No apparent gain from continuing migration for more than 3000 kilometres: willow warblers breeding in Denmark winter across the entire northern Savannah as revealed by geolocators
© The Author(s). 2017
Received: 5 April 2017
Accepted: 21 August 2017
Published: 30 August 2017
For most Afro-Palearctic migrants, particularly small songbirds, spatiotemporal migration schedules and migratory connectivity remain poorly understood. We mapped migration from breeding through winter of one of the smallest Afro-Palearctic migrants, the willow warbler Phylloscopus trochilus, using geolocators (n = 15).
Birds migrated from North European breeding grounds to West Africa via the Iberian Peninsula following a narrow corridor along the West Coast of Africa. Birds then dispersed across the northern Savannah with termination of migration highly variable among individuals. The termination of migration appeared not to be related to timing, current and previous years’ vegetation conditions or biometrics. During winter, most birds moved southwards to improved vegetation.
The willow warblers showed a large, unexpected longitudinal spread in winter sites of more than 3000 km between individuals breeding within a 500 m range resulting in a low degree of connectivity. The large wintering area may well be related to generalist behaviour in the species. Our findings contribute to understanding the link between breeding and wintering ecology in long-distance migratory birds.
Migration is an adaptation to seasonal environments [1, 2] and billions of small songbirds travel vast distances from Palearctic breeding to sub-Saharan wintering grounds every year . Understanding the drivers of these travels and the links to the seasonally changing environments requires detailed knowledge of the spatiotemporal migration pattern. Yet, for most of these species, our knowledge of migration routes, non-breeding stop-overs and wintering sites is incomplete. Gaining knowledge and understanding of these patterns is crucial in the light of their general declines [4, 5] and if we are to forecast potential effects of climate change .
Migratory connectivity, how much individuals from the same breeding site mix during the non-breeding season with birds from other sites within the same population , is essential for understanding ecology and population dynamics [8–11]. Connectivity is defined as a relative measure: It is high if little mixing occurs, i.e. individuals from different breeding sites stay separated in winter, and it can be low even if wintering occurs within a limited area as long as the whole population winters within the same area. Thus, connectivity can be inferred to be low if birds from a single site spread out over a large part of the known wintering area. Connectivity is commonly low in long-distance migratory birds  but some notable exceptions have been reported [9, 11].
Many European bird species wintering in West Africa follow a southwestern migration route into Africa [13, 14]. In West Africa, migration along the coast apparently occurs in several species, including passerines [11, 15, 16], only turning inland after reaching the Sahel [17–19]. Circumventing the Sahara along the coast is a relatively short route  and potentially allows a safe Sahara crossing with opportunities for foraging, drinking and predator avoidance. However, Biebach  concluded that the coast was unlikely to be a major flyway for most passerines because no mass migration had been observed.
Individuals of several species move between sedentary periods in Africa (itinerancy ). Itinerancy is thought to be a response to changing food conditions though direct links such as in raptors  are rarely documented. Tracking has revealed both sedentary behaviour [11, 15, 24, 25] and itinerant behaviour [26, 27] in species wintering in West and Central Africa.
The insectivorous willow warbler Phylloscopus trochilus is among the smallest of the Afro-Palearctic migrants. It breeds in a variety of woody habitats across the Palearctic and winters in the entire Savannah zone covering most of sub-Saharan Africa . However, there is a considerable variation among subspecies and presumably a high degree of connectivity [29, 30] with ring recoveries supporting partial separation of subspecies also in winter . The subspecies P. t. trochilus occurs in West Europe and southern East Europe, wintering in West Africa. Little is known about winter movements and both residency  and itinerancy  have been suggested.
Despite importance for understanding links to the seasonal environments as well as conservation , information about connectivity, migration routes and within-winter itinerancy  is generally scarce or lacking. Furthermore, few studies have evaluated the causes of connectivity at the behavioural level, for example migration direction or termination of migration . Among-individual spread is often assumed to result from variation in migration directions  but several other factors could also be involved. Here, we use geolocators to describe migration routes, stopovers and wintering sites of male willow warblers P. t. trochilus breeding in Denmark. Despite the low spatial resolution, estimation of timing of movements and mapping of stopping-over locations at a regional scale is possible [37, 38]. We focus on the resulting migratory connectivity and the behaviours causing it at the individual level. Overall, we find surprisingly little connectivity (large longitudinal spread compared to the wintering distribution of the studied subspecies) and test a range of possible causes, such as filling up of suitable habitat, dominance hierarchy, and vegetation conditions. Furthermore, we describe itinerancy (movement between sedentary stays) and explore the link with environmental conditions. We use arrival data as a proxy for the potential filling up of suitable habitat. A potential dominance hierarchy is explored based on measurements of weight and wing length. To investigate effects of vegetation and environmental conditions, we explore patterns of the Normalized Difference Vegetation Index (NDVI) based on the assumption that vegetation greenness is ultimately related to food availability though the link is not direct .
We fitted male willow warblers with Intigeo–W30 geolocators (Migrate Technology LTD, 0.3 g, c. seven months capacity) in East Denmark (55.61°N, 12.57°E; catching range 500 m) from May to mid-June (n = 17 in 2014, n = 20 in 2015) using leg-loop harnesses  made of 1 mm braided nylon cord. 17 birds were recaptured the year after tagging (n = 11 in 2015, n = 6 in 2016). Two loggers from 2014 contained no data.
Positions were estimated using the GeoLight package  in R . A threshold of 3 lx was used and sun elevation angles between −3° and 0° provided the best fit using Hill-Ekström calibration  (breeding area calibration produced similar spatiotemporal patterns, Additional file 1: Appendix S2-S4).
Periods of no overall change in longitude for ≥5 days were considered staging. We excluded latitude from positions within ten days of equinox. Position outliers >10° from median longitude/latitude at each staging site were excluded (Additional file 1: Appendix S1).
Normalized Difference Vegetation Index (NDVI) was used to estimate vegetation conditions . NDVI was obtained from the MODIS satellite product MOD13C1 . Mean NDVI within a radius of 50 km for each wintering site were extracted with the adehabitat R package .
Data were pooled for all analyses because t-tests revealed no differences between the two years in average latitude (p = 0.44), longitude (p = 0.79) or NDVI (p = 0.23).
The western ‘detour’ between the staging sites before and after the Sahara coincided with Equinox. We estimated average westernmost latitude projecting from the mean position of the last European staging sites assuming a speed of 300 km/day (daily migration speed of willow warblers ringed in Denmark ).
Longitudinal spread of birds in winter was estimated using the loxodromic distance between longitude of the centre of mass for all individuals and the latitude and longitude of the centre of mass for each individual in five-day intervals in R using SDMTools  and geosphere .
We correlated arrival date, body mass, wing length and NDVI with longitude to evaluate causes of winter spread using Pearson’s r (Note that a weak relationship with arrival date is expected because of extra travel time). We tested for consistent north-south or east-west directional changes and direction of change in NDVI between consecutive winter sites using Sign tests. Lastly, we investigated trends over time in NDVI within sites using Pearson’s r. Potential effects of variation in longitudinal distribution of NDVI in earlier years on termination of migration were investigated by correlating site-specific NDVI among the last three winters before capture.
Summary of migration timing and staging
69 ± 14 days
Number of staging sites (of 5 to 12 days duration)
1.7 ± 1.3
Duration of staging before Sahara
13.6 ± 7 days
Duration of staging after Sahara
4.3 ± 6.9 days
Most birds used at least two wintering sites, moving between late October and mid-December (Fig. 1a–d). The second site was south (p = 0.0005) and east (p = 0.0005) of the first. Moving to another site resulted in increased vegetation greenness (Fig. 2f; p = 0.02). Greenness generally decreased during an individual’s stay at each site (Fig. 2f; NDVI slope = −0.0025 ± 0.0002 day−1) until mid-January. From mid-January greenness increased for three of six stationary individuals.
Our results highlight considerable individual variation in spatiotemporal migration schedules of a small long-distance migrant, the willow warbler. Despite migration to sub-Saharan Africa on a narrow front, we found individuals dispersing widely in winter (>3000 km) by variable termination of migration, resulting in low connectivity (individuals spread over the known wintering area of the subspecies) and considerable difference in migration length among individuals. Vegetation conditions generally declined at each wintering site but within-winter movements toward southeast increased vegetation greenness.
The southwest autumn migration route from Europe to West Africa which turns anticlockwise after reaching the Sahel and with stopovers before and after the desert crossing is likely a common migration pattern for West European birds [15, 17–19] and following the coast rather than crossing the Sahara directly is presumably safer. The willow warblers wintered in the Savannah zone, which is well documented as the main wintering area . However, the easternmost sites were further east of the range considered for this subspecies  though separation of subspecies on the wintering grounds is not well known.
The very large longitudinal spread is surprising and in contrast to the high connectivity and limited west-east movement seen in for example common nightingale Luscinia megarhynchus . However, similar variable termination of migration also caused large within-population variation in wintering longitudes in some swift populations  but not others . Northern and eastern willow warblers migrate via the eastern Mediterranean  but it is unknown whether a similar spread westwards into western birds’ ranges occurs in winter. Such spread is assumed in lesser whitethroats Sylvia curruca that cross the eastern Mediterranean , turning westwards after crossing the Sahara, to winter in the Sahel zone from Ethiopia to Senegal .
We did not find support for a temporal progression in settling patterns that would suggest ‘filling up’ of arriving migrants as cause. Nor did we find a relationship between body size and longitude that could be expected if a dominance hierarchy or flying capability were determinant. Furthermore, the similar seasonal vegetation conditions in eastern and western wintering sites indicate that travelling further did not generally result in improved habitat conditions. Potentially, birds could be wintering where they successfully wintered in their first year as suggested by Cresswell . However, we found no support for termination of migration resulting from variation among years in geographic distribution of favourable vegetation conditions, which could potentially influence survival, nor did we find a difference between longitudes of tracked birds in the two years with tracks. Experience-dependent migration routes are known in other species (for example Eurasian honey buzzards Pernis apivorus [51, 52]) and because we only tracked experienced adults, birds on their first migration could potentially use a more direct route to the winter grounds without detouring along the coast in which case the spread would be a result of spread in innate migration directions . However, ring-recoveries indicate a general southwest migration direction  consistent with the route documented here and indicating that this route is also followed by first-time migrants. Furthermore, such differences between age classes are presumably a result of social learning in larger, diurnally migrating species [51, 52] which is unlikely to happen in the solitary migrating willow warblers.
Migrants are often considered generalists  and, compared to for example the common nightingale with high connectivity , the willow warbler is associated with a larger variety of more open habitats. Possibly, the spread of individuals could be attributed to generalist species  or species foraging in widely distributed open areas. Willow warbler populations appear to be in decline throughout Europe . The low connectivity is consistent with an overall decline associated to factors operating outside the breeding season combined with pronounced regional differences in trends related to breeding area conditions . Itinerant behaviour has also been reported in great reed warblers Acrocephalus arundinaceus  and common nightingales  but not in common redstarts Phoenicurus phoenicurus  and pied flycatchers Ficedula hypoleuca . Movement during winter resulted in improved vegetation conditions when conditions deteriorate in northern Savannah and Sahel areas, as found in Montagu’s harriers . Because spring migration could not be tracked, it remains unknown whether the autumn route is retraced or a more direct route is chosen (as indicated by more easterly ring recoveries in the Mediterranean in spring than autumn; ). Such tracking is dependent on further development of device capabilities.
Willow warblers migrate from North Europe to sub-Saharan Africa on a narrow front, but disperse widely over the Savannah zone after reaching the Sahel. During winter, birds moved southward tracking greenness in vegetation. Wintering sites were spread out with >3000 km between the most eastern and most western individual resulting in low connectivity. The low connectivity in this small, long distance generalist migrant wintering in the Savannah zone may be related to the species being a generalist and is consistent with factors operating outside the breeding area being responsible for the species’ decline.
We thank Troels Eske Ortvad and Jesper Johannes Madsen for fieldwork assistance and Bruno Correia de Freitas Cardoso for assisting with data processing and maps.
The Danish Council for Independent Research supported the MATCH project (1323-00048B) and Danish National Research Foundation supported Center for Macroecology, Evolution and Climate (DNRF96).
Data are available as Additional file 2.
All authors conducted the fieldwork, MLJ, KT, APT, MW performed the analyses. MLJ, KT wrote the manuscript. All authors approved the final version and agree to be held accountable for the manuscript’s content.
Tagging was approved by the Copenhagen Bird Ringing Centre (SN302–009) under permission from the Danish Forest and Nature Agency.
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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- Alerstam T. Bird migration. Cambridge Univ. Press; 1990.Google Scholar
- Greenberg R, Marra PP, Eds. Birds of Two Worlds. Johns Hopkins Univ. Press; 2005.Google Scholar
- Hahn S, Bauer S, Liechti F. The natural link between Europe and Africa – 2.1 billion birds on migration. Oikos. 2009;118:624–6.View ArticleGoogle Scholar
- Sanderson FJ, Donald PF, Pain DJ, Burfield IJ, van Bommel FPJ. Long-term population declines in afro-Palearctic migrant birds. Biol Conserv. 2006;131:93–105.View ArticleGoogle Scholar
- Bairlein F. Migratory birds under threat. Science. 2016;354:547.View ArticlePubMedGoogle Scholar
- Thorup K, Tøttrup AP, Willemoes M, Klaassen RHG, Strandberg R, Lomas Vega M, Dasari HP, Araújo MB, Wikelski M, Rahbek C. Resource tracking within and across continents in long-distance bird migrants. Sci Adv. 2017;3:e1601360.View ArticlePubMedPubMed CentralGoogle Scholar
- Webster MS, Marra PP, Haig SM, Bensch S, Holmes RT. Links between worlds: unraveling migratory connectivity. Trends Ecol Evol. 2002;17:76–83.View ArticleGoogle Scholar
- Finch T, Saunders P, Avilés JM, Catry I, de la Puente J, Emmenegger T, Mardega I, Mayet P, Parejo D, Račinskis E, Rodríguez-Ruiz J, Sackl P, Schwartz T, Tiefenbach M, Valera F, Hewson C, Franco A, Butler SJ. A pan-European, multi-population assessment of migratory connectivity in a near-threatened migrant bird. Divers Distrib. 2015;21:1051–62.View ArticleGoogle Scholar
- Hahn S, Amrhein V, Zehtindijev P, Liechti F. Strong migratory connectivity and seasonally shifting isotopic niches in geographically separated populations of a long-distance migrating songbird. Oecologia. 2013;173:1217–25.View ArticlePubMedGoogle Scholar
- Martin TG, Chadès I, Arcese P, Marra PP, Possingham HP, Norris DR. Optimal conservation of migratory species. PLoS One. 2007;2:e751.View ArticlePubMedPubMed CentralGoogle Scholar
- Ouwehand J, Ahola MP, Ausems ANMA, Bridge ES, Burgess M, Hahn S, Hewson CM, Klaassen RHG, Laaksonen T, Lampe HM, Velmala W, Both C. Light-level geolocators reveal migratory connectivity in European populations of pied flycatchers Ficedula hypoleuca. J Avian Biol. 2016;47:69–83.View ArticleGoogle Scholar
- Finch T, Butler SJ, Franco AMA, Cresswell W. Low migratory connectivity is common in long-distance migrant birds. J Anim Ecol. 2017;86:662–73.View ArticlePubMedGoogle Scholar
- Hilgerloh G. Radar observations of transsaharan migrants in southern Portugal. Ardeola. 1988;35:221–32.Google Scholar
- Hilgerloh G. Autumn migration of trans-Saharan migrating passerines in the straits of Gibraltar. Auk. 1989;106:233–9.Google Scholar
- Kristensen MW, Tøttrup AP, Thorup K. Migration of the common redstart (Phoenicurus phoenicurus): a Eurasian songbird wintering in highly seasonal conditions in the west African Sahel. Auk. 2013;130:258–64.View ArticleGoogle Scholar
- Arizaga J, Willemoes M, Unamuno E, Unamuno JM, Thorup K. Following year-round movements in barn swallows using geolocators: could breeding pairs remain together during the winter? Bird Study. 2015;62:141–5.View ArticleGoogle Scholar
- Gatter W. Vogelzug in Westafrika: Beobachtungen und Hypothesen zu Zugstrategien und Wanderrouten. Vogelwarte. 1987;34:80–92.Google Scholar
- Gatter W. Birds of Liberia. Sussex: Pica Press; 1997.Google Scholar
- Liechti F, Komenda-Zehnder S, Bruderer B. Orientation of passerine trans-Sahara migrants: the directional shift (Zugknick) reconsidered for free-flying birds. Anim Behav. 2012;83:63–8.View ArticleGoogle Scholar
- Valverde JA. Some observations on the migration through the occidental Sahara. Bull Br Ornithol Club. 1958;78:1–5.Google Scholar
- Biebach H. Strategies of trans-Sahara migrants. In: Gwinner E, editor. Bird migration. Berling: Springer-Verlag; 1990. p. 352–67.View ArticleGoogle Scholar
- Moreau RE. The Palaearctic-African bird migration systems. London: Academic Press; 1972.Google Scholar
- Trierweiler C, Mullié WC, Drent RH, Exo KM, Komdeur J, Bairlein F, Harouna A, de Bakker M, Koks BJ. A Palaearctic migratory raptor species tracks shifting prey availability within its wintering range in the Sahel. J Anim Ecol. 2013;82:107–20.View ArticlePubMedGoogle Scholar
- Emmenegger T, Hahn S, Bauer S. Individual migration timing of common nightingales is tuned with vegetation and prey phenology at breeding sites. BMC Biol. 2014;14:9.View ArticleGoogle Scholar
- Schmaljohann H, Buchmann M, Fox JW, Bairlein F. Tracking migration routes and the annual cycle of a trans-Sahara songbird migrant. Behav Ecol Sociobiol. 2012;66:915–22.View ArticleGoogle Scholar
- Hahn S, Emmenegger T, Lisovski S, Amrhein V, Zehtindjiev P, Liechti F. Variable detours in long-distance migration across ecological barriers and their relation to habitat availability at ground. Ecol Evol. 2014;4:4150–60.View ArticlePubMedPubMed CentralGoogle Scholar
- Lemke HW, Tarka M, Klaassen RHG, Åkesson M, Bensch S, Hasselquist D, Hansson B. Annual cycle and migration strategies of a trans-Saharan migratory songbird: a Geolocator study in the great reed warbler. PLoS One. 2013;8:e79209.View ArticlePubMedPubMed CentralGoogle Scholar
- Urban EK, Fry CH, Keith S. The birds of Africa. Vol. V: Academic Press; 2002.Google Scholar
- Bensch S, Bengtsson G, Åkesson S. Patterns of stable isotope signatures in willow warbler Phylloscopus trochilus feathers collected in Africa. J Avian Biol. 2006;37:323–30.View ArticleGoogle Scholar
- Chamberlain CP, Bensch S, Feng X, Åkesson S, Andersson T. Stable isotopes examined across a migratory divide in Scandinavian willow warblers (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) reflect their African winter quarters. Proc R Soc Lond B. 2000;267:43–8.View ArticleGoogle Scholar
- Hedenström A. Migration routes and wintering areas of willow warblers Phylloscopus trochilus (L.) ringed in Fennoscandia. Ornis Fenn. 1987;64:137.Google Scholar
- Curry-Lindahl K. Bird migration in Africa - movements between six continents. London: Academic Press; 1981.Google Scholar
- Salewski V, Bairlein F, Leisler B. Different wintering strategies of two Palearctic migrants in West Africa: a consequence of foraging strategies? Ibis. 2002;144:85–93.View ArticleGoogle Scholar
- Runge CA, Martin TG, Possingham HP, Willis SG, Fuller RA. Conserving mobile species. Front Ecol Environ. 2014;12:395–402.View ArticleGoogle Scholar
- Cresswell W. Migratory connectivity of Palaearctic-African migratory birds and their responses to environmental change: the serial residency hypothesis. Ibis. 2014;156:493–510.View ArticleGoogle Scholar
- Reilly JR, Reilly RJ. Bet-hedging and the orientation of juvenile passerines in fall migration. J Anim Ecol. 2009;78:990–1001.View ArticlePubMedGoogle Scholar
- Fudickar AM, Wikelski M, Partecke J. Tracking migratory songbirds: accuracy of light-level loggers (geolocators) in forest habitats. Methods Ecol Evol. 2012;3:47–52.View ArticleGoogle Scholar
- Lisovski S, Hewson CM, Klaassen RHG, Korner-Nievergelt F, Kristensen MW, Hahn S. Geolocation by light: accuracy and precision affected by environmental factors. Methods Ecol Evol. 2012;3:603–12.View ArticleGoogle Scholar
- Naef-Daenzer B. An allometric function to fit leg-loop harnesses to terrestrial birds. J Avian Biol. 2007;38:404–7.View ArticleGoogle Scholar
- Lisovski S, Hahn S. GeoLight - processing and analysing light-based geolocation in R. Methods Ecol Evol. 2013;3:1055–9.View ArticleGoogle Scholar
- R Core Team. R: A Language and Environment for Statistical Computing. Vienna, Austria: R Foundation for Statistical Computing;2016.Google Scholar
- Tucker CJ. Red and photographic infrared linear combinations for monitoring vegetation. Remote Sens Environ. 1979;8:127–50.View ArticleGoogle Scholar
- Didan K. MOD13C1 MODIS/Terra Vegetation Indices 16-Day L3 Global 0.05Deg CMG V006. NASA EOSDIS Land Processes DAAC; 2015. https://doi.org/10.5067/modis/mod13c1.006.
- Calenge C. The package adehabitat for the R software: a tool for the analysis of space and habitat use by animals. Ecol Model. 2006;197:516–9.View ArticleGoogle Scholar
- Bønløkke J, Madsen JJ, Thorup K, Pedersen KT, Bjerrum M, Rahbek C. The Danish Bird Migration Atlas. Copenhagen: Rhodos; 2006.Google Scholar
- Van Der Wal J, Falconi L, Januchowski S, Shoo L, Storlie C. SDMTools: Species Distributions Modelling Tools: Tools for processing data associated with species distribution modelling exercises. Copenhagen; 2014. R package version 1.1–221. https://CRAN.R-project.org/package=SDMTools
- Hijmans RJ. Geosphere:Spherical Trigonometry. 2016. R package version 1.5–5. https://CRAN.R-project.org/package=geosphere
- Jacobsen LB, Jensen NO, Willemoes M, Hansen L, Desholm M, Fox AD, Tøttrup AP, Thorup K. Annual spatiotemporal migration schedules in three larger insectivorous birds: European nightjar, common swift and common cuckoo. Anim Biotelem. 2017;5:4.Google Scholar
- Åkesson S, Klaassen R, Holmgren J, Fox JW, Hedenström A. Migration routes and strategies in a highly aerial migrant, the common swift Apus apus, Revealed by Light-Level Geolocators. PLoS ONE. 2012;7:e41195.Google Scholar
- Fransson T, Jakobsson S, Kullberg C. Non-random distribution of ring recoveries from trans-Saharan migrants indicates species-specific stopover areas. J. Avian Biol. 2005;36:6–11.View ArticleGoogle Scholar
- Hake M, Kjellén N, Alerstam T. Age-dependent migration strategy in honey buzzards Pernis apivorus tracked by satellite. Oikos. 2003;103:385–96.View ArticleGoogle Scholar
- Agostini N. Additional observations of age-dependent migration behaviour in western honey buzzards Pernis apivorus. J Avian Biol. 2004;35:469–70.View ArticleGoogle Scholar
- Ivande ST, Cresswell W. Temperate migrants and resident bird species in afro-tropical savannahs show similar levels of ecological generalism. Ibis. 2016;158:496–505.View ArticleGoogle Scholar
- Gregory RD, van Strien A, Vorisek P, Gmelig Meyling AW, Noble DG, Foppen RPB, Gibbons DW. Developing indicators for European birds. Phil Trans R Soc B. 2005;360:269–88.View ArticlePubMedPubMed CentralGoogle Scholar
- BirdLife International. The BirdLife checklist of the birds of the world: version 8. 2015.Google Scholar
- Morrison CA, Robinson RA, Clark JA, Gill JA. Spatial and temporal variation in population trends in a long-distance migratory bird. Diversity Distrib. 2010;16:620–7.View ArticleGoogle Scholar
- Arbonnier M. Trees, shrubs and lianas of west African dry zones. Paris: CIRAD; 2002.Google Scholar