131
Great flexibility in autumn movement patterns of European gadwalls Anas strepera
Andrea Gehrold, Hans-Günther Bauer, Wolfgang Fiedler and Martin Wikelski
A. Gehrold (andrea.gehrold@hotmail.de), H.-G. Bauer, W. Fiedler and M. Wikelski, Dept of Migration and Immuno-ecology, Max Planck Inst.
for Ornithology, Am Obstberg 1, DE-78315 Radolfzell, Germany, and Dept of Biology, Univ. of Konstanz, DE-78457 Konstanz, Germany.
The annual migration cycle of waterbirds often involves several distinct movement stages, for example within-winter movements or the moult migration during summer, which require a high degree of individual flexibility in migration direction. Here, we investigate whether such flexibility is a common characteristic of waterbird migration by analysing movement behaviour of a dabbling duck, the gadwall Anas strepera, during the little studied, intermediate autumn period. The tracking of individuals via satellite transmitters (n 7) as well as the ring re-encounter analysis of three European gadwall populations (Germany, England, Russia) revealed that autumn movements were multidirectional.
Furthermore, the comparison with winter re-encounters suggested that autumn movements were partly independent of the movements towards subsequently used south to southwestern wintering areas. Some individuals even travelled long distances north- or eastwards. Accordingly, some autumn locations were characterized by a harsh climate, thus serving as temporary staging sites but necessitating further movements when wetlands freeze during winter. The occurrence of such detours or reversals of migration was confirmed by the transmitter data. Inter-individual variability in distance and direction of autumn movements was found for both sexes and age-classes indicating that gadwalls, in general, followed flexible movement strategies. Based on the extent of multidirectional autumn movements, we hypothesize important benefits of such flights and suggest that the analysis of year-round movement patterns of individual animals during their distinct life-history stages is essential to understand how they can successfully reproduce and survive.
The power of flight enables birds to exploit distant areas that offer optimal temporary breeding and survival habitats at different stages of the annual cycle (Alerstam and Högstedt 1982). When moving between these areas, most habitat generalists and terrestrial birds can navigate along an environmental gradient, while others, particularly habitat specialists and waterbirds, depend on a patchy distributed resource (Bensch 1999). This dependence is, for example, pronounced in anatid species (ducks, geese and swans) which spend almost all their life on or along water bodies.
Even in areas where water bodies appear to be abundant, for instance in Europe, only a few wetland types may fulfil the seasonal demands of a certain species (Nilsson and Nilsson 1978, Suter 1994). Furthermore, temporary events such as rainfall, drought or ice cover may further alter the avail- ability of wetlands.
Anatids may therefore strongly benefit from a goal- oriented migration resulting in the encounter of distinct, seasonally suitable wetlands. This strategy implies that indi- viduals acquire the essential knowledge about the location of suitable sites and about the pathways that connect them by following experienced conspecifics or by performing inde- pendent exploratory movements (Wolff 1970). Furthermore,
this strategy requires a relatively high degree of flexibility, thus enabling individuals to adjust the direction and distance of migration according to the distribution of habitat patches that offer suitable conditions during a particular life history stage (Van Toor et al. 2013).
Several studies indicate that such flexibility is indeed common in anatids, particularly in duck species: 1) paired male ducks follow females during their migration towards breeding sites (Schüz et al. 1971, Greenwood and Harvey 1982); 2) dependent on breeding status, males and females may perform undirected movements to well-defined moulting sites during summer (Salomonsen 1968, Yarris et al. 1994, Oppel et al. 2008); and 3) dependent on local weather and feeding conditions, both sexes may perform large-scale, sometimes multidirectional within-winter movements (Keller et al. 2009, Sauter et al. 2010). A recent study on common pochards Aythya ferina and tufted ducks Aythya fuligula suggested that multidirectional movements may occur as early as the pre-winter season (from October onwards; Gourlay-Larour et al. 2012). Here we go one step further and start the movement analysis in August, i.e. during the post-fledging period of juvenile ducks and the post-moulting period of adult ducks (Pyle 2005).
Erschienen in: Journal of Avian Biology ; 45 (2014), 2. - S. 131-139
Considering the generally patchy distribution of inland waters as well as the seasonal changes in their suitability between autumn and winter (when food supply decreases and high latitude waters freeze over), we hypothesize that autumn movements (Aug–Oct) may represent a discrete part of the annual migration cycle of ducks that is not necessarily consistent with movements towards final winter- ing grounds. In support of this hypothesis, we expect vari- ability in the direction and destination of autumn movements to be more pronounced than during subsequent winter movements, when harsh climatic conditions should restrict movements towards warmer, generally more southerly locations. In addition, we expect that undirected autumn movements are 1) more frequent in juveniles which cannot rely upon prior knowledge about suitable autumn and winter habitats; and 2) less frequent in females than in males, because females moult later in the season (Ringelman 1990, Gehrold and Köhler 2013) and consequently suffer stronger temporal constraints during the subsequent move- ments towards wintering sites.
We test these predictions in the European gadwall Anas strepera by means of ring re-encounter data of different populations and by tracking individual birds continuously via GPS satellite transmitters. The gadwall has been continu- ously increasing in Europe during recent decades (Cramp and Simmons 1977, Bauer et al. 2005), presumably in response to the eutrophication of wetlands and the creation of artificial reservoirs (Fox 2005). It therefore seems to be a good candidate to evaluate flexible movement strategies that allow for the detection and utilization of seasonal, high quality habitats.
Methods
Study species
The gadwall prefers shallow, eutrophic waters and mainly feeds on submerged and emergent vegetation (Fox 2005). In Europe, its migration towards breeding grounds peaks in March–April (Bauer et al. 2005). After breeding, a distinct moult migration may lead males as well as females to well-defined moulting areas where large aggregations can be found (Köhler and Köhler 2009). The ‘Ismaning reservoir with fish ponds’ in Germany and the Volga Delta in Russia belong to the most frequented moulting areas of the European gadwall (Cramp and Simmons 1977, Scott and Rose 1996, Köhler and Köhler 2009) and both these key sites were considered as the origin of subsequent autumn movements in our analysis. Autumn movements take place as early as September and most birds will arrive at
their final wintering sites by December (Bauer et al. 2005).
At this stage, many individuals have already formed pairs for the next breeding season (Paulus 1983, Köhler 1991).
Tagging and tracking of individuals
Gadwalls were trapped at the wetland complex ‘Ismaning reservoir with fish ponds’ in southeastern Germany (48o12aN, 11o43aE; hereafter referred to as ‘Ismaning’) during summer (2009–2011). This area is extraordinary in support- ing primarily moult migrants and daily maxima of about 15 000 gadwalls are regularly recorded during the wing moulting season (Köhler and Köhler 2009).
Overall, 95 gadwalls were caught in un-baited swim-in traps (Köhler 1986), sexed, aged and ringed. All of them were adults ( 1 yr). In 2009 and 2010, 23 individuals were equipped with backpack satellite transmitters by use of the harness described by Roshier and Asmus (2009).
However, because of equipment failure, which is common in ducks (Van Toor et al. 2013), only six males and one female could be tracked after leaving the moulting site. Note that the initial sample was already male-biased (18 males, 5 females), as we tried to avoid using the heaviest device (type 1, see Table 1) for lightweight females. The transmitters eventually accounted for 2.3–4.1% of body weight and an overview of the three types of deployed devices is given in Table 1.
Location data were automatically downloaded from the ARGOS webpage ( www.argos-system.org ) and stored in Movebank ( www.movebank.org , Fiedler and Davidson 2012). Type 1 transmitters (Table 1) provided highly accurate GPS data. For type 2 and 3, accuracy of each satellite fix was given by the Argos location class (LC).
We included all data accurate to within 1.5 km (LC 1, 2, 3; CLS 2011). Although location deviations can be higher for LC 0, A, B (CLS 2011), we accepted these coordinates if verified by other fixes. We manually checked the data for outliers and exported the cleaned tracks to a Movebank library (doi: 10.5441/001/1.26dg08hv).
All movements of birds with transmitters were evaluated with regard to season and, if possible time of day.
Migration directions and distances were calculated using loxodromes (Imboden and Imboden 1972) and staging sites were identified. Furthermore, minimum stopover times were calculated based on the first and last satellite fix at each staging site.
The remaining 72 trapped gadwalls (38 males and 34 females) were tagged with nasal saddles labelled with individual alpha-numeric codes, following the method described by Rodrigues et al. (2001). Annual resighting activities at the moulting site Ismaning confirmed that
Table 1. Microwave satellite transmitters deployed on the seven gadwalls (six males, one female) that could be tracked after leaving the moulting site. Note that 16 other transmitters (13 type 1, 2 type 2, 1 type 3) failed before departure, i.e. within two months.
Type Device Weight (g) No. of tags Duty cycle Maximum location accuracy (m) No. of days operating
1 Solar Argos/GPS PTT 32 4 4 fixes d1 18 69–233
2 Solar GPS PTT 18 1 10 h on, 2 d off 250 78
3 Battery GPS PTT 22 2 10 h on, 10 d off 250 87–322
PTT Platform transmitter terminal.
Maximum location accuracy: Microwave Telemetry (www.microwavetelemtry.com), CLS (2011).
most birds were still alive and in good condition after wearing the nasal saddle for one year or longer; a result that will be addressed in a subsequent study. After departure from the moulting area, 19 of the 72 individuals were resighted by birdwatchers during the subsequent autumn and winter period. However, only one re-encounter location per bird could be obtained, precluding the identi- fication of real ‘tracks’ by means of resighting data. We therefore treated resighted birds in the same way as ringed birds. The exception was one male with nasal saddle that was recorded at several locations. The movement of this male will be described in detail in the results part.
Autumn and winter migration in three European gadwall populations
European ringing data were obtained from the EURING database (1930–2009, www.euring.org ). We selected gadwalls that were ringed during the astronomical summer (after 21 June) and re-encountered at distances 40 km during the following months. Direct re-encounters between August and October were classified as movements towards autumn staging sites, whereas direct re-encounters between November and 21 March, when harsh winter conditions may occur, were classified as movements towards wintering grounds.
The dataset was restricted to individuals ringed in southern Germany (n 146), England (n 98) and the European part of Russia (n 60) because data for other regions only contained single records. All German birds were ringed at the moulting site Ismaning. Here, we added recent information on the movements of gadwalls tagged with nasal saddles at Ismaning in 2009–2011 (n 19). We also included the data obtained from transmitters (n 7) using the first verified relocation ( 40 km) of each bird. Two Russian birds were excluded from the Russian sub-sample after first inspection of the data. These two March recoveries came from western Siberia and did not reflect the location of winter residence but an early return to the breeding grounds.
Re-encounters were accurate in time to 2 weeks and in space to 5 km (except one bird that was recovered within 20 km). Whenever possible, we differentiated between males and females and between juveniles (1st year of life) and adults ( 1 yr).
Temperature data
We aimed to investigate whether autumn movements either led birds to suitable wintering sites or were chosen by use of other criteria that might involve instantaneous or future advantages but necessitate further movements when tem- peratures drop and wetlands start to freeze. Therefore, cur- rent climate data (1950–2000) at the highest available spatial resolution (^ 1 km) were downloaded from WorldClim ( www.worldclim.org/bioclim ). The bioclimatic variable
‘mean temperature during the coldest quarter of the year’
was chosen as a measure of local winter conditions with regard to the occurrence of cold spells and the resulting emergence of ice cover on water bodies. Corresponding val- ues were extracted for the autumn re-encounter locations,
i.e. potential wintering sites, and for actual winter re- encounter locations. These temperature data were trans- formed into a binary response to represent values below and above freezing ( 0oC versus 0oC).
Data analysis
Statistical analyses were performed in R 2.15.0 (R Develop- ment Core Team 2012). Migration directions of gadwalls ringed in Germany (including birds with nasal saddle or transmitter), England and Russia were investigated on the population-specific level due to differences in wintering dis- tributions (Scott and Rose 1996) and in local geography (e.g. occurrence of natural barriers). The mean migration direction was calculated for autumn and winter re-encoun- ters and tested for significance with Rao’s spacing test (Batschelet 1981). The circular statistics program Oriana (Kovach 2004) was implemented to calculate the length of the mean vector (‘r-Vector’) as a measure of directional con- centration and to compare the distribution of bearings dur- ing autumn and winter with a Mardia–Watson–Wheeler test (Batschelet 1981).
Next, we set a 90o sector around the mean direction towards wintering areas and classified all autumn movements as either consistent or inconsistent with this main winter bearing. To investigate whether the age classes (adults versus juveniles) or sexes differ in their affinity to perform autumn movements that are directed away from the main wintering sector, we selected birds of known age and/
or sex. For this analysis, the German and the English samples were combined to increase overall sample size (Germany:
nage 49, nsex 45; England: nage 26, nsex 19). The complete sample of Russian birds had to be excluded from this analysis due to missing information. Testing the interaction of age and sex was not feasible due to a small sample of birds for which both these traits were known.
Consequently, age- and sex-specific effects were tested in independent generalized linear mixed models (GLMMs), using Bayesian statistics (Supplementary material Appendix 1, Table A1).
Similarly, distances of autumn movements were com- pared between adults and juveniles and between males and females. Distances were log-transformed and analysed dependent on direction, age/sex and the interaction direction:age/sex. We fit general linear mixed models (LMMs) to the data and tested individual effects via likeli- hood ratio tests (LRTs, Supplementary material Appendix 2, Table A2).
Finally, generalized linear models followed by LRTs were used to compare the proportion of autumn and winter recoveries that were located in areas of a relatively cold ( 0oC) or mild ( 0oC) winter climate.
Results
Tracking of individuals
The data obtained from satellite transmitters revealed that movements from the moulting location to an autumn stag- ing site were multidirectional. Some gadwalls moved to the
later, it migrated 360 km southeast to Lake Neusiedl, Austria/Hungary. Another male (no. 91770, Fig. 1c) moved 205 km to the southwest. After two months at Lake Constance, it flew 80 km back to the northeast to the river Iller. Finally, it continued its migration with a southwest bearing to reach its wintering site at the High Rhine, Switzerland (150 km). An even more pronounced reversal of movement directions and distances was observed in a male tagged with a nasal saddle (no. O8, Fig. 1a). Post- moulting, it was recorded southwest at Lake Neuchâtel, Switzerland. It subsequently returned to the moulting area – a total journey of at least 830 km. Finally, it moved again to Switzerland, this time to Lake Lucerne (276 km).
Three gadwalls with transmitters were also tracked during migratory flights. These flights were rapid and highly target-oriented, following an almost straight line between origin and destination (Fig. 1b, c; doi: 10.5441/001/
1.26dg08hv). Records of three other gadwalls tracked earlier in the season, during departure from their breeding grounds, confirmed this pattern of highly directed move- ments (transmitters no. 91736, no. 91784, no. 91801;
doi: 10.5441/001/1.26dg08hv).
Whenever gadwalls could be tracked after arrival at an autumn staging site, they stayed for at least two weeks (Fig. 1; doi: 10.5441/001/1.26dg08hv). Birds that could be tracked during winter (transmitters no. 41731, no. 77200, no. 91770) were stationary and used specific wetlands for extended periods ( 2 months; Fig. 1b, c; doi: 10.5441/
001/1.26dg08hv).
Autumn and winter migration in three European gadwall populations
The majority of gadwalls originating from southeastern Germany wintered in France, northern Italy and Spain (Fig. 2). Gadwalls ringed during summer in England stayed on the island or wintered along the northwestern to southwestern part of the European continent (Fig. 2).
Russian gadwalls, especially those that were ringed at the Volga Delta, wintered along the western coast of the Caspian Sea (Fig. 2). During autumn (Aug–Oct), some birds could already be recorded south to southwest, close to the identified wintering areas. However, as shown above in the individual tracking data, several individuals also per- formed autumn movements in opposing directions, some- times travelling hundreds of kilometres to the northwest, north and northeast (Fig. 2, 3).
There was a clear directional preference during both autumn and winter (Rao’s spacing test: all p 0.05), except for English autumn migrants (p 0.1; Fig. 3). How- ever, the distribution of bearings significantly changed between seasons in all three populations (Mardia–Watson–
Wheeler test: Germany W 10.42, p 0.005; England W 6.22, p 0.03; Russia W 8.27, p 0.016; Fig. 3).
Autumn re-encounters were in general less directionally concentrated than winter re-encounters, yet English birds exhibited little directional concentration during both seasons (see r-vector, Fig. 3).
There was no difference in the likelihood of adults and juveniles or of males and females to deviate from subsequent wintering areas during autumn (GLMM: 0 Figure 1. Movements of gadwalls after departure from the moulting
site in southeastern Germany (‘M’). (a) Overview of the recorded movements of seven gadwalls tracked via satellite transmitters (including one female no. 77200, pink track) and of one gadwall male tagged with nasal saddle (no. O8, orange track marked by asterisk). Large filled dots indicate verified stopovers of 2 weeks. (b)–(c) Birds tracked during migration (solid lines). Dashed lines represent unverified movement paths ( 3 d without localization). Large filled dots show satellite fixes during stopover periods ( 2 weeks) and small open dots show satellite fixes during migration.
southwest, whereas others left to the north or northeast (Fig. 1a). Continuous tracking of several individuals revealed that the wetlands chosen as staging sites were not necessarily located en route to subsequently used autumn habitats or final wintering grounds. For example, one male (no. 41731, Fig. 1b) moved 242 km northeast to a small pond area in the Czech Republic in September. Two weeks
Figure 2. Migration of gadwalls ringed during summer in Germany, England and Russia and re-encountered in 40 km distance during the following autumn (Aug–Oct) or winter period (Nov–21 Mar). Note the different latitudinal and longitudinal scales for Russian migrants.
Figure 3. Directions of autumn and winter movements 40 km of gadwalls ringed during summer in Germany, England and Russia.
The arrows point to significant mean directions. Arrow lengths represent the standardized lengths of the r-vectors, a measure of the concentration of taken bearings. Grey sectors depict the mean 90o sector of winter re-encounters.
than autumn locations for German and Russian gadwalls (LRT: all p 0.001). During autumn, 35% of the German birds and 89% the Russian birds were located in areas that would later experience mean winter temperatures below freezing, i.e. areas inappropriate for wintering ducks (Fig. 5).
During winter itself, German and Russian gadwalls gathered at warmer areas (mean temperature 0oC; Fig. 5). In con- trast to the continental populations, English birds were exclusively recorded in mild climatic areas during all periods investigated (Fig. 5).
Discussion
The overall annual movement of animals involves different types of movements such as migrations between breeding and non-breeding grounds which represent a response to predictable seasonal changes in requirements and environ- mental conditions (Alerstam and Högstedt 1982, Ramenof- sky and Wingfield 2007). The migratory cycle of anatid birds is often more complex and may involve distinct movements in advance of wing moult or during winter ( Salomonsen 1968, Berthold 1990, Oppel et al. 2008).
Here, we observed another type of movement in a water- bird species: autumnal movements that were apparently characterized by high inter-individual variability in migra- tion direction and destination. In addition, as suggested by the comparison of autumn (Aug–Oct) and winter re-encounters (Nov–Mar) of European gadwalls, autumn movements were partly independent of the prevalent, south to southwesterly direction towards final wintering grounds, including some individuals that moved large distances in opposite directions.
On the population-specific level, we found that gadwalls originating from England exhibited the lowest differentia- tion between autumn and wintering locations. This was largely due to the fact that undirected winter re-encounters were more common in this population. For example, some birds wintered in the north while staying on the British Isles. Nevertheless, all English gadwalls experienced included in 95% credible intervals of posterior distribution,
Supplementary material Appendix 1, Table A1; Fig. 4a).
Indeed, 38–58% of individuals of one age class or sex were re-encountered at autumn locations that did not cor- respond to the route towards main wintering locations.
This finding could not be attributed to an increased direc- tional variability of English migrants, in general, but held also true when German birds were tested separately (42–50%
deviated).
Similarly, there was no age- or sex-specific effect on the distances covered during autumn (LRT: all p 0.05, Supplementary material Appendix 2, Table A2; Fig. 4b).
However, the distances covered were shorter when their direction deviated from the direction to the main wintering distribution (LRT: p 0.001, Supplementary material Appendix 2, Table A2).
The role of winter temperature
The analysis of mean temperatures during the coldest quarter of the year at re-encounter locations revealed that final win- ter locations were characterized by a milder winter climate Figure 4. Movement behaviour of the two age classes and sexes during autumn (Aug–Oct). The German and English samples were combined. (a) Percentage of gadwalls moving towards (grey) or deviating from the mean 90o sector of the final winter distribu- tion (white, see Fig. 3). (b) Mean distance ( CI) travelled during autumn.
Figure 5. Autumn and winter locations of gadwalls ringed in Germany, England and Russia that are in general characterised by a harsh or a mild winter climate (average temperature 0oC versus 0oC). The numbers in the column sections show the number of re-encounters.
autumn independent of age and sex? We suggest, based on the large extent of multidirectional autumn movements, that individuals may derive a direct advantage from them.
For example, gadwalls may exploit temporarily superior feeding grounds. They would thus be able to recover body stores depleted during the preceding breeding and/or moulting period (Oring 1969, King and Fox 2012, Gehrold and Köhler 2013) or to accumulate fat stores in preparation for winter when resources are most restricted for dabbling ducks (Alerstam and Högstedt 1982).
Not mutually exclusive, gadwalls that fly towards final wintering grounds during autumn may approach known wetlands or may sample the quality of potential wintering locations (e.g. wetland characteristics, food supply, com- petition). Such ‘exploratory’ winter movements have, for instance, been observed in common pochards and tufted ducks (Gourlay-Larour et al. 2012). Similarly, Lewis et al.
(2010) could show that pre-moulting brent geese Branta bernicla nigricans visited several wetlands before choosing their final moulting site.
A test of the hypotheses we propose above requires mea- surements of habitat conditions at individually chosen water bodies. Furthermore, it would be an important future avenue for research to link specific migratory habits to indi- vidual fitness consequences. This task was impossible to address in our study because most transmitters had failed by late winter and most ringed birds were shot. The latter point, however, raises another important issue: the impact of the local hunting pressure on the temporal and spatial distribu- tion of ring recoveries (Perdeck 1977, Hofer et al. 2005). For example, most gadwalls were re-encountered in southern and western European countries where hunting pressure is much more intense than in northern or eastern countries (Fox and Mitchell 1988, Hirschfeld and Heyd 2005). Simi- larly, resighting data may be biased according to the density of human settlements and the local intensity of birdwatching activities. Since the equipment with nasal saddles has started at the German moulting site in 2009, there was, for instance, no resighting in eastern European countries although this region is supposed to support a good part of the gadwall’s breeding population (Köhler 1991, Scott and Rose 1996, Bauer et al. 2005). Based on these local differences, it seems likely that the proportion of gadwalls that move to the north or east (i.e. to staging sites beyond the defined non-breeding range) may be even higher than reported in our study.
In summary, the great flexibility observed in migrating gadwalls during autumn appears to be consistent with, and likely surpasses, individual, variable movements of ducks during other parts of the annual cycle (Salomonsen 1968, Keller et al. 2009, Sauter et al. 2010). Furthermore, ring recoveries as well as genetic analyses indicate that European dabbling and diving ducks frequently connect flyways that were formerly thought to be separated (Guillemain et al.
2005, Keller et al. 2009, Kraus et al. 2011, Liu et al. 2012).
This flexibility in migration behaviour may strongly enhance the ability of duck species to discover suitable water bodies, despite the spatially and temporally patchy distribution of this habitat type. The success of this movement strategy, however, depends on the general availability of wetlands that meet the species-specific demands – a prerequisite that becomes increasingly important in densely settled areas consistently mild temperatures. There are apparently fewer
cold areas within a given distance from an English than from a continental ringing site, facilitating counter- intuitive, northward winter movements and confirming the role of Great Britain as a favoured wintering area of European gadwalls (Fox and Mitchell 1988, Fox and Salmon 1989).
In contrast, German and Russian gadwalls that moved to the north and east in autumn would have experienced sub- zero temperatures during the subsequent winter months.
Associated changes in the availability of ice-free water bod- ies would thus have forced the birds to leave these locations later on. Furthermore, the observed concentration of German and Russian gadwalls in sufficiently warm (non- freezing) areas during winter indicated that even individuals that first move north- or eastwards in autumn may subse- quently fly south or westwards during their migration towards wintering sites.
Direct evidence for such movement detours came from some individuals that were tracked after departure from the German moulting site. Continuous tracks or repeated resightings could only be obtained for few individuals.
Nevertheless, even within this small sample of birds, the directions of autumn movements scattered from southwest to northeast. Some autumn movements also included a par- tial or even complete reversal of movement directions and of distances up to several 100 km. In other avian species, reverse movements of this kind occur primarily when migrants are confronted with adverse weather conditions or an ecological barrier (Schüz et al. 1971, Åkesson et al. 1996).
However, gadwalls with transmitters 1) moved rapidly and highly target-oriented independent of direction; 2) neglected numerous water bodies en route; and 3) used a targeted wetland for at least two weeks. Therefore, it is conceivable that the tracked birds, all of them being adults, used prior experience to navigate to specific wetlands (see also Roshier et al. 2008a). Wikelski et al. (2003) showed that migratory flight is not as costly as generally expected, indicating that the benefits of using specific, profitable staging sites may outweigh the relative costs of movement detours.
The ring re-encounter analysis of German and English gadwalls further suggested that multidirectional autumn movements were as common in experienced adults as in inexperienced juveniles. Similarly, both sexes were shown to have a broad, individually variable movement distribution during autumn. However, the interaction of age and sex could not be tested due to small sample size. Furthermore, it was impossible to determine whether individuals tended to move independently or rather followed other (experi- enced) individuals in space and time. Considering the early pair formation of the gadwall, which starts as early as summer (Köhler 1991), we hypothesize that at least paired individuals migrate together. This hypothesis is consistent with the fact that males and females of other early-pairing ducks, e.g. mallard Anas platyrhynchos and wood duck Aix sponsa, also gather in the same non-breeding areas (Perdeck and Clason 1983, Hepp and Hines 1991), whereas males and females of later pairing species often show a sex-specific spatial differentiation (Perdeck and Clason 1983, Owen and Dix 1986, Carbone and Owen 1995).
How can we consequently explain the high inter- individual variability in movement behaviour during
Fox, A. D. and Mitchell, C. 1988. Migration and seasonal distribution of gadwalls from Britain and Ireland: a preliminary assessment. – Wildfowl 39: 145–152.
Fox, A. D. and Salmon, D. G. 1989. The winter status and distribution of gadwall in Britain and Ireland. – Bird Study 36: 37–44.
Gehrold, A. and Köhler, P. 2013. Wing-moulting waterbirds maintain body condition under good environmental conditions:
a case study of gadwalls (Anas strepera). – J. Ornithol. 154:
783–793.
Gourlay-Larour, M.-L., Schricke, V., Sorin, C., l’Hostis, M. and Caizergues, A. 2012. Movements of wintering diving ducks:
new insights from nasal saddled individuals. – Bird Study 59: 266–278.
Greenwood, P. J. and Harvey, P. H. 1982. The natal and breeding dispersal of birds. – Annu. Rev. Ecol. Syst. 13: 1–21.
Guillemain, M., Sadoul, N. and Simon, G. 2005. European flyway permeability and abmigration in teal Anas crecca, an analysis based on ringing recoveries. – Ibis 147: 688–696.
Hepp, G. R. and Hines, J. E. 1991. Factors affecting winter distribution and migration distance of wood ducks from southern breeding populations. – Condor 93: 884–891.
Hirschfeld, A. and Heyd, A. 2005. Jagdbedingte Mortalität von Zugvögeln in Europa: Streckenzahlen und Forderungen aus Sicht des Vogel- und Tierschutzes. – Ber. Vogelschutz 42: 47–74.
Hofer, J., Korner-Nievergelt, F., Korner-Nievergelt, P., Kestenholz, M. and Jenni, L. 2005. Herkunft und Zugverhalten von in der Schweiz überwinternden Reiherenten Aythya fuligula:
eine Ringfundanalyse. – Ornithol. Beob. 102: 181–204.
Hohman, W. L., Ankney, C. D. and Gordon, D. H. 1992. Ecology and management of post-breeding waterfowl. – In: Batt, B. D.
J., Afton, A. D., Anderson, M. G., Ankney, C. D., Johnson, D. H., Kadlec, J. A. and Krapu, G. L. (eds), Ecology and management of breeding waterfowl. Univ. of Minnesota Press, pp. 128–189.
Imboden, C. and Imboden, D. 1972. Formel für Orthodrome und Loxodrome bei der Berechnung von Richtung und Distanz zwischen Beringungs- und Wiederfundort. – Vogelwarte 26:
336–346.
Keller, I., Korner-Nievergelt, F. and Jenni, L. 2009. Within-winter movements: a common phenomenon in the common pochard Aythya ferina. – J. Ornithol. 150: 483–494.
King, R. and Fox, A. D. 2012. Moulting mass dynamics of female gadwall Anas strepera and male wigeon A. penelope at Abberton Reservoir, southeast England. – Bird Study 59:
252–254.
Köhler, P. 1986. Die Entenfanganlage am Ismaninger Speichersee.
– Anz. Orn. Ges. Bayern 25: 1–10.
Köhler, P. 1991. Mauserzug, Schwingenmauser, Paarbildung und Wegzug der Schnatterente Anas strepera im Ismaninger Teichgebiet. – Orn. Anz. 30: 115–149.
Köhler, U. and Köhler, P. 2009. Saisonale Dynamik und Bestandsentwicklung von mausernden Wasservögeln (Anatidae, Podicipedidae, Rallidae) am Ismaninger Speichersee mit Fischteichen. – Orn. Anz. 48: 205–240.
Kovach, W. 2004. Oriana v. 2.02a. – Kovach Computing Services, Anglesey.
Kraus, R. H. S., Zeddeman, A., van Hooft, P., Sartakov, D., Soloviev, S. A., Ydenberg, R. C. and Prins, H. H. T.
2011. Evolution and connectivity in the world-wide migration system of the mallard: inferences from mitochondrial DNA.
– BMC Genet. 12: 99.
Lewis, T. L., Flint, P. L., Schmutz, J. A. and Derksen, D. V. 2010.
Pre-moult patterns of habitat use and moult site selection by brent geese Branta bernicla nigricans: individuals prospect for moult sites. – Ibis 152: 556–568.
where wetlands are strongly affected by anthropogenic disturbance (Hohman et al. 1992). Hence, we encourage further studies on the year-round migratory habits of waterbirds. Such knowledge could be crucial for the con- servation of wetlands and waterbird populations as well as for predicting the emergence and spread of avian-induced diseases (Takekawa et al. 2010). In addition, waterbirds and other habitat specialists seem to be promising candidates to analyse individual differences in behavioural and migratory strategies (Roshier et al. 2008a, b) and may provide impor- tant insights into bird migration in general.
Acknowledgements – We thank Christine Gehrold, Andreas Schmidt, Peter Köhler, Ursula Köhler, Doris Matthes, Christian Hemmer, Karin Haas, Eberhard von Krosigk, Erwin Taschner and Hanna Prüter for field assistance. We also thank the involved field orni- thologists, coordinators and ringing schemes for communicating the resighting data. Petr Musil, David Roshier and Lukas Jenni contributed to fruitful discussions during several stages of the focal study. David Roshier was also heavily involved in the development of a species–specific harness and the corresponding attachment method. Fränzi Korner-Nievergelt provided valuable support dur- ing statistical analysis and Sarah Davidson during processing of tracking data and proof-reading. A. Gehrold is member of the International Max Planck Research School for Organismal Biology.
This work was supported by the Max Planck Society. Experiments were approved by Landratsamt München, Sachgebiet 5.3 (Az.: 5.3- 751-4/Hei).
References
Åkesson, S., Karlsson, L., Walinder, G. and Alerstam, T. 1996.
Bimodal orientation and the occurrence of temporary reverse bird migration during autumn in south Scandinavia. – Behav.
Ecol. Sociobiol. 38: 293–302.
Alerstam, T. and Högstedt, G. 1982. Bird migration and reproduction in relation to habitats for survival and breeding.
– Ornis Scand. 13: 25–37.
Batschelet, E. 1981. Circular statistics in biology. – Academic Press.
Bauer, H.-G., Bezzel, E. and Fiedler, W. 2005. Das Kompendium der Vögel Mitteleuropas – Alles über Biologie, Gefährdung und Schutz. Band 1: Nonpasseriformes – Nichtsperlingsvögel.
– AULA-Verlag, Wiebelsheim.
Bensch, S. 1999. Is the range size of migratory birds constrained by their migratory program? – J. Biogeogr. 26:
1225–1235.
Berthold, P. 1990. Vogelzug. – Wissenschaftliche Buchgesellschaft, Darmstadt.
Carbone, C. and Owen, M. 1995. Differential migration of the sexes of pochard Aythya ferina: results from a European survey.
– Wildfowl 46: 99–108.
CLS 2011. Argos user`s manual. – CLS, Toulouse.
Cramp, S. and Simmons, K. E. L. 1977. Handbook of the birds of Europe, the Middle East and North Africa – the birds of the Western Palearctic. Vol. 1: ostrich to ducks. – Oxford Univ.
Press.
Fiedler, W. and Davidson, S. 2012. Movebank – eine offene Internetplattform für Tierwanderungsdaten. – Vogelwarte 50: 15–20.
Fox, A. D. 2005. Gadwall Anas strepera. – In: Kear, J. (ed.), Bird families of the World: ducks, geese and swans. Oxford Univ. Press, pp. 491–494.
Roshier, D. A., Asmus, M. and Klaassen, M. 2008a. What drives long-distance movements in the nomadic grey teal Anas gracilis in Australia? – Ibis 150: 474–484.
Roshier, D. A., Doerr, V. A. J. and Doerr, E. D. 2008b. Animal movement in dynamic landscapes: interaction between behavioural strategies and resource distributions. – Oecologia 156: 465–477.
Salomonsen, F. 1968. The moult migration. – Wildfowl 19:
5–24.
Sauter, A., Korner-Nievergelt, F. and Jenni, L. 2010. Evidence of climate change effects on within-winter movements of European mallards Anas platyrhynchos. – Ibis 152: 600–609.
Schüz, E., Berthold, P., Gwinner, E. and Oelke, H. 1971. Grundriß der Vogelzugskunde. – Paul Parey, Berlin.
Scott, D. A. and Rose, P. M. 1996. Atlas of Anatidae populations in Africa and Western Eurasia. – Wetlands International, Wageningen.
Suter, W. 1994. Overwintering waterfowl on Swiss lakes: how are abundance and species richness influenced by trophic status and lake morphology? – Hydrobiologia 279/280: 1–14.
Takekawa, J. Y., Prosser, D. J., Newman, S. H., Bin Muzaffar, S., Hill, N. J., Yan, B. P., Xiao, X. M., Lei, F. M., Li, T. X., Schwarzbach, S. E. and Howell, J. A. 2010. Victims and vectors: highly pathogenic avian influenza H5N1 and the ecology of wild birds. – Avian Biol. Res. 3: 51–73.
Van Toor, M. L., Hedenström, A., Waldenström, J., Fiedler, W., Holland, R. A., Thorup, K. and Wikelski, M. 2013. Flexibility of continental navigation and migration in European mallards.
– PloS One 8: e72629.
Wikelski, M., Tarlow, E. M., Raim, A., Diehl, R. H., Larkin, R. P.
and Visser, G. H. 2003. Costs of migration in free-flying songbirds. – Nature 423: 704.
Wolff, W. J. 1970. Goal orientation versus one-direction orientation in teal Anas c. crecca during autumn migration. – Ardea 58: 131–141.
Yarris, G. S., McLandress, M. R. and Perkins, A. E. H. 1994. Molt migration of postbreeding female mallards from Suisun Marsh, California. – Condor 96: 36–45.
Liu, Y., Keller, I. and Heckel, G. 2012. Breeding site fidelity and winter admixture in a long-distance migrant, the tufted duck (Aythya fuligula). – Heredity 109: 108–116.
Nilsson, S. G. and Nilsson, I. N. 1978. Breeding bird community densities and species richness in lakes. – Oikos 31: 214–221.
Oppel, S., Powell, A. N. and Dickson, D. L. 2008. Timing and distance of king eider migration and winter movements.
– Condor 110: 296–305.
Oring, L. W. 1969. Summer biology of the gadwall at Delta, Manitoba. – Wilson Bull. 81: 44–54.
Owen, M. and Dix, M. 1986. Sex ratios in some common British wintering ducks. – Wildfowl 37: 104–112.
Paulus, S. L. 1983. Dominance relations, resource use, and pairing chronology of gadwalls in winter. – Auk 100: 947–952.
Perdeck, A. C. 1977. The analysis of ringing data: pitfalls and prospects. – Vogelwarte 29: 33–44.
Perdeck, A. C. and Clason, C. 1983. Sexual differences in migration and winter quarters of ducks ringed in the Netherlands.
– Wildfowl 34: 137–143.
Pyle, P. 2005. Molts and plumages of ducks (Anatinae). – Waterbirds 28: 208–219.
R Development Core Team 2012. R: a language and environment for statistical computing. – R Foundation for Statistical Computing, Vienna.
Ramenofsky, M. and Wingfield, J. C. 2007. Regulation of migration. – BioScience 57: 135–143.
Ringelman, J. K. 1990. Habitat management for molting waterfowl. – US Fish and Wildlife Service, Fish Wildlife Leaflet 13.4.4.
Rodrigues, D. J. C., Fabião, A. M. D. and Figueiredo, M. E. M.
A. 2001. The use of nasal markers for monitoring Mallard populations. – In: Field, R., Waren, R. J., Okarma, H. and Sievert, P. R. (eds), Wildlife, land, and people: priorities for the 21st century. Proc. 2nd Int. Wildlife Manage. Congress.
The Wildlife Society, Bethesda, MD, pp. 316–318.
Roshier, D. A. and Asmus, M. W. 2009. Use of satellite telemetry on small-bodied waterfowl in Australia. – Mar. Freshwater Res. 60: 299–305.
Supplementary material (Appendix JAV-00248 at www.
oikosoffice.lu.se/appendix ). Appendix 1–2.
