deviations did not accumulate across the entire year and were not consistently detected among individuals across years. Furthermore, neither the accumulation of lateness during previous portions of the annual cycle nor arrival date at the breeding grounds resulted in individuals suffering reductions in their breeding success or survival. Given their extreme life history, such a lack of carry-over effects suggests that strong selection exists on godwits at each stage of the annual cycle and that carry-over effects may not be able to persist in such a system, but also emphasizes that high-quality stopover and wintering sites are critical to the maintenance of long-distance migratory populations.

The authors present results from a study to look for the accumulation and dissipation of carry-over effects across an entire annual cycle, using data from 26 adult Hudsonian godwits (Limosa haemastica) from the Beluga River region in Alaska. The birds carried British Antarctic Survey geolocation-tracking devices that recorded data from three consecutive years of migration (2009-2011). These data were then compared with four consecutive years of data on breeding success, measured as producing young living to past 28 days. 

Godwits migrate the entire length of the Western Hemisphere: Beluga River - breeding site; central Saskatchewan - staging site during southward migration; Amazon Basin, Colombia - stopover site during southward migration; Buenos Aires Province, Argentina - stopover site during southward migration; Isla Chiloe, Chile - nonbreeding site; and Rainwater Basin, Nebraska - staging site during northward migration (see Figure 1). They then must breed within a short, nine-week summer. This constrained time-frame was therefore thought likely to be affected by delays in migration which may in turn increase the likelihood of detrimental carry-over effects on events such as nesting. 

The authors predicted that the godwits falling behind during one portion of their annual cycle would either not recoup this lost time or would  subsequently fall further behind, resulting in a late arrival on the breeding grounds, and a failure to produce a new brood.

Figure 1.
Map showing the migration routes of Hudsonian godwits breeding at Beluga River, Alaska. Twenty-six individuals were tracked across three years 2009–2012. Each red triangle denotes the location of an individual on one day, but does not necessarily indicate that the individual stopped in that location. Each blue circle denotes a region in which the majority of godwits stopped and congregated in both years.

Figure 2 shows the variation in timing of arrival and departure from the migration points, used as a measure of carry over effect. Each bar represents the average relative difference in timing of departure or rates of change for all individuals migrating either ahead or behind the population mean.

This is one of the first studies to document how individual birds of a migratory species control carry-over effects due to delays in the timing of migration events, by analysing changes in reproductive success and survival. The authors found that in spite of having one of the most extreme migrations of any migratory bird, returning godwits that migrated later than the population mean during one portion of their annual cycle did not remain behind for the entirety of their annual cycle, nor did they suffer reduced breeding success or survival.

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