Spring migration dynamics and sex-specific patterns in stopover strategy in the Wood Sandpiper Tringa glareola
author: Yoko Muraoka yoko.muraoka@gmail.com Department of Population Ecology, Faculty of Life Sciences University of Vienna 1090 Vienna Austria; Christian H. Schulze Department of Population Ecology, Faculty of Life Sciences University of Vienna 1090 Vienna Austria; Mihaela Pavličev Department of Anatomy and Neurobiology Washington University St Louis MO 63108 USA; Gábor Wichmann Auring Society 2273 Hohenau an der March Austria
source date: Online
edition published: 2008
language: en

Abstract

Due to being a virtually monomorphic wader species, migration dynamics and sex-related migration patterns in the Wood Sandpiper (Tringa glareola) have rarely been investigated. We captured spring migrants at an important stopover site in northeastern Austria. Birds were individually color-marked, and sex was determined by an analysis of DNA from tail feather material. Among temporary residents (birds seen again after day of capture), males migrated on average 3 days earlier than females. However, since sexes did not differ in fat score, the length of stay and the proportion of transients (birds not seen again after day of capture) and temporary residents, we suggest that males and females adopt similar migration strategies in the spring. The large number of transients captured as well as shorter stopover durations in later temporary residents indicate that Wood Sandpipers minimize time at this stage of their northbound migration. Temporary residents earlier in the season exhibited lower fat stores than later ones. Nevertheless, since the fat stores of transients and temporary residents were similar even after the progress of the season had been accounted for, we assume that Wood Sandpipers may afford to exhibit individual flexibility in migration strategy and the use of stopover sites, especially early in the season. This variability may be a necessary adaptation to cope with possible varying environmental conditions at dynamic and unpredictable inland stopover sites. After having reached North Mediterranean regions, mean body mass of spring migrants gradually increases during successive stopovers, indicating that Wood Sandpipers follow a ‘hopping’ migration strategy. This emphasizes the high conservation value of even small artificial mudflat pools as important stepping stones in order to maintain a continuous network of wetland habitats for this continental migrant.

Introduction

The Wood Sandpiper Tringa glareola is a common migrant in inland Central Europe (Kirchner 1978) and found in abundance at freshwater habitats during its north- and southbound migration. In the course of their annual migratory journeys between sub-Saharan Africa and Fennoscandian breeding grounds, they depend on the availability of suitable stopover sites. Piersma (1987) hypothesized that waders follow either a ‘hop, skip or jump’ migration strategy. ‘Jumping’ is characterized by long flights and may involve as few as one stopover in-between. As such, jumpers need to put on and carry large fat loads for covering the long distances between stopovers. The ‘skipping’ migration strategy involves a series of flight bouts with few stops, whereas ‘hoppers’ fly only short distances between numerous brief stopovers. Hopping is energetically less costly than performing long jumps or intermediate skips and entails lower risks of delays in fattening and timing (Piersma 1987).

Among waders, males generally precede females in the spring (Ketterson and Nolan 1983), and the latter leave breeding grounds ahead of males and juveniles in many species (Glutz von Blotzheim et al. 1986). As in all Tringa species, female Wood Sandpipers are on average slightly larger than their male counterparts (Jehl and Murray 1986; Remisiewicz and Wennerberg 2006). However, due to being a virtually monomorphic species, sex-related differences in the migration dynamics of the Wood Sandpiper have rarely been investigated. Remisiewicz and Wennerberg (2006) recently used genetic analyses to reveal sex differences in the morphology and spring migration phenology of Wood Sandpipers in northeastern Poland. These researchers found that differential timing of migration occurred between sexes (males ahead of females) and age classes (second year birds after older birds). In the study reported here, we applied molecular sexing in order to examine sex-specific patterns in migration dynamics in the Wood Sandpiper at an important stopover site in northeastern Austria, which is further south along the spring migration route than that studied by Remisiewicz and Wennerberg (2006). Due to the small size and the openness of our study site, color-ringed birds can be easily detected (Wichmann et al. 2004). Consequently, minimum stopover durations of migrants can be estimated quite precisely. Studies involving individual color-ringing therefore provide an ideal opportunity for more detailed investigations of migration dynamics in this locally abundant migrant species.

Methods

Study area and field methods

Our study site, the ‘Absetzbecken Hohenau-Ringelsdorf’ (48°35′N, 16°55′E), is located in northeastern Austria and consists of several small pools (‘Absetzbecken’, total area 38 ha) and a 17-ha pond (‘Kühlteich’) belonging to the local sugar factory. The site contains a mosaic of shallow water pools, semi-submerged vegetation and muddy areas (Wichmann et al. 2004) and represents an important stopover site for waders in Austria (Zuna-Kratky and Rössler 1993). The Wood Sandpiper is one of the most abundant wader species migrating through this area (Zuna-Kratky et al. 2000).

Spring migrants were caught with walk-in traps (Meissner 1998) from 18 April until 12 May 2004 (4 days a week, 16 days in total) and from 22 April until 17 May 2005 (7 days a week, 26 days in total). The captured birds were individually marked with metal and plastic color rings. The size of the visible fat deposits was determined according to a 0–8 scale developed by Kaiser (1993), which enables the detection of small differences in visible fat stores independent of absolute body size or mass fluctuations, such as those that occur during the course of day (Kaiser 1993). We therefore used fat score as a direct measure of body condition (Green 2001).

During the trapping period, all Wood Sandpipers in the area were counted once a day for 5 days per week in 2004 and 7 days per week in 2005. We estimated the minimum length of stay by assuming that birds arrived on the day of initial capture and departed on the day when they were last observed. Color-ringed birds that were not seen again after the day of capture were assumed to have departed from the study site the day they were ringed. Since the stopover length of this latter group was classified as 0, we will refer to those birds as ‘transients’. Migrants seen again after the day of capture were defined as ‘temporary residents’.

Sex determination

We used the feathers of ringed birds for sexing as feather bases provide a good source of DNA (Bayard de Volo et al. 2003). Of the breast feathers collected during ringing in 2004 and stored in dry envelopes, only 13 out of 28 samples were successfully sexed. Consequently, in 2005, tail feathers were collected and stored in 96% ethanol. Over the spring passage period, 71 individuals were randomly chosen, of which 67 were successfully sexed based on DNA extracted from the tail feathers.

In birds, females carry two different copies of the sex chromosome (WZ), while males are homogametic (ZZ). This difference is employed in genetic sex determination. We used a PCR-based method to amplify sex-specific DNA fragments of CHD-gene located on the sex chromosomes (Griffiths and Tiwari 1993, 1995; Ellegren 1996; Griffiths et al. 1996). In the avian genome, the two copies of this gene (CHD-W and CHD-Z) differ in the lengths of their introns (Griffiths and Korn 1997). We used primers P2 and P8 by Griffiths et al. (1998), which bind in the CDH gene at exon sequences flanking an intron that usually differs in length between W and Z copies. Hence, when the section of the CHD-gene including the length-variable intron is PCR amplified, the sexes can be distinguished by the presence of either (1) two bands showing two fragments of different lengths or (2) fragments of uniform length (males) constituting a single band on the electrophoresis gel. For the screening of total genomic DNA by PCR analysis, we extracted DNA from feathers using the DNA mini Kit (QIAGEN, Valencia, CA). The amplification of PCR fragments was performed in 25-μl reaction mixtures containing PCR buffer with 1.5 mM MgCl2, 0.2 mM of each dNTP, 0.4 μM of each primer and 0.5 U of Taq polymerase (Amersham Biosciences, Piscataway, NJ). The PCR reaction conditions comprised an initial denaturation step of 2 min at 94°C, 35 cycles of 10 s at 95°C, 15 s at 52°C, 50 s at 72°C and a final extension step of 7 min at 72°C. Negative and positive controls were included in all runs. All screens were repeated three times to verify the results. Amplified fragments were separated by length on a 2% agarose gel.

Data analysis

To determine intraspecific variation among spring migrant Wood Sandpipers, we examined differences in (1) all individuals and (2) temporary residents. We chose the latter for separate analyses because transient birds were assumed to have departed from the study area on the day of arrival and, therefore, did not actually use Hohenau as a stopover site. Since median dates of passage differed between study years (see Results), we investigated sex-related differences in the timing of migration by classifying the median date of passage of each study year as day 0.

Statistical analyses were performed using STATISTICA ver. 7.1 (StatSoft Inc 2005). We used nonparametric and randomization tests if the assumptions, such as the normal probability distribution of data, could not be met. Randomizations were performed using the program MUREG (H.L. Nemeschkal, University of Vienna). Results were taken to be statistically significant if P < 0.05.

Results

Spring migration phenology

The number of male Wood Sandpipers (47) migrating through the area exceeded the number of female migrants (33), but the difference was not significant (binomial test, P = 0.15). Due to the low sample size of sex-determined individuals in 2004 (eight males, five females), we were unable to test whether the median dates of passage within either sex differed between the study years. However, the median date of passage of all birds captured did differ between years (2004: May 3, n = 28; 2005: May 7, n = 183; Mann–Whitney U-test U = 755.5, P < 0.001). Birds of each sex were recorded during the entire trapping season. The median date of passage was May 5 in both sexes (range April 25–May 17, n = 211). Within temporary residents, the median date of passage differed between males (median May 4, range April 26–May 9, n = 11) and females (median May 7, range May 2–May 12, n = 11; Mann–Whitney U-test U = 26, P = 0.023).

Use of stopover site and body condition

Of the 211 first captures in both study years, 159 birds (73.5%) were transients and 52 birds (26.5%) stayed in the study area at least overnight (temporary residents). The number of temporary residents and transients did not differ between males and females (χ 2 = 0.96, P = 0.234; temporary residents: n ♂ = 11, n ♀ = 11; transients: n ♂ = 36, n ♀ = 22).

Since stopover duration and fat score of the spring migrants did not differ between study years (Mann–Whitney U-test; length of stay: ♂ U = 136.5, P = 0.581, ♀ U = 42.5, P = 0.167; fat score: ♂: U = 136.0, P = 0.571, ♀: U = 68.0, P = 0.920), data were pooled in order to increase the sample size. Fat scores of spring migrants were similar between transients and temporary residents (ANCOVA F 1,208 = 0.01, P = 0.914) and varied slightly (albeit not significantly) during the course of the season (covariate: Julian day, ANCOVA F 1,208 = 3.53, P = 0.062). This result is founded on variations in fat deposits of the temporary residents: whereas the fat stores of transients did not vary during the course of the season (regression equation: y = 8.69x + 2.08, r 2 = 0.003, P = 0.54; Fig. 1a), those of temporary residents significantly increased as the season progressed (regression equation: y = 0.122x − 12.19, r 2 = 0.24, P < 0.001, n = 155; Fig. 1b). Differences in fat score could not be detected between males and females (median fat score = 3 in both sexes; Mann–Whitney U-test U = 694.0, P = 0.426).

[Fig. 1
Temporal variation in fat scores of (a) transient (n = 131) and (b) temporarily resident Wood Sandpipers (n = 52) during spring passage in 2005]

The median length of stay of temporarily resident birds was 4 days and did not differ between sexes (Mann–Whitney U-test U = 54.0, P = 0.944, n ♂ = 47, n ♀ = 33). The stopover duration of the temporary residents decreased as the season progressed (Spearman rank correlation r s = −0.31, P = 0.025, n = 52). No such correlation was observed when pooling all spring migrants captured (r s = −0.05, P = 0.527, n = 183) due to the large number of transients that left the study area on the day of capture. We found no relationship between length of stay and fat score (r s = −0.06, P = 0.672, n = 52).

Migration strategy

When we compared the mean body mass of the birds captured at Hohenau with data extracted from the literature, we found that a quadratic regression best described changes in mean body masses and latitudinal occurrence of Wood Sandpipers during the spring migration (r 2 = 0.675, F 2,8 = 8.3, P = 0.011). After crossing the Mediterranean Sea, the body masses of the spring migrants increase in the course of their northbound journey (Fig. 2).

[Fig. 2
Mass gains of Wood Sandpipers during the northward migration. Line fitted quadratic regression curve, open circle location of the study area, filled circles literature data (1, 2, 6, 8 references in Glutz von Blotzheim et al. 1986, 3 Yosef et al. 2002, 4 Scebba and Moschetti 1996, 5 Licheri and Spina 2005, 7 present study, 9 Anthes et al. 2002, 10 Remisiewicz and Wennerberg 2006, 11 Persson 1998)]

Discussion

Birds traveling long distances are subject to severe physiological and ecological constraints and depend on suitable stopover sites for refuelling in order to complete their migration (e.g. Alerstam 1993; Berthold 1990). The exact timing of resource availability at the time of passage is critical to their survival. Timing is especially important during the spring, when resources must be available during a rather narrow time window. Unlike coastal staging sites, interior wetlands are dynamic and unpredictable habitats (Skagen and Knopf 1993; Amezaga et al. 2002). As such, waders travelling along an inland migration route need to be more flexible in their choice of stopover sites. This is reflected by the lower degree of site fidelity observed in continental migrants than in coastal shorebirds, with the latter being highly depend on the availability of traditional staging posts (Colwell and Oring 1988; Skagen and Knopf 1994).

The spring phenology of Wood Sandpipers at our study site corresponds well with phenology patterns reported from other inland stopover sites (Winkler and Herzig-Straschil 1981; Glutz von Blotzheim et al. 1986; Leuzinger and Jenni 1993; Scebba and Moschetti 1996; Anthes et al. 2002; Laber 2003; Remisiewicz et al. 2007). Differences in median dates of passage between years may have occurred due to shifting environmental conditions (Anthes 2004) and/or varying catching efforts and periods. However, the numbers of migrant waders recorded in 2004 were overall low, both at Hohenau and at Austria’s most important stopover site for waders, the Seewinkel region in the southeastern part of the country (Zuna-Kratky 2004). Considerable between-year variation in the spring migration intensity of Wood Sandpipers has also been reported recently for stopover sites in Poland and Belarus (Remisiewicz et al. 2007). The tight timing of spring passage as well as the large number of transients suggest that Wood Sandpipers minimize their time during the northbound migration. While the median stopover duration of spring migrants at Hohenau was 4 days, the overall median length of stay at study sites in Poland and Belarus was 2 days (M. Remisiewicz, personal communication), indicating that the migration schedule becomes tighter as birds get closer to the breeding grounds (see references in Battley et al. 2004).

Since neither the number of transients and temporary residents nor the stopover duration differed between males and females, our data implies that sex-related differences in migration speed did not occur. Differences in fat stores could also not be detected between sexes. In northeastern Poland, Remisiewicz and Wennerberg (2006) found that adult (>2nd year) males migrated ahead of females. That the same pattern applied only to temporarily resident birds in our study may be due to the fact that we did not distinguish between age classes. Our findings suggest that, except for differences in the timing of migration, males and females adopt similar migration strategies during the northbound migration. Differential migration with males preceding females is a commonly observed pattern among many bird species during the spring migration and may result from latitudinal segregation of sexes on the wintering grounds (Cristol et al. 1999). In species in which sexes are not spatially segregated during the non-breeding season, male-biased earlier arrival on the breeding grounds (protandry) in migratory birds has primarily been attributed to (1) male–male competition for high-quality territories and (2) benefits arising from increased opportunities to mate (Morbey and Ydenberg 2001). In the monogamous Wood Sandpiper, males establish breeding territories and females choose a mate among territorial males (Jehl and Murray 1986; Remisiewicz and Wennerberg 2006). Therefore, males should strive to reach the breeding grounds early in order to obtain high-quality territories. For females, however, it may be convenient to arrive at breeding sites after males to maximize the pool of available mates. The high overlap of biometric measurements between sexes (Remisiewicz and Wennerberg 2006; own data, unpublished) may be one reason why only minor sex-specific differences in migration patterns could be detected. In wader species with a more pronounced sex dimorphism, the constraints acting on sexes and resulting in different migration patterns of males and females may be more important, as demonstrated for the European Bar-tailed Godwit Limosa lapponica (Ketterson and Nolan 1983; Scheiffarth 2003).

The long migratory journeys between widely separated wintering and breeding areas require several refuelling stops en route. The ‘jumping’ and ‘skipping’ migration strategies are characterized by long flights with one or few stops in-between (Piersma 1987). The alternative strategy, ‘hopping’ (i.e. flying short distances between frequent stops), is energetically cheaper than performing long jumps or intermediate skips (Piersma 1987). Whereas hoppers should be expected to show little variation in body condition during stopovers, jumpers should arrive at a staging site in a fat-depleted condition. In comparison to hoppers, jumpers have to stay longer at stopovers to refuel and to depart with higher levels of reserves for the next long-distance bout. In this case, a negative correlation between fat loads upon arrival at a stopover site and length of stay should exist. The missing relationship between fat stores and stopover duration among birds captured at Hohenau indicates that Wood Sandpipers follow a ‘hopping’ migration strategy, as has been suggested for the autumn passage of this species (Meissner 1997; Wichmann et al. 2004). This assumption is further confirmed by the comparison of mean body masses of birds captured at Hohenau with data found in the literature. After crossing the Mediterranean Sea, the body masses of spring migrants increase during the course of their northbound journey.

Overall, earlier birds tended to have lower fat stores than later migrants, but a significant increase in fat deposits could only be recorded in temporary residents. Birds arriving at Hohenau earlier in the season thus exhibited lower fat stores and stayed at Hohenau longer than later temporary residents, which carried larger fat deposits and stopped over only for a short time. The reasons for the shorter stopovers recorded in later temporarily resident migrants may be twofold. The first is that the time pressure to arrive at breeding grounds on time in order to obtain a high-quality territory, mate and complete the breeding cycle presumably increases as the season progresses. Spring migrants earlier in the season may have the ‘luxury' to stay longer and encounter variable environmental conditions in a fairly flexible manner. The second is that later migrants may have used other sites for refuelling, as indicated by the relatively larger fat deposits in birds arriving later in the season, and thus stayed in Hohenau only for a short time. Since fat stores of transients and temporary residents were similar, even after accounting for the progress of the season, we assume that Wood Sandpipers may exhibit individual variability in migration strategy and the use of stopover sites (Iverson et al. 1996; Meissner 1997; Remisiewicz et al. 2007), as reflected by variation in the residency patterns and body condition of birds captured at Hohenau.

The short-hop migration strategy emphasizes the importance of maintaining a network of abundant wetlands along the migration route. Being wetland specialists, Wood Sandpipers, as well as many other waders migrating through or breeding in continental wetlands, are highly dependent on the preservation of these habitats. During the last few decades, wetlands in Europe have drastically declined (van Vessem et al. 1997). The Hohenau sugar factory was closed in the year after our study was conducted; the Auring Society (a local non-governmental organization) has since struggled to maintain the Absetzbecken as artificial wetlands for waders and waterfowl. Small disjunct patches of temporally dynamic wetlands provide less predictable resources than large permanent wetlands or coastal intertidal areas. The latter in particular accommodate many species that show consistent year-to-year use of traditional staging posts. For jumpers, the loss of only one site could cause a net reduction of the entire population if feeding and resting areas become too far apart. In contrast, hoppers may be capable of locating habitats opportunistically (Skagen and Knopf 1993, 1994) and may better be able to cope with the loss of a few individual sites. However, the summation of the effects, which in our case would result from the loss of several suitable inland wetlands, has proven to be one of the major problems encountered in nature conservation (EEA 2006). While stopover sites that traditionally support large populations during migration have received the most recognition, complexes of small wetlands have often been neglected by recent conservation efforts. Yet the existence of an adequate density of small freshwater stopover sites must be ensured for those long-distance migrants depending on a continuous network of wetland habitats as stepping stones along their migratory route.

Zusammenfassung

Zugdynamik und geschlechtsspezifische Muster im Rastverhalten des Bruchwasserläufers Tringa glareola im Frühjahr

Als nahezu monomorphe Limikolenart wurden geschlechtsspezifische Unterschiede in der Zugstrategie des Bruchwasserläufers bisher kaum untersucht. Wir fingen Frühjahrszieher an einem wichtigen Rastplatz in den March-Thaya—Auen im nordöstlichen Niederösterreich und markierten sie individuell mit Farbringen. Die molekulare Geschlechtsbestimmung von 80 Individuen erfolgte anhand von Schwanzfederproben. Unter jenen Vögeln, die nach dem Fangtag erneut im Gebiet gesichtet wurden (“temporary residents”) zogen Männchen durchschnittlich 3 Tage früher als Weibchen. Da jedoch weder bezüglich Fettdepots, Verweildauer noch des Anteils an “transients” (Vögel, die nach dem Fangtag nicht erneut im Gebiet gesichtet wurden) und temporary residents geschlechtsspezifische Unterschiede nachweisbar waren, deuten die gewonnenen Daten darauf hin, dass Männchen und Weibchen ähnliche Zugstragien im Frühjahr anwenden. Der hohe Anteil an transients sowie kürzere Verweildauern später durchziehender temporary residents weisen darauf hin, dass Bruchwasserläufer in diesem Zugabschnitt Zeit minimieren. Früher durchziehende temporary residents wiesen geringere Fettdepots auf als spätere. Nachdem sich jedoch das Ausmaß der angelagerten Fettdepots bei “transients” und temporary residents auch unter Berücksichtigung der fortschreitenden Saison ähnelte, vermuten wir, dass Bruchwasserläufer eine gewisse individuelle Flexibilität in der Zugstrategie und dem Gebrauch von Rastgebieten vor allem früh in der Zugsaison an den Tag legen können. Diese Variabilität könnte eine notwendige Anpassung sein, um mit möglichen variierenden Umweltbedingungen in dynamischen und unvorhersagbaren Binnenland-Rastgebieten zu Rande zu kommen. Durch den Vergleich mit Ergebnissen anderer Fangstationen zeigte sich, dass das Körpergewicht der Vögel im Laufe ihres Zuges in die nördlichen Brutgebiete sukzessive zunimmt. Nach Überquerung des Mittelmeeres werden während zahlreicher Zwischenstopps die für die Bewältigung der verbleibenden Zugstrecke und das Brutgeschäft notwendigen Energiedepots nach und nach aufgebaut. Dem Vorhandensein und der Erhaltung geeigneter Rasthabitate im Binnenland kommt damit eine besondere Bedeutung zu.

Acknowledgments

We thank M. Hillbrand and H. Klosius who helped with the fieldwork. K.H. Lux, director of the former AGRANA Sugar Factory Hohenau, who granted us permission to catch Wood Sandpipers on the Absetzbecken. We are also grateful to E. Haring, K. Fiedler, W. Meissner, M. Remisiewicz and two anonymous reviewers who provided helpful comments. The study was supported by the Austrian Academy of Sciences and the German Ornithologists’ Society.

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