Sex differences in the stopover ecology of Curlew
Sandpipers Calidris ferruginea at a refuelling area
during autumn migration
JORDI FIGUEROLA* and ALBERT BERTOLERO Department of Animal
Biology (Vertebrate), Faculty of Biology, University of Barcelona, Avda.
Diagonal 645, E-08028 Barcelona, Spain
We investigated the stopover patterns of male and female Curlew Sandpipers at a
stopover area in northeast Spain. Curlew Sandpipers were trapped and
colour-ringed during autumn migration in 1992 and 1993. Stopover length
was similar to those reported previously for this and other waders that migrate
using a small number of widely separated staging areas, but were greater than
stopovers reported for other waders that migrate using a large number of
staging areas separated by short distances. The differences in stopover length
between the birds using these two strategies could be related to the fuel reserves
that have to be accumulated to reach the next staging area. Males stayed longer
in the area than females. Seasonal changes in prey availability or sex differences
in moulting and migratory patterns do not account for these differences in
stopover ecology. Following a time-selected model of optimal migration, sex
differences in stopover ecology could be related to a dominance of the larger
females over the males or to a higher foraging efficiency or a shorter search and
settling time in females. Whether these differences are restricted to the studied
area or are widespread in other staging areas used by the species could be
important for assessing the possible differences in the migration speed of
Curlew Sandpipers. The finding that males leave the breeding grounds 21—35
days before females but arrive at the study area with only a 10-day difference
supports the hypothesis that females migrate faster than males at least in the
first half of their migration.
D
uring migration, long-distance migrants
have to replenish their energetic reserves
several times at a number of staging areas.1 The
overall time needed to complete migration is
limited by the time necessary to accumulate
new reserves2 (short periods of flight alternate
with longer ones of fat accumulation) and, to a
lesser extent, by flight speed. Two strategies
of migration have been described and documented mainly in waders.3 The first strategy is
used by ‘hoppers’ which make short flights and
use a large number of staging areas along their
*Correspondence author.
Email: jordif@porthos.bio.ub.es
migratory route. The second is used by
‘jumpers’ which fly large distances between the
small number of staging areas they use to complete their migration.
Alerstam & Lindström2 produced a model
based on the optimization criteria of the
decisions faced by a bird accumulating fat at a
staging area. These models assumed that a bird
assessed the moment to leave a staging area in
terms of its present fat reserves, rate of fat
accumulation and its expected future fat gain.2,4
These authors presented different models to
explain the decisions of a bird at a stopover
area. The differences between these models
rely on the variables the birds try to optimize.
‘Time-selected’ migrants are expected to
maximize migration speed and so are very
sensitive to variations in their fat deposition
rate. According to this model, birds with
higher intake rates or which select habitats
with greater food resources will migrate
faster than other birds that do not forage as
efficiently or use less favourable habitats.
‘Energy-selected’ migrants try to complete their
migration with lower energetic costs by
maximizing the relationship between flight
range and fat load. The utility of accumulating
additional fat decreases with increasing fat
reserves because heavy body mass increases
flight costs.5 Alerstam & Lindström’s model
predicts that migration decisions of energyselected migrants are less sensitive to variations in fat deposition rate.
According to the time-selected migration
model, dominant individuals or more efficient
foragers should depart earlier and/or with
greater fat reserves than subdominants because
dominants will be able to forage in more
favourable areas. In species with energyselected migration, dominants should depart
with lower fat loads than subdominants and,
according to this model, no difference in
stopover length is to be expected between these
two groups. In this model the primary benefit
for dominants is a reduced cost for search and
settling.2 The effect of social status on departure
mass and stopover length in time-selected
migrants could explain the differences in
stopover ecology of young and adults of
several species during autumn migration6,7 and
of males and females in spring.8,9
Despite the fact that most studies of the
stopover ecology of waders have found agerelated differences in the residence patterns in
autumn,7,1O no study has focused on the
stopover patterns of males and females at a
staging area during the autumn migration.
Such studies could provide basic information
to determine the possible intraspecific differences in the costs of migration, information
necessary to determine the factors responsible
for the winter distribution of waders.11 In this
study we analyse the stopover patterns of adult
male and female Curlew Sandpipers Calidris
ferruginea at a staging area in autumn. The
stopover ecology of this species was examined
in relation to the optimization models of
Alerstam & Lindström2 to test the hypothesis
that the sexes differ in their stopover length.
METHODS
Curlew Sandpiper migration was studied at
Les Salines de la Trinitat (4O°37′N OO°35′E;
northeast Spain). The study area consisted of an
area of mud and sand-flats in a saltpan near the
sandy coast of the Mediterranean Sea. Little
tidal oscillation occurs on this coast and the
greatest variation in water level in the saltpan
was produced by changes in wind force and
direction, or when salt extraction was in
progress. Birds were trapped at night by
mist-net and by day using walk-in-traps from
27 July to 12 October 1992 and 25 July to
24 September 1993. Birds were colour-ringed
and body mass, wing, bill and tarsus lengths of
most birds were measured.12 Individuals were
aged12 and the sex of adult birds (more than
one-year-old) was determined from wing and
bill length using the discriminant formula of
Wymenga et al.13 Body mass at ringing was corrected to remove the effect of water loss and the
emptying of the gut occurring between
capture and data recording, assuming a rate of
mass loss of O.88% per hour of captivity.14 Data
from moulting birds were not included in
the analysis because moulting increases the
energetic requirements of birds15,16 and the rate
of reserve accumulation at the study area was
lower in moulting than in non-moulting
Curlew Sandpipers. 17 Because the rate of
reserve accumulation was one of the variables
that affect stopover decisions,2 the inclusion of
these birds would have affected the results of
our analysis.
Estimation of stopover length
Approximately every other day a transect of 2.2
km was surveyed to locate and identify colourringed birds present in the area. From this
transect we censused 44 ha of mud and sandflats, covering most of the suitable habitat for
Curlew Sandpipers in the saltpan. Additional
records of colour-marked birds obtained from
casual observations were also considered.
Estimating true stopover length is difficult
because birds were usually not trapped immediately after arrival nor just before leaving the
study area. To estimate stopover length we
used the method described in Holmgren et al.7
This method is based on a mark—recapture
model derived from Jolly18 and Seber.19 The
assumptions of this model are that each
individual has a constant probability of staying
in the area and that the probability of resighting
was the same for all birds. Using this model we
calculated the maximum likelihood estimate of
the stopover probability (ϖ) and the estimate of
mean stopover length (τ) separately for each
sex on each year (see Holmgren et al.7 for a
detailed description of the model and parameter estimation).
Statistical analyses
Differences in stopover probabilities were tested by calculating the standard normal deviate
(d) and the associated P value.7 Differences in
body condition among groups were tested
with square-root-transformed body mass
using a trifactorial ANCOVA. In this analysis
sex (male/female), control (resighted/not
resighted) and year (1992/1993) were used as
factors. Culmen length was introduced as a
covariate to remove the effect of body size on
body mass. When significant interactions
between the factors were detected we used
Scheffè multiple range tests to examine the
differences between particular levels of the
factors.2O
RESULTS
Relationship of body mass and stopover
length
Although females were larger than males,21
differences in body mass were not significant
after controlling for the effect of bird size
(three-way ANCOVA, F1,9O1 = O.27, P = O.61) and
none of the interactions of sex with the other
two factors was significant (sex—control: F1,9O1 =
2.11, P = O.15; sex—year: F1,9O1 = 1.27, P = O.26).
Body mass of marked birds was greater in 1992
than in 1993 (three-way ANCOVA, F1,9O1 = 17.O7, P
< O.OOO1). Body mass of birds resighted did
not differ from those that were not resighted
(three-way ANCOVA, factor control, F1,9O1 = 1.27,
P = O.26).
However, a significant interaction was
detected between two of the factors (control—
year, F1,9O1 = 4.29, P = O.O4). This significant
interaction appeared because birds resighted in
1993 were leaner than birds trapped in 1992
(both resighted and not resighted, Scheffè tests,
P < O.OO3 for both comparisons and P > O.2O for
the rest of contrast). The relationships between
body mass at ringing and stopover length
were also analysed separately for each sex and
year. None of the linear regressions between
body mass and stopover lengthwas significant
(r ranging from —O.O5 to —O.12 and P from O.3O
to O.71).
Frequency and length of stopover
In 1992 we colour-marked 517 individuals, of
which 99 were later resighted (19.1%). In 1993,
89 out of 393 marked individuals were resighted after the date of ringing (22.6%). Numbers of
marked and resighted individuals did not
differ between years (χ2 = 1.67, 1 df, P = O.2O),
nor between sexes (1992: χ2 = 1.43, 1 df, P = O.23;
1993: χ2 = O.45, 1 df, P = O.5O). The estimated
stopover length of males was longer than
females in 1992 (d = 1.84, P < O.O5), in 1993
(d = 1.87, P < O.O5), and when data of both years
were combined (d = 2.64, P < O.OO5), see Table 1.
Table 1. Observed mean stopover length of males and females resighted at least once (t) and estimated mean
stopover length of the Curlew Sandpiper population (τ) both expressed in days. Daily stopover probability (ϖ)
and its standard deviation.
Males
Females
Year
t
n
τ
ϖ
sd
t
n
τ
ϖ
sd
1992
15.O
56
13.O
O.93
O.OO9
1O.8
49
8.8
O.9O
O.O1O
1993
16.8
5O
14.O
O.93
O.OO9
12.O
44
9.2
O.9O
O.O1O
Total
15.9
1O6
13.5
O.93
O.OO6
11.4
93
9.O
O.9O
O.O1O
DISCUSSION
Stopover length and migration strategy
Comparing the stopover lengths reported in
different studies is difficult because authors
usually use different methods to estimate mean
stopover length. Accounting for this shortcoming, the stopover lengths presented in this
study were similar to the approximately 14
days reported at a staging area on the German
Waddensea in autumn 1992,22 and fall within
the range of 7—21 days for individuals
refuelling on a British estuary.23 Similar
stopover lengths were also reported for the
Semipalmated Sandpiper Calidris pusilla in
North America.24,25 Both species were considered to use a jumping strategy.24,26 However, all
these estimates were higher than the stopover
lengths reported for other waders of similar
size that were considered to use a hopping
strategy.1O,27,28 All Dunlins Calidris alpina migrating through Ottenby stayed in the area for less
than nine days (a mean of O.55—1.44 days7) and
very short stopovers were also reported for the
Western Sandpiper Calidris mauri and Least
Sandpiper Calidris minutilla during autumn
migration along the western coast of North
America.27,29
The differences in flight distance between
species that jump and those that hop suggest
that jumpers have to depart with higher levels
of reserves and have to stay longer in the
staging areas to replenish their fat reserves.
Although theoretical flight ranges of jumpers
and hoppers ready to depart in spring did not
differ at a wintering area in Mauritania,14 the
comparison of stopover lengths of several
species in different areas suggests that jumpers
will stop for longer periods in the staging areas
to replenish their fat reserves. The apparent
discrepancy between these two results could
occur because both jumping and hopping
strategies might occur in the same species and
thus among the individuals using the same
refuelling area.
This pattern has been demonstrated by
Skagen and Knopf3O with the use of radiotracking to describe the residency patterns of
Semi-palmated Sandpipers and White-rumped
Sandpipers Calidris fuscicollis at a staging area
in spring. Their study showed that residency
patterns were intraspecifically variable and
although some birds departed with large fat
loads and in a position to carry out long uninterrupted flights, others leave with small
reserves and were only capable of short hops.
The occurrence in one species of both migration
strategies could explain why differences in
flight range were not found by Zwarts et al.14
and that reported estimates of stopover length
(a variable probably more influenced by birds
staying longer in the area) were greater in
jumpers than in hoppers.
Optimal models of bird migration predict
that in time-selected migrants, leaner birds will
make longer stopovers than fatter birds. We did
not find this relationship in our study nor in
other studies of several species of waders
with time-selected migration7,25,29,31,32 but see
Mascher.33 As Holmgren et al.7 pointed out,
these apparently random stopovers could
appear if birds tend to wait for suitable
weather conditions in order to resume migration and if weather changes randomly. In this
situation time-selectedmigrants would continue to deposit fat, but energy-selected migrants
would not.
Sex differences in stopover ecology
Our results show that, in autumn, male Curlew
Sandpipers stayed for longer than females at
the studied stopover area. Several factors could
have caused these differences. Male Curlew
Sandpipers migrate ahead of the females which
remain in the breeding grounds taking care of
the brood.21,26 The density of prey available for
birds at a staging area could change during
autumn migration and prey depletion by early
migrants has been reported in several studies
(reviewed in Szèkely & Bamberger34). Males
arrived at our study area ten days before
females in 1993 and prey density available for
each sex could have changed in this time.
However, in 1992 the breeding success of
arctic waders was very low35 and in this year
probably led to males arriving only four days
before females.36
If temporal variation in prey availability was
the cause of the differences in stopover ecology
reported in this study, differences in stopover
length of males and females should appear in
1993 but not in 1992 (when both sexes migrated
approximately at the same time and stayed in
the study area under the same biotic and
abiotic conditions). This, however, is not the
case and so we reject the hypothesis that temporal variation in prey availability may have
produced the differences in stopover length
reported in this study.
Another possible source of bias in our estimates of stopover length could be related to the
inclusion in the data set of birds in active wing
moult. Although we have excluded from our
analysis birds trapped in active wing moult, it
is possible that some birds started primary
moult just after being ringed, thereby increasing our estimates of stopover length. Some
studies have reported that male Curlew
Sandpipers start to moult before females,26,37,38
although others failed to find any difference.39,4O
More males starting primary moult in our
study area could have increased the estimated
stopover length of this sex. However, the
number of birds that started wing moult in the
study area was small (6.2% of the adult birds
ringed in autumns 1992 and 1993), and
although more males started to moult in the
study area in 1993, no difference in the number
of males and females moulting in the area was
detected in 1992.17 Longer stopovers of males
were reported in both years, so we also reject
the possibility that differences in moult
patterns were the cause of the reported sex
differences in stopover length (at least of those
found in 1992).
According to the models of optimal migration,2 a variety of factors could explain the sex
differences in stopover length. Females could
stay for a shorter period in the study area if
they leave with lower levels of accumulated
reserves. To examine this issue, Figuerola &
Bertolero41 compared theoretical flight ranges
of males and females staying at the Ebro Delta.
Arrival body mass was calculated as the
average body mass of the 1O% of leanest birds
trapped in the area, and departure body mass
was calculated as the average body mass of the
1O% of heaviest birds. These approximate
calculations suggested that males accumulated
approximately 3O.4 g and females 32.5 g of
body reserves. If these figures reflect the real
situation, the sexes did not differ greatly in
their estimated flight range (3171 versus 3288
km) and the differences were not consistent
with the hypothesis that females leave with
smaller fat loads than males.
The time-selected model of optimal migration suggests that birds with higher fat
deposition rates or a lower search-settling time
(time between arrival at the area and initiation
of efficient fat deposition) could depart earlier
from the staging areas.2 These differences in fat
deposition rate and search costs could appear
if one sex dominates the other or if sexes use
ecologically different resources. In a field
experiment at a staging area in Scandinavia,
Lindström et al.42 demonstrated that dominant
Bluethroats Luscinia svecica accumulated fat at a
higher rate than subdominant ones. Although
information on the dominance structure of
Curlew Sandpiper populations is scarce (but
see Puttick43), it is common among birds that
the larger sex dominates the smaller one.44,45
Female Curlew Sandpipers are larger than
males so it is fair to expect that they dominate
males. Sexual dimorphism in bill length is also
acute and could allow females to capture prey
buried more deeply in the mud. According to
these differences Puttick46 found in a wintering
area in South Africa that the diet of the sexes
differed and that females dominated males,
foraged more successfully and needed less
time foraging to cover their energetic requirements.43
All these observations lend support to the
hypothesis that the sex differences in stopover
length reported in this study could be
produced by differences between sexes in the
capacity to accumulate fat in the study area
and/or because females can start to deposit fat
earlier than males (i.e. have a shorter searchsettling time). Our study area could be
especially suitable for females, whereas other
areas could be more favourable for males.
If the differences we found in the stopover
ecology of Curlew Sandpiper also occur in the
other staging areas used by the species, this
would mean that the females had a higher
overall migration speed, because this sex will
stay at the stopover areas for less time than
males. Males leave breeding grounds over
three weeks before females21 but arrived at our
study area in 1993 only 1O days ahead of
females.36 This reduction of the differences in
the timing of passage for each sex gives
support to the hypothesis that sexes differ in
their migration speed. Obviously, the results
presented here do not demonstrate that in
Curlew Sandpipers sexes differ in their
migration efficiency. However, they are consistent with this hypothesis. We hope that this
paper might stimulate further research on sex
differences in stopover ecology and searchsettling patterns in species with significant
normal or reverse sexual dimorphism in body
size.
ACKNOWLEDGEMENTS
This work was funded by the Ebro Delta
Natural Park, Diputació de Tarragona and
Grup Català d ‘Anellament. The Generalitat de
Catalunya (Catalan Autonomous Government)
provided the permits necessary for carrying
out the fieldwork. L.M. Copete and L.
Gustamante made an indispensable contribution to the fieldwork. We would also like to
thank J.M. Arcos, L. Brotons, L. Carrera, D.
Escobar (Fonoteca del Museo de Zoologia de
Barcelona), M.A. Franch, D. Froelich, S. Galan,
R. Marine, R. Marti, A. Martinez, R. Mateo, A.
Motis, D. Oró, X. Riera, A. Salmerón, J. Solans,
F. Vicents and R. Vidal for their help and friendship. M. Lockwood improved the English and
J.A. Amat, B.J. Ens, J.D. Goss-Custard, A.
Lindström, T. Piersma, J.C. Senar and D.A.
Stroud made valuable comments on earlier
drafts of this manuscript.
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