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1 January 2014 Post-Breeding Migration of Dutch-Breeding Black-Tailed Godwits: Timing, Routes, Use of Stopovers, and Nonbreeding Destinations
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Abstract

Conservation of long-distance migratory shorebirds is complex because these species use habitats spread across continents and hemispheres, making identification of critical habitats and potential bottlenecks in the annual cycle especially difficult. The population of Black-tailed Godwits that breeds in Western Europe, Limosa limosa limosa, has declined precipitously over the past few decades. Despite significant efforts to identify the root causes of this decline, much remains unclear. To better understand the migratory timing, use of stopover and nonbreeding sites, and the potential impact of breeding success on these parameters, we attached 15 Argos satellite transmitters and 10 geolocation tracking devices to adult godwits nearing completion of incubation at breeding sites in southwest Friesland, The Netherlands during the spring of 2009. We successfully tracked 16 adult godwits for their entire southward migration and two others for part of it. Three migration patterns and four regions of use were apparent. Most godwits left their breeding sites and proceeded south directly to stopover sites in the Mediterranean — e.g. Spain, Portugal, and Morocco — before flying on to non-breeding sites in West Africa. Other individuals spent the entire nonbreeding season in the Mediterranean. A third pattern included a few individuals that flew nonstop from their Dutch breeding sites to nonbreeding sites in West Africa. Tracking data from this study will be immediately useful for conservation efforts focused on preserving the dispersed network of sites used by godwits during their southward migration.

Long-distance migratory shorebirds are a compelling group: the past two decades have brought a wealth of new information regarding the capability of shorebirds to sustain flight over many thousands of kilometers (Gill et al. 2009, Battley et al. 2012); adjust their phenotypes to meet the strict requirements of their long-distance movements (Buehler et al. 2012, Vezina et al. 2012); navigate across open oceans and between hemispheres (Gill et al. 2009); and precisely time movements between widely spaced stops at which they make use of resources that occur in brief peaks (Baker et al. 2004). What makes long-distance migratory shorebirds compelling, however, also makes them complex. Accordingly, the past few decades have also seen an increasing recognition that much remains unknown about the life cycles of these organisms (Buehler & Piersma 2008). For instance, what kinds of resources do these species require to enable such extreme flights (van Gils et al. 2005) and what cues do they use to time their precise movements (Senner 2012)? Unfortunately, what makes long-distance migratory shorebird life cycles complex may also make them vulnerable. Across all avian taxa, populations of migratory shorebirds are among those most uniformly and dramatically in decline (International Wader Study Group 2003). For instance, of 85 North American taxa, 40 have populations that are declining (Andres et al. 2012).

European-breeding Black-tailed Godwits, Limosa l. limosa (hereafter ‘godwits’), are in many respects emblematic of long-distance migratory shorebirds. We know they are able to fly great distances, moving between northwestern Europe and the river deltas of West Africa (Meltofte 1996, Beintema & Drost 1986, Zwarts et al. 2009). We know they are able to time these movements, with high repeatability, year after year (Lourenço et al. 2011). We also know they are declining precipitously — continental populations are half of what they were 30 years ago (Gill et al. 2007, Sovon 2013). The decline itself is emblematic in that extensive study has yet to explain its cause (s), especially potential cross-seasonal interactions (Schroeder et al. 2012, Kentie et al. 2013).

To better understand the flight behaviour and use of stopover sites by godwits, as well as to obtain more precise information on the contemporary use of wintering areas, we used two remote tracking methods — satellite telemetry and geolocation — to track adult Blacktailed Godwits of the limosa subspecies captured on their breeding grounds in The Netherlands. Specifically we wanted to understand the southward migration of godwits by asking the following questions. How soon do adult godwits leave the breeding area after successful and failed breeding attempts? When do they depart the breeding areas? Where do they stop during migration? And, where are their final nonbreeding destinations? We discuss our findings in the context of life-history theory and the current conservation situation.

METHODS

We studied the post-breeding migration of godwits using adults from an intensively studied breeding population in an 8480 ha area of southwest Friesland, The Netherlands, between Makkum (53°02.41′N, 05°23.14′E) in the north and Laaksum (52°50.59′N, 05°25.16′E) in the south (Schroeder et al. 2008, Groen et al. 2012). This area predominantly consists of grasslands (88.5%) and arable land (11%; mostly maize fields), most of which is intensively managed for dairy farming (Groen et al. 2012). About 10% of the grasslands exist as nature reserves that are specially managed for godwits and other meadow-bird species.

We captured 15 adults on nests between 10 and 17 May 2009 using walk-in traps set during the final few days of incubation or as eggs were hatching. We implanted satellite transmitters into the coelom of these birds following the protocol of Mulcahy et al. (2011) and as present