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Male-Biased Sex-Ratio of Dunlins Calidris alpina in the Gulf Of Gdańsk (Southern Baltic) During Autumn Migration



SUMMARy.—This long-term study (10 years) aimed to check if the sex-ratio of dunlins Calidris alpina at a stopover site in the southern Baltic region was biased. Two age classes among non-juvenile dunlins were recognised: immatures (2nd calendar year) and adults (> 2nd calendar year). There was a significant male bias in the sample of 4,406 non-juvenile dunlins captured during their southward migration. Overall, 60.3% of immatures and 59.4% of adults were males. Particularly among adults, the proportion of males increased significantly after the start of autumn migration. The annual sex-ratios were consistently male biased, but varied somewhat and fluctuated in parallel for adults and immatures. One plausible explanation for the male bias is that males and females differ in migration strategy. Females may make longer flights and avoid stopover sites with unpredictable feeding conditions, such as the southern Baltic coasts, which provide low quality habitat. Assuming a balanced non-juvenile population sex ratio, the ‘missing’ females could stopover elsewhere in the Baltic or fly directly to the tidal areas of the Wadden Sea. The sex ratio in the study area may depend on wind conditions during the early phase of autumn migration. In some years, adverse weather may force more females than usual to stopover in the study area.
Short Communications
Włodzimierz MEISSNER1*
SUMMARY.—This long-term study (10 years) aimed to check if the sex-ratio of dunlins Calidris alpina
at a stopover site in the southern Baltic region was biased. Two age classes among non-juvenile dunlins
were recognised: immatures (2nd calendar year) and adults (> 2nd calendar year). There was a significant
male bias in the sample of 4,406 non-juvenile dunlins captured during their southward migration.
Overall, 60.3% of immatures and 59.4% of adults were males. Particularly among adults, the proportion
of males increased significantly after the start of autumn migration. The annual sex-ratios were
consistently male biased, but varied somewhat and fluctuated in parallel for adults and immatures. One
plausible explanation for the male bias is that males and females differ in migration strategy. Females
may make longer flights and avoid stopover sites with unpredictable feeding conditions, such as the
southern Baltic coasts, which provide low quality habitat. Assuming a balanced non-juvenile population
sex ratio, the ‘missing’ females could stopover elsewhere in the Baltic or fly directly to the tidal areas
of the Wadden Sea. The sex ratio in the study area may depend on wind conditions during the early
phase of autumn migration. In some years, adverse weather may force more females than usual to
stopover in the study area.
Key words: migration, Polish coast, stopover site, waders.
RESUMEN.—En este estudio a largo plazo (10 años) se trata de comprobar si la razón de sexos en el
correlimos común Calidris alpina presenta desviaciones en un sitio de parada migratoria. Se distinguieron
dos clases de edad entre los correlimos no juveniles: inmaduros (2º año calendario) y adultos (> 2º año
calendario). Hubo un predominio de machos en una muestra de 4.406 correlimos no juveniles durante
la migración hacia el sur. Del total, el 60,3% de inmaduros y el 59,4% de adultos fueron machos. Entre
los adultos en particular la proporción de machos se incrementó significativamente tras el inicio de la
Ardeola 62(2), 2015, 335-342 DOI: 10.13157/arla.62.2.2015.335
1Avian Ecophysiology Unit, Department of Vertebrate Ecology and Zoology, University of Gdańsk,
Wita Stwosza 59, 80-308 Gdańsk, Poland.
*Corresponding author:
The dunlin Calidris alpina is the most nu-
merous wader species staging on sandy shores
of the southern Baltic during autumn migra-
tion (Kube et al., 1994; Meissner et al., 2009).
Despite unpredictable feeding conditions for
waders in these non-tidal areas (Kube, 1994),
the southern Baltic coasts gather about 1-2%
of adults and more than 10% of juveniles of
the East Atlantic migratory dunlin population
(Kube et al., 1994). The eastern limit of the
dunlin population that migrates through the
Baltic region remains unknown but there is
evidence that some of them originate from
as far east as the Ob estuary (Gromadzka,
1989; Gromadzka and Ryabitsev, 1998).
These birds spend the winter in Western and
Southern Europe, and also in North Africa
(Gromadzka, 1983, 1989). At stopover sites
along the Polish Baltic shoreline, autumn
migrant dunlins have low fat reserves and
stay only for a few days (Meissner, 1998a).
Moreover the number of dunlins retrapped in
subsequent years is very low (WRG KULING
ringing database). These findings strongly
suggest that sandy coasts in this region are
used as stopovers rather than as regular
staging sites (Warnock, 2010).
Sex bias among migrating waders at the
stopover sites along the southern Baltic/
North Sea migratory route was found in the
Siberian knot Calidris canutus canutus, with
more females staging in the Wadden Sea
than on the southern Baltic coasts (Nebel et
al., 2002; Meissner, 2005). Despite quite
numerous studies of dunlin migration phe-
nology (Brenning, 1987; Pettersson, 1994;
Meissner and Strzałkowska, 2006; Meissner,
2015), stopover strategy (Meissner, 1998a),
moult (Gromadzka, 1989; Holmgren et al.,
1993) and analyses of ringing recoveries
(Gromadzka, 1983, 1989), there are no analy-
ses of dunlin sex-ratios during their autumn
migration through Europe. Studies at North
American and East Asian wintering grounds
suggest that there is a modest latitudinal cline
in the sex-ratio of wintering dunlin popu-
lations, with a higher proportion of males
staying at higher latitudes (Page, 1974;
Shepherd et al., 2001; Yang et al., 2012; Gill
et al., 2013). It remains unknown if birds of
different sexes also differ in their migration
strategy, e.g. exploit different feeding sites
on their route. Thus, this study was aimed at
determining whether the sex-ratio of dunlins
at one of the southern Baltic stopover sites
was biased.
The study was conducted near the mouth
of the Reda river (Puck Bay) during 1991-
2000 (fig. 1). Waders were captured with
walk-in traps (Meissner, 1998b) at a narrow
Ardeola 62(2), 2015, 335-342
migración otoñal. Las razones de sexos anuales estuvieron consistentemente desviadas a favor de los
machos, aunque variaron, y fluctuaron en paralelo en inmaduros y adultos. Una explicación plausible
para esta razón de sexos desviada es que machos y hembras difieren en sus estrategias migratorias. Las
hembras harían vuelos más largos y sobrepasarían sitios de parada con unas condiciones alimenticias
impredecibles, como las costas del Báltico meridional, que constituyen un hábitat de baja calidad. Su-
poniendo una razón de sexos balanceada en los no juveniles, las hembras harían paradas durante la mi-
gración en otras partes del Báltico, o volar directamente a zonas intermareales del mar de Wadden. La
razón de sexos en el área de estudio puede depender de las condiciones del viento durante la fase tem-
prana de la migración otoñal. Las condiciones climáticas adversas algunos años pueden forzar a un nú-
mero mayor de hembras de lo normal a efectuar paradas migratorias en el área de estudio.
Palabras clave: aves limícolas, costas de Polonia, migración, sitio de parada migratoria.
sandy spit (about 800 m long) near Rewa
village and about 2.5 km westwards near the
Reda mouth, where the study area consisted
of small sandy islets, a narrow sandy beach
and an electric power station ash dumping
site. Birds were counted throughout the area
daily around midday, where they could feed
or rest. The main period of the fieldwork
lasted from 16 July to 28 September, with
daily trapping each year. This period covered
almost the whole migration period of non-
juvenile dunlins, but only about 70% of the
passage period of juveniles (Meissner and
Sikora, 1995; Meissner et al., 2009). Therefore
the sex-ratios of juveniles were not analysed.
Two age classes could be recognised
among dunlins in breeding plumage: ‘imma-
tures’ birds in their second calendar year,
andadults’ – older than two years (Gro-
madzka, 1985; Meissner and Skakuj, 2009).
This distinction is unusual relative to many
other arctic breeding waders, because imma-
ture birds of most of these species do not
return to the breeding grounds during their
first spring (Hockey et al., 1998). In this
paper, analyses were performed separately
for immature and adult dunlins.
The sex of trapped dunlins was assessed
using discriminant functions based on the
bill and wing lengths derived for birds older
than one year (Meissner and Pilacka, 2008).
Bill length was measured with callipers with
accuracy 0.1 mm, while wing length was
measured with a stopped ruler to the nearest
1 mm (Meissner, 2000). Although the classi-
fication accuracy of these discriminant func-
tions is quite high (over 97% in the case of
females and 100% in males), to increase
reliability of sexing, 5% of males and 5%
of females of the lowest discriminant value
were excluded from the analysis (in total,
169 immatures and 456 adults).
The probability that an individual bird was
male or female was analysed treating sex as
a binary response variable. A generalised
linear model (GLZ) with binomial errors and
a logistic link function was used to investi-
gate the relationship of sex with age, year
(categorical) and date of capture (linear, day
number in the season) as predictor variables.
All statistical procedures were performed
using STATISTICA 10 (StatSoft, 2011).
Akaike’s Information Criterion (AIC) was
used to evaluate which of the GLZ models
had the highest level of support from the
data. Differences in AIC values (∆AIC) were
calculated by subtracting the minimum AIC
for the best-fitted model from the AIC of all
subsequent candidate models. These differ-
ences were used to determine which model
provided the best description of the data on
the basis of the fewest model parameters (k).
Only models with a ∆AIC value lower than
Ardeola 62(2), 2015, 335-342
FIG. 1.—Study area. Dots show the sites where
bird trapping was conducted.
[Área de estudio. Los puntos muestran los sitios
donde se capturaron las aves.]
9 were presented, because they are con-
sidered to be fairly similar in their ability to
describe the data, whereas the models with
larger ∆AIC have considerably less support
(Burnham and Anderson, 2002; Burnham et
al., 2011). The Akaike weight (AICw), which
gives the relative support of the data for
each model, was reported for all the models
to determine the relative likelihood of each
(Burnham and Anderson, 2002; Burnham et
al., 2011).
The nu mber of d unli ns in b reeding
plumage increased at the beginning of the
studied period and fluctuated considerably
till the end of second ten-day period of
August with no well-defined migration peak.
Later, their number was low with only a very
few birds recorded in the second half of
September (fig. 2). The first single juveniles
were observed in July, but these were birds
from the local Baltic population. Distinct
migration of juveniles began just after mid-
August and the maximum numbers were
recorded on 21-24 September (fig. 2).
During 10 years of field studies, 1,186
immature and 3,220 adult dunlins were cap-
tured and sexed within the study period.
The whole sample consisted of 60.3% males
among immatures and 59.4% males among
adults. Both groups differed statistically
from sex equality (goodness-of-fi
2= 25.4 and χ2
2= 57.9 with P < 0.0001 for
immatures and adults, respectively). The
number of males captured exceeded those
of females except for immatures in 1991 and
adults in 1994 (fig. 3).
The best GLZ model indicated a signifi-
cant effect for date of capture, age and year
on sex of captured dunlins, with no signifi-
cant interactions among variables (table 1:
model 1). This model was supported twice
as well as the second-best model, which in-
cluded the interaction between age and year
(table 1: model 2), and more than five times
Ardeola 62(2), 2015, 335-342
FIG. 2.—Mean numbers of dunlins in breeding
plumage (solid line) and juveniles (dashed line)
in the study area during July-September. Verti-
cal lines show the beginning and end of the bird
trapping period.
[Número medio de correlimos comunes en pluma-
je nupcial (línea continua) y juveniles (línea de
trazos) en el área de estudio durante julio-sep-
tiembre. Las líneas verticales muestran el inicio y
el final del periodo de captura.]
FIG. 3.—Changes in the proportion of adult
(black circles and solid line) and immature (grey
squares and dashed line) males in successive
years; error bars show 95% confidence intervals.
[Cambios en la proporción de machos adultos
(círculos negros y línea continua) e inmaduros
(cuadrados grises y línea de trazos) en años suce-
sivos, con los intervalos de confianza al 95%.]
as well as the next two models (table 1:
model 3 and 4). Subsequent models (not
listed in table 1) were much less supported
by the data (∆AIC lower than -50).
All variables included in the first model
had a significant effect predicting the sex of
captured dunlins (table 2). The interaction
between age and year in model 2 had no sig-
nificant sex effect (GLZ: Wald statistic =
16.320, P = 0.060). This confirmed the se-
lection of model 1, which excluded this
interaction, as the best model.
The selected model showed that the pro-
portion of males increased significantly with
the progress of autumn migration (GLZ:
table 2, fig. 4) and that immatures migrated
on average earlier than adults (GLZ: table 2,
fig. 2). Males predominated for both age
classes in all years, except for immatures in
1991 and adults in 1994, when slightly more
females than males were recorded. Year had
a significant effect on the proportion of males
(table 2, fig. 4), and adult and immature birds
fluctuated among years in parallel (t
coefficient = 0.60, N = 10, P = 0.016, fig. 3).
Ardeola 62(2), 2015, 335-342
Model selection using Akaike’s Information Criteria (AIC) to determine the effect of date, age and year
on the sex of dunlins caught during autumn migration at Puck Bay.
[Selección de modelo usando el Criterio de Información de Akaike (AIC) para determinar el efecto de
la fecha, edad y año sobre el sexo de los correlimos comunes capturados durante la migración otoñal
en la bahía de Puck.]
Model no Model structure AIC ∆AIC AICw
1 Date + Age + Year 5953.85 0.000 0.065
2 Date + Age + Year + Age*Year 5955.20
1.351 0.129
3 Date + Year 5957.22
3.364 0.352
4 Date + Year + Age*Year 5957.73
3.876 0.454
FIG. 4.—Proportion of males in consecutive five-
day periods among adult (solid line) and imma-
ture (dashed line) dunlins captured in Puck Bay
during autumn migration. Mean migration date
(black squares) and standard deviation (horizon-
tal box: white - adults, grey - immatures) of each
age class were shown above.
[Proporción de machos de correlimos común adul-
tos (línea continua) e inmaduros (línea de trazos)
capturados en la bahía de Puck en periodos de cin-
co días durante la migración otoñal. En la parte
superior se muestran las fechas medias de migra-
ción (cuadrados negros) y desviaciones típicas
(rectángulos) de cada clase (blanco: adultos; gris:
Females dominate in the flocks appearing
at Puck Bay before 16 July −the first day of
the main trapping period. Indeed, before 16
July adult males constitute only 27% of 69
birds captured during that early migration
stage (data from all years pooled). This bias
may reflect the sex difference in parental
care. Male dunlins stay with the brood for
longer periods of time, while females leave
breeding grounds around 10-12 days after
the hatch (Soikkeli, 1967; Jamieson, 2011,
2014). However, according to the results of
daily counts conducted in this area, the total
number of dunlins at that time is very low
and never exceeded 50 individuals (Meissner
et al., 2009). Also data from other staging
areas in the Baltic region (Brenning, 1987;
Pettersson, 1994) confirmed that this species
appeared only in small numbers before mid-
July. Thus, females clearly dominate in the
earliest phase of autumn migration, but
the total number of dunlins in this period is
very low and excluding these birds from the
analysis may have only slight influence on
the results obtained.
Biased sex-ratios on wintering grounds are
a quite common phenomenon in migrating
birds, including waders (e.g. Barter, 1987;
Figuerola and Bertolero, 1996; McCloskey
and Thompson, 2000; Nebel et al., 2002).
Five hypotheses have been proposed to
explain differential migration of males and
females, which might produce such segrega-
tion (see Nebel, 2007 for a short summary).
Among them, the resource partitioning hy-
pothesis may be fully applied to stopover
sites, where migrants spend only few days
before departure to the next step of their
migration. This hypothesis predicts that in
species with sexual dimorphism in bill size,
sexual differences in foraging niche might
lead to spatial segregation of males and fe-
males (Nebel, 2005), which may occur also
on a small-scale at stopover sites (Nebel et
al., 2000; Both et al., 2003). The slightly
larger females can exploit more deeply buried
invertebrates and may stay in deeper water
than males. In this study area, however,
dunlins forage along sandy shores and, since
there are no tides in the Baltic Sea, walk-in
traps were set such that the lead-line fences
crossed the entire foraging area utilised by
dunlins, from dry sand to deeper water.
Hence, small differences in habitat selection
seem unlikely to influence the sex bias among
captured dunlins.
The second possibility is that both sexes
differ in migration strategy, such that one
sex may omit some staging areas by making
longer flights, which may lead to unequal
proportions of males and females at sub-
sequent stopover sites, as seen in Siberian
knots (Nebel et al,. 2000; Meissner, 2005).
Hence, the biased sex-ratio observed at our
site may not reflect real migrant population
sex-ratios. If so, the ‘missing’ females should
produce compensating female-biased sex ra-
tios elsewhere in the Baltic Sea, or should
fly directly to the tidal areas of the Wadden
Sea, avoiding the unpredictable feeding
conditions in non-tidal coastal areas in the
southern Baltic.
Assuming that females make longer flights
than males, and that most of them omit the
Gulf of Gdańsk during the early phase of mi-
gration, their annual proportion in the study
area may depend on wind conditions. Thus,
in some years adverse weather may force
more females than usual to stopover in the
study area. As a result, the proportion of
males among captured dunlins would fluc-
tuate, as observed, with similar patterns in
adult and in immature birds.
A third possibility is that the dunlin fe-
males in the Gulf of Gdańsk stay for shorter
times than males, which decreases the proba-
bility of catching them in walk-in traps. The
longer bills of females allow them to capture
prey buried more deeply, as occurs in other
Calidris species (Puttick, 1981; Summers
Ardeola 62(2), 2015, 335-342
et al., 1990; Fernández and Lank, 2008).
Thus, they can forage more efficiently than
males, exploiting a wider range of prey
(Puttick, 1981). Site selection and differen-
tial stopover times are not mutually exclusive,
but dunlins migrate through the southern
Baltic with small fat reserves and majority
of non-juvenile birds stay in the study area
no longer than two days (Meissner, 1998a).
Hence, the potential difference in foraging
efficiency might have a minor influence on
the length of stay in this feeding area.
ACKNOWLEDGEMENTS.—I would like to thank
all colleagues who participated in the fieldwork of
the Waterbird Research Group KULING, and par-
ticularly the chiefs of the ringing station: R. Kru-
pa, M. Kozakiewicz, A. Włodarczak-Komosińska,
D. Górecki, P. Zięcik, M. Polak, M. Remisiewicz,
M. Ściborski, C. Wójcik, P. Rydzkowski and N.
Pokorski. Special thanks to David Lank and Ag-
nieszka Ożarowska for helpful comments on the
earlier version of the manuscript and language
correction. Comments by an anonymous reviewer
greatly improved an earlier version of this manu-
script and are appreciated. This is a paper of WRG
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Received: 6 October 2014
Accepted: 20 April 2015
Editor: José A. Masero
Ardeola 62(2), 2015, 335-342
... To ensure the survival, reproduction and growth of their populations, animals have evolved a series of adaptive life history strategies, such as sex differences in migratory strategies. Sex ratio bias is common in migratory animals and affects population development [1][2][3] . sex differences in reproductive behaviour and life history lead animals to adjust the sex structure during migration to change population structure. ...
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Sex ratio bias is common in migratory animals and can affect population structure and reproductive strategies, thereby altering population development. However, little is known about the underlying mechanisms that lead to sex ratio bias in migratory insect populations. In this study, we used Cnaphalocrocis medinalis, a typical migratory pest of rice, to explore this phenomenon. A total of 1,170 moths were collected from searchlight traps during immigration periods in 2015–2018. Females were much more abundant than males each year (total females: total males = 722:448). Sex-based differences in emergence time, take-off behaviour, flight capability and energy reserves were evaluated in a laboratory population. Females emerged 0.78 days earlier than males. In addition, the emigratory propensity and flight capability of female moths were greater than those of male moths, and female moths had more energy reserves than did male moths. These results indicate that female moths migrate earlier and can fly farther than male moths, resulting more female moths in the studied immigratory population.
... A male-biased sex ratio was evident in non-juveniles (immatures-68.5%, adults-65.7%) and is well known in dunlin at the chicks stage (61,1%, [54]), at the stopover during migration (immatures-60,3%, adults-59,4%, [55]) and at various winter quarters (Portugal-75% [56], Pacific Coast-61% and 65% [57]). The measurement data were carefully checked prior to analysis and all individuals with missing measurements, and a further 17 birds with extreme measurements, most probably due to observer errors, were excluded. ...
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Weather and predation constitute the two main factors affecting the breeding success of those Arctic waders whose productivity is highly variable over the years. We tested whether reproductive success is associated with the post-breeding condition of adults, in which in ‘good’ years (with warm weather, plentiful food and low predation pressure) the condition of breeders and their productivity is high. To verify this hypothesis, we used a 10-year dataset comprising 20,792 dunlins Calidris alpina, trapped during migration at a stopover site on the southern Baltic Sea shore. Males were consistently in a slightly worse condition than females, likely due to male-biased parental investment in brood rearing. Annual productivity indices were positively correlated with the respective condition indices of breeders from the Eurasian Arctic, indicating that in ‘good’ years, despite great effort spent on reproduction, breeders leave the breeding grounds in better condition. The pattern did not hold for 1992: productivity was low, but the average condition of adults during migration was the highest noted over the decade. We suggest that the delayed effect of the Mount Pinatubo eruption in the Philippines in 1991, could be responsible for the unexpected high condition of Arctic breeders in 1992. High population-level average condition, coupled with the low productivity could stem from severe weather caused by the volcano eruption a year before and strong predation pressure, which in turn lead to a reduced investment in reproduction. The importance of large-scale episodic phenomena, like this volcano eruption, may blur the statistical associations of wildlife with local environmental drivers.
... Another possible explanation of the lack (or very low number) of tundrae females is that sexes differ in migration strategy such that one sex may skip some staging areas by making longer flights. This may lead to an unequal proportion of males and females in subsequent stopover sites, which was found in Siberian Knots, Calidris canutus canutus, (Nebel et al. 2000, Meissner 2005 and Dunlins (Meissner 2015) in the southern Baltic coast. If so, tundrae females probably migrate longer distances than males and pass over the Pripyat floodplain. ...
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Biometric data on 161 adult males and 166 adult females of the Ringed Plover Charadrius hiaticula captured in the southern Belarus in spring between 2002 and 2014 were analysed. In males, a significant decrease in total head length and wing length occurred from April to May, while there was no such trend in females. As individuals of the C. h. tundrae subspecies are generally smaller than birds of the nominate subspecies, this indicates that mainly C. h. tundrae males migrated through the study area in May. A possible explanation of the lack (or very low number) of tundrae females is that the sexes differ in migration strategy, such that one sex may skip some staging areas and make longer flights, leading to an unequal proportion of males and females at some stopover sites. Low body mass indices of males in May and a very low number of retrapped birds suggested that these birds arrived to the study area with low energy reserves and have to use additional stopover sites to reach their breeding areas.
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During southward migration the Wadden Sea is the meeting place of Red Knots Calidris canutus of two subspecies that breed in either western Siberia (C. c. canutus) or north Greenland and north-east Canada (C. c. islandica), but the details of their co-occurrence have not been described. In 1995-98 numbers of Red Knots in our study area in the western Dutch Wadden Sea usually built up in late July towards maxima of 10 000-20 000 individuals in August and early September. In each of these four years we attached tiny (1.3-1.8 g) radiotransmitters to a total of 95 molecularly sexed adults to determine the length of stay of different categories of birds. The 65 females (68%) predominated the samples, and among the females the majority (48 birds) was captured without traces of wing moult. In females, but not in males, birds caught in wing moult stayed significantly longer than non-moulting birds. Non-moulting females weighed up to 200 g and disappeared within three weeks after being marked. The timing of their disappearance corresponded with observed departures of flocks towards the southwest, and published departure times of canutus. The relationship between length of stay and mass at capture of these early departing non-moulting females suggests a daily mass gain of about 2.84 g d-1. These birds had a mean bill length that was 1 mm (yet significantly) longer than those of the other female categories; a relatively long bill is a well known attribute of canutus. The much smaller sample of males with similar mass, moult and staging time characteristics did not show longer bill lengths and we are thus unable to unambiguously confirm the presence of canutus males in late July and early August; this bias remains to be functionally explained. Sex ratios were even in birds assignable to islandica.
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Walk-in traps are widely used by ringers to catch waders, both at inland and at coastal sites where there are no tides. There are several types of such traps (Bub 1971) and many of them have been used at Waterbird Research Group KULING ringing stations in the Puck Bay region (southern Baltic coast) since 1983. They differ in shape, dimensions, position of capturing chamber, types of entrance and material used for their construction (wire netting or thick fish netting). The traps have been set in many different habitats e.g. sandy seashore, small shallow muddy bays, sewage farms, wet meadows. We have ringed a total of over 30,000 waders up to 1997. Efficiency of catching depends greatly on the way the walk-in traps are set and also on their type. Walk-in traps are very convenient to use and they are safer for waders than mist-nets (Meissner 1992). Moreover, catching in walk-in traps is almost independent of weather conditions. Below, I sum up KULING's experience of wader catching using walk-in traps and also give a piece of advice on their use. WRG KULING has used several types of walk-in traps, but we have developed at least two models which in our opinion provide best catching results in different habitats (Figure 1). The first model (A) was considered safe, limiting the mortality of trapped birds (Meissner 1992), whilst the second one (B) has been used in Poland since the sixties. Both of them are
There is little information concerning differences in migration chronology between male and female Common Snipe (Gallinago gallinago) and virtually no accounts of sex-related differences in winter habitat use. We collected 372 Common Snipe in five different habitat types during the non-breeding period along the central Gulf Coast of Texas. Proportions of male and female snipe collected on wintering areas during the beginning of the fall period (i.e., between 6 and 21 October) indicated a tendency for females to arrive ahead of males. Sex ratios during the latter part of spring (16 March-10 April 1998) suggested male snipe leave wintering areas before females. During the winter period (14 November 1997-4 February 1998), female snipe were more common than males along the Texas Gulf Coast. Differences in sex ratios during winter are likely due to sex-related differences in habitat use. During winter, females were more common than males in heavily vegetated habitats (e.g., coastal marshes and cultivated rice fields). Conversely, males were more common in open habitats (e.g., mud flats). Male snipe may begin spring migration before females to establish territories on the breeding grounds. Sex-specific differences in winter habitat use may be related to reverse sexual size dimorphism of Common Snipe.
The sexes of many migratory shorebird species differ in timing and distance of their migrations, but this phenomenon has been little studied for birds in the East Asian-Australasian Flyway (EAAF). Using molecular sexing techniques (but also taking morphometric data), we compared the sex ratio of Dunlin Calidris alpina sampled on the west coast of Taiwan between September 2007 and May 2008. We found an overall sex bias (59.2%) toward females, including juveniles. A discriminant function deduced from three morphological measurements correctly identified 89.0% of Dunlin, but performed better in identifying males than females. Since Taiwan is at the southern end of the Dunlin's East Asian winter range, our results support the 'arrival-time' hypothesis whereby the sex that arrives earlier on the breeding grounds winters closer to them. To maintain stable populations, conservation must take account of this differential use of wintering grounds by the two sexes.
Two main questions are discussed: (1) the eastern limit of Dunlin Calidris alpina populations migrating through the Baltic and (2) migration between the Baltic and the Mediterranean/Black Sea. Ringing and moult data show that some Dunlins migrating to the Baltic orignate from more easterly regions than previously presumed. Many Dunlins migrating in autumn through the Baltic are in active moult. Some of them probably start their moult while breeding and originate from areas east of the Urals. These birds show some easily recognized plumage patterns: their new median coverts (usually only some of them) are of "adult buff" type. This is characteristic of Central and Eastern Siberia populations which start moulting very early, while still on their breeding grounds. Ringing data show that these birds winter in western Europe as well as in the Mediterranean. Some Dunlins from the Baltic migrate in autumn in a SE direction - to the Mediterranean and the Black Sea regions. The origin of these birds is not known. The SE direction may be used also by Polish breeding schinzii. Some Dunlins of the sub-species alpina, ringed in the Baltic in autumn, are controlled in spring at the Black Sea; in autumn they seem to migrate along a more northern route - through the Baltic, while in spring they choose a more southern route - through the Black Sea (loop migration).