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

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Abstract

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
MALE-BIASED SEX-RATIO OF DUNLINS
CALIDRIS ALPINA IN THE GULF OF GDAŃSK
(SOUTHERN BALTIC) DURING AUTUMN MIGRATION
RAZÓN DE SEXOS FAVORABLE A LOS MACHOS
EN EL CORRELIMOS COMÚN CALIDRIS ALPINA
EN EL GOLFO DE GDAŃSK (BÁLTICO MERIDIONAL)
DURANTE LA MIGRACIÓN OTOÑAL
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: w.meissner@ug.edu.pl
INTRODUCTION
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.
MATERIAL AND METHODS
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
MEISSNER, W.
336
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
MALE-BIASED SEX-RATIO OF DUNLINS 337
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).
RESULTS
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
test,
χ2
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
MEISSNER, W.
338
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
Kendall
coefficient = 0.60, N = 10, P = 0.016, fig. 3).
Ardeola 62(2), 2015, 335-342
MALE-BIASED SEX-RATIO OF DUNLINS 339
TABLE 1
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:
inmaduros).]
DISCUSSION
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
MEISSNER, W.
340
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
KULING no 152.
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Received: 6 October 2014
Accepted: 20 April 2015
Editor: José A. Masero
Ardeola 62(2), 2015, 335-342
MEISSNER, W.
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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