ArticlePDF Available

Stopover Strategy of Adult and Juvenile Red Knots Calidris c. canutus in the Puck Bay, Southern Baltic

Authors:

Abstract and Figures

For Red Knots Calidris c. canutus sandy coasts of the southern Baltic seem to be a low quality stopover site, because of unpredictable feeding conditions and a low density of molluscs. Observation collected in the Puck Bay, Poland, suggest that Knots feed mainly on Nereis sp., Gammarus sp., small sized Hydrobia sp. and small insects taken from wet sand. Among 1471 Knots ringed 27.8% of juveniles and 10.5% of adults were retrapped few days after the first capture, which indicates that the majority had a short stay and apparently departed with small energetic reserves. In spite of poor feeding conditions the fuel deposition rate of the Knots in autumn (adults: 2.4 g per d, juveniles: 2.7 g per d) was similar to that observed in the Dutch Wadden Sea and the Baltic coast of SE Sweden. It is suggested that Knots are able to accumulate body stores despite low food quality and density because the lack of tides allow them to forage without interruption, and because there is no additional energetic cost related to increasing gizzard size.
Content may be subject to copyright.
INTRODUCTION
The occurrence of Knot Calidris canutus outside
the breeding season is restricted to soft-sediment
intertidal areas, where it mainly feeds on small
molluscs. Red Knots C. canutus canutus appear
only during autumn migration towards African
wintering grounds in the Baltic region (Meissner
2005a). This species undertakes long-distance
flights between the arctic breeding grounds and
temperate or tropical wintering areas (Piersma &
Davidson 1992), and for fast refuelling it needs to
stop in sites with high densities of benthic inverte-
brates (Piersma et al. 1992). The sandy coasts of
the southern Baltic represent a low quality habitat
for migrating Knots, where the birds might
encounter difficulties to find sufficient food to
meet energy demands. Bivalves, which are the
main prey of Knot outside the breeding period, are
inaccessible for most of the time due to a lack of
regular tides. Even when the water level drops
during periods of strong wind and the bottom in
shallowest places is exposed, the density of mol-
luscs within reach of the waders is much lower
than in the Dutch Wadden Sea (Piersma et al.
1993, Kube 1994, Górecki 2003, Wlodarczak-
Komosi´nska 2004). Moreover, feeding conditions
for waders on the Baltic coasts are very change-
able and unpredictable due to variable water level
(Kube 1994, Wlodarczak-Komosi´nska 2004).
Stopover strategy of adult and juvenile Red Knots
Calidris c. canutus in the Puck Bay, southern Baltic
Meissner W. 2007. Stopover strategy of adult and juvenile Red Knots
Calidris c. canutus in the Puck Bay, southern Baltic. Ardea 95(12):
97–104.
For Red Knots Calidris c. canutus sandy coasts of the southern Baltic
seem to be a low quality stopover site, because of unpredictable feeding
conditions and a low density of molluscs. Observation collected in the
Puck Bay, Poland, suggest that Knots feed mainly on Nereis sp.,
Gammarus sp., small sized Hydrobia sp. and small insects taken from
wet sand. Among 1471 Knots ringed 27.8% of juveniles and 10.5% of
adults were retrapped few days after the first capture, which indicates
that the majority had a short stay and apparently departed with small
energetic reserves. In spite of poor feeding conditions the fuel deposi-
tion rate of the Knots in autumn (adults: 2.4 g per d, juveniles: 2.7 g per
d) was similar to that observed in the Dutch Wadden Sea and the Baltic
coast of SE Sweden. It is suggested that Knots are able to accumulate
body stores despite low food quality and density because the lack of
tides allow them to forage without interruption, and because there is no
additional energetic cost related to increasing gizzard size.
Key words: Knot, southern Baltic, autumn migration, stopover strategy
1
Avian Ecophysiology Unit, Department of Vertebrate Ecology &
Zoology, University of Gda´nsk, Al. Legionów 9, 80-441 Gda´nsk, Poland;
(w.meissner@univ.gda.pl)
Wlodzimierz Meissner1
98
ARDEA 95(1), 2007
Additionally, invertebrates in the brackish water of
the Baltic are smaller than their congeners in inter-
tidal areas of the North Atlantic (Kautsky 1998).
Furthermore, there is a strong competition from
other waders, especially Dunlin Calidris alpina and
Curlew Sandpiper Calidris ferruginea, which are
numerous on the Baltic coasts at the same time. In
mixed flocks Knots lose the majority of interspe-
cific aggressive interactions even with smaller
waders (Stawarczyk 1984).
The irregular occurrence of adult Knots and
their very low body mass (Meissner 2005a,
Meissner & Kamont 2005) indicate that the south-
ern Baltic is rather an emergency feeding place,
than a regular stopover site on the route from
Siberia to Africa. The mean body mass of adult
Knots caught in autumn in the southern Baltic is
among the lowest reported for the subspecies C. c.
canutus (Piersma et al. 1992, Meissner & Kamont
2005). Autumn records of juvenile Knots in the
southern Baltic are dispersed along the migration
route, and are less concentrated than adults at tra-
ditional stopover sites (Gromadzka 1992, Diersch-
ke 1995). Thus, it is possible that juveniles migrate
in small hops rather than in long-distance flights,
as adults do, using the southern Baltic as one of
many stopover sites along their migration route. If
true, it might be expected that adults and juveniles
behave differently in use of stopover sites.
The aim of this paper is to compare the
stopover pattern of adult and juvenile Knots at the
Polish Baltic coast in autumn, on their way
towards the wintering grounds. Particular focus is
on key features of stopover strategies, i.e. the
length of stay and body mass changes.
METHODS
Studies were conducted between 1988 and 1995
in the western part of the Gulf of Gda´nsk (Puck
Bay), on the narrow sandy spit in Rewa village
(Fig. 1). The length of the spit depended on water
level and varied from c. 500 to 1000 m. Waders
were caught and ringed on a regular basis, using
walk-in traps (Meissner 1998). Fieldwork lasted
from mid-July through the end of September. This
period covered almost the whole period of Knot
migration in the study area (Meissner & Sikora
1995). In total, 1071 juvenile and 400 adult Knots
were ringed. Each Knot was aged (Prater et al.
1977) and weighed with an accuracy of 1 g.
Lengths were measured of wing, total head, bill
and tarsus (Meissner 2000). To estimate energy
reserves, it was necessary to correct for body size
because the size of adults and juveniles decreased
progressively through the season (Meissner &
Kamont 2005). Among the measurements taken,
the wing length in adults and total head length in
juveniles showed the highest correlation coeffi-
cient with body mass (r=0.32 and r=0.33,
respectively, both P< 0.05). However, wing length
might vary within the season due to abrasion of
Gda´nsk
10 km
RM
VM
Vistula
Reda
PUCK BAY
RE
GULF OF GDA ´
NSK
Figure 1.
Location of the ringing site at Rewa (RE) and
other places mentioned in the text: RM = Reda mouth,
VM = Vistula mouth.
Meissner: STOPOVER STRATEGY OF KNOTS
99
the longest primary (Pienkowski & Minton 1973),
potentially affected by food quality during the
moult (Pehrsson 1987). Therefore, both in juve-
niles and adults the total head length was used to
correct body mass (correlation coefficient between
total head length and body mass in adults r=
0.29, P< 0.05). The adjusted body mass of each
bird was calculated from the residuals of the
regressions of body mass on total head length for
juveniles and adults.
Body mass changes were expressed as the dif-
ference between body mass at the first and subse-
quent captures. To obtain an instantaneous mea-
sure of the rate of body mass increase birds
trapped within four days after first capture were
selected. The length of stay was estimated using
capture–recapture techniques following Schaub et
al. (2001).
Along the coast of the Puck Bay only few sites
are suitable for waders to feed. The most impor-
tant ones are close to the river mouths of the
Vistula and Reda (where the main study area was
located). Within the study period, waders were
also ringed at the mouth of the Vistula river
(about 45 km apart in a straight line) (Gromadzka
1998). Some birds used only a single area, while
others were recorded in both regions. During
autumn migration the majority of wader move-
ments within Puck Bay were directed from the
eastern to the western part (Brewka et al. 1987).
To show the intensity of passages of Knots within
the Puck Bay, the Index of Direct Recoveries (IDR)
between two ringing sites was calculated (Busse
1982). It was deduced from the number of recov-
eries per 100 birds ringed at one site and 100 birds
caught at the second as:
IDR = V
1–2
x
10
4
NR
1
x
NC
2
where V
1–2
=number of birds ringed at the ring-
ing site 1 and recovered within the same season at
ringing site 2, NR
1
=number of ringed birds at
site 1, NC
2
=number of caught birds at site 2
(including retraps and recoveries). All other statis-
tical methods followed Zar (1996). Analyses were
done in STATISTICA 6.0 software (StatSoft 2001).
RESULTS
Number of retraps and length of stay
In total 298 (27.8%) juveniles were retrapped
within the same autumn, and only 42 (10.5%)
adults (comparing the percentages, χ
2
=45.89,
P<0.0001). The mean length of stay of adults
(2.98 days) was about three times shorter than in
juveniles (9.36 days).
Body mass changes
Two-way ANOVA indicated significant differences
in mean adjusted body mass among years (F
7,1327
=6.15, P<0.0001), but not age (F
1,1327
=0.03,
P=0.86) and not year-age interaction (F
7,1327
=
0.83, P=0.56; Fig. 2).
During the first four days after capture, juve-
niles exhibited a similar body mass increase
among years (ANOVA, F
4,208
=1.25, P=0.29).
The number of retraps of adults was much lower
and comparing two years with a reasonable sam-
ple size (1991 and 1995, when ten and eleven
retraps occurred) indicated no year effect (t-test,
P>0.05). Thus, within each age class samples
from all years were combined for subsequent
analyses.
Knots from both age classes increased body
mass significantly during their stay (Fig. 3).
160
40
80
200
120
body mass (g)
1988
79
1990 1992 1994
42
32
27 164
29
420
56 6
88
123
14 43
21
122
77
adult
juvenile
1989 1991 1993 1995
Figure 2.
Changes in the adjusted body mass in adult and
juvenile Knots caught in consecutive seasons. Indicated
are mean, SD (box), range (whiskers) and sample size.
100
ARDEA 95(1), 2007
Juveniles that were retrapped had a lower body
mass at first capture than birds caught only once.
There was no such difference in adults (Fig. 3).
There were large differences among individuals
in body mass increment. Some juveniles gained
more than 60 g after 10 days, while others hardly
changed their body mass after this period (Fig. 4).
The overall body mass increase during the first
four days was similar in adults (mean 2.4 g day
–1
±3.70 SD, n=38) and juveniles (mean=2.7 g
day
–1
±4.13 SD, n=223; t-test, t=0.39, P=
0.70). However, adults did not stay as long as
juveniles and their body mass at last capture was
on average about 7 g lower than that of juveniles
(Cochran-Cox test, t'=3.55, P=0.0007; Fig. 3).
Juveniles which lost body mass during the first
day after catching had a significantly higher
adjusted body mass than birds that put on mass
from the beginning onwards (Fig. 5) (t-test, t=
3.20, P=0.002). Small sample sizes of retrapped
adults did not allow a similar comparison,
although the few data indicate a similar pattern as
in juveniles; three adults which lost body mass
160
40
80
200
120
body mass (g)
no retr retr 2
244
retr 1
42 42
no retr retr 2
697
retr 1
300
300
adults juveniles
Figure 3.
Comparison of mean body mass of Knots caught
only once (No retr) and in retraps at the first (Retr 1) and
the last capture (Retr 2) in adults (ANOVA, F2,325 = 3.76,
P= 0.002) and in juveniles (ANOVA, F2,1294 = 56.27, P<
0.0001). Arrows indicate significant difference according
to post-hoc Tukey test at P<0.05. Indicated are mean,
SD (box), range (whiskers) and sample size.
25
63
160
40
80
120
body mass (g)
increasing
body mass decreasing
body mass
t-test, p=0.002
Figure 5.
Mean adjusted body mass at first capture in
juveniles increasing (dark grey) and loosing (light grey)
weight after one day. Indicated are mean, SD (box), range
(whiskers) and sample size.
0
-20
20
0
40
60
80
body mass (g)
10 122468 20
days
14 16 18
juveniles
-20
20
0
40
60
80
body mass (g)
adults
Figure 4.
Changes in the body mass in following days bet-
ween first and subsequent capture of adult and juvenile
Knots retrapped in Rewa.
Meissner: STOPOVER STRATEGY OF KNOTS
101
during the first day after capture, weighed 112 g,
119 g and 134 g, which was much higher than the
mean adjusted body mass of adults that increased
their weight after one day (mean 101.0 ± 12.27
SD, n=12).
Ringing recoveries
Between 1988 and 1995 movements of 16 juve-
niles and 4 adults between Vistula and Rewa were
recorded. In juveniles the period between ringing
at Vistula and recapture in Rewa ranged between
0 and 33 days (median 3). Five of them (31%)
were retrapped after two days. In adults, three
birds were recorded the same day at both sites,
while the fourth was recorded at the second site
after three days. The Index of Direct Recoveries
between Vistula and Rewa was noticeably higher
in adults (IDR = 0.67) than in juveniles (IDR =
0.29). In the opposite direction only one juvenile
bird moved (IDR = 0.02).
Of the Knots ringed at Rewa, several were
recovered at a distance of more than 100 km; 24
juveniles and 6 adults. This indicates recovery
rates of 2.2% and 1.5%, respectively, which are
not significantly different (χ
2
-test, χ
2
=0.82, P=
0.37). Among these recoveries, six juveniles and
one adult were retrapped in the western Baltic at
the Langenwerder Island (450 km in a straight line
from the study area) no later than after 9 days
(one juvenile after 21 days), while other records
came from the intertidal area of the North Sea and
Atlantic.
DISCUSSION
Fuel deposition rates of adult Red Knots in the
Puck Bay were similar to those observed in
autumn in the Dutch Wadden Sea (2.8 g d
–1
) and
in the Baltic coast of SE Sweden (2.7 g d
–1
, calcu-
lated for both age classes) (Nebel et al. 2000,
Helseth et al. 2005). However, in the Wadden Sea
Knots stay much longer than in the Baltic area and
the departure body mass of adult females is esti-
mated at about 200g (Nebel et al. 2000). During
their stay at the southern Baltic coasts Knots spend
most of their time on foraging (Dierschke & Rippe
1997, own observations), similar to other Calidris
species (Górecki 2003, Wlodarczak-Komosi´nska
2004). They are able to accumulate fat stores
despite low food quality and density because the
lack of tides makes long periods of foraging with-
out interruption possible. Another reason that
Knots achieve a similar fuelling rate on poor qual-
ity Baltic sandy coasts as in rich intertidal mudflats
might be the gizzard size, which in Knots varies
considerably during the year (Dekinga et al. 2001,
van Gils et al. 2005). After the breeding season the
gizzard of Knots increases rapidly in response to a
shift in diet from rather soft-bodied arthropods to
hard-shelled molluscs (Dekinga et al. 2001, van
Gils et al. 2005). These adjustments of digestive
organs are important for permitting the high feed-
ing rate during migration and wintering (van Gils
et al. 2003, 2005). However, a large gizzard
entails higher maintenance and transport cost and
hence overall energetic costs increase with gizzard
size (Piersma et al. 2003, van Gils et al. 2003).
There are no empirical data on the gizzard size of
Knots from Baltic stopover sites, but it is likely that
birds arrive there directly from the breeding
grounds (Meissner 2005a). Thus, when landing in
the southern Baltic area, both adults and juveniles
might still have a small stomach to increase later
in tidal areas when the birds change diet to hard-
shelled molluscs (Dekinga et al. 2001, Battley &
Piersma 2005). This assumption is in agreement
with data on small stomachs of Knots arriving in
autumn in the Dutch Wadden Sea and islandica
Knots making stopover in Iceland after leaving
their breeding grounds (Battley & Piersma 2005).
Data on stomach contents of two Knots obtained
by stomach pumping in 2001 and observations of
foraging birds also strongly suggest that they feed
on the same prey as Dunlins in this area, e.g.
Nereis sp., Gammarus sp., small sized Hydrobia sp.
and different small insects taken from wet sand
(A. Wlodarczak-Komosi´nska, unpubl. data).
The percentage of retraps was lower and the
mean length of stay was shorter in adults than in
juveniles, and similar differences were found in
other wader species in Puck Bay during autumn
102
ARDEA 95(1), 2007
migration (Meissner & Koziróg 2001, Meissner &
´
Sciborski 2002, Meissner 2005b, Meissner &
Górecki 2006). The very low number of retraps
(Meissner 1992, this study) indicates that the
majority of Knots probably departed with small
energetic reserves after a short stay. However,
some juvenile birds (especially those with small
energetic reserves at arrival) stopped for few days
and they were able to achieve a considerable gain
of weight. Thus, body mass at departure varied,
being larger in birds after some days of refuelling.
The decision to depart immediately or to stay and
gain body mass must be taken individually accord-
ing to information gathered from the environment.
Weather conditions might be an additional factor
which strongly influences the stopover time
(Alerstam 1979, Weber & Hedenström 2000).
However, in the period of Knot autumn migration,
weather conditions along the southern Baltic coast
are usually favourable for birds flying in western
direction and there are only few days with strong
head winds at low altitudes making westward
flight impossible (Remisiewicz 1996).
Adults and juveniles that stop at the non-tidal
areas of the southern Baltic were recorded later in
tidal areas of Western Europe (Gromadzka 1992),
and the recovery rate of Knots ringed in the Puck
Bay is higher than in many other wader species
(Meissner & Remisiewicz 1998). Thus, it seems
that Knots cope with poor feeding conditions at a
low quality stopover site, yet accumulate sufficient
energetic reserves for further migration. It is possi-
ble that the lack of additional energetic costs
related to a large gizzard is one of the reasons of
the high refuelling rate in Knots feeding on low
quality food on the southern Baltic sandy coasts.
Despite of a longer period of stay, juveniles
seemed to search for better feeding places less
often than adults. One of the reasons might be
that juveniles lack previous experience consider-
ing high profits of foraging on molluscs and about
the relative richness of tidal mudflats of the
Wadden Sea.
ACKNOWLEDGEMENTS
This study was conducted by the Waterbird Research
Group KULING (paper no. 125). Jaga Gromadzka kindly
provided data on the number of ringed and caught Knots
in the Vistula mouth. We are grateful to all colleagues
from WRG KULING and to many volunteers who helped
in collecting the data in the field. Special thanks to Anna
Wlodarczak-Komosi´nska, who provided data on prey of
Knots and to Robert Krupa and Maciej Kozakiewicz who
spent many days in Rewa creating unforgettable atmos-
phere of this ringing station. And final thanks to
Magdalena Remisiewicz for help in preparation of this
paper and to two anonymous referees for helpful com-
ments.
REFERENCES
Alerstam T. 1979. Wind as selective agent in bird migra-
tion. Ornis Scand. 10: 76–93.
Battley P.F. & Piersma T. 2005. Adaptive interplay
between feeding ecology and features of the digestive
tract in birds. In: Starck J.M. & Wang T. (eds)
Physiological and ecological adaptations to feeding in
vertebrates: 201–227. Science Publ., Enfield.
Busse P. 1982. Finding of local passage direction as the
result of an analysis of retraps and short distance
direct-recoveries. Not. Orn. 22: 31–39. (in Polish
with English summary).
Brewka B., Meissner W., Sikora A. & Skakuj M. 1987.
Four years of the activity of Waterbird Research
Group “KULING”. Ring 11: 339–347.
Dekinga A., Dietz M.W., Koolhaas A. & Piersma T. 2001.
Time, course and reversibility of changes in the giz-
zards of red knots alternately eating hard and soft
food. J. Exp. Biol. 202: 2831–2837.
Dierschke V. 1995. Der Knutt Calidris canutus. Ber.
Vogelw. Hiddensee 12: 129–132.
Dierschke V. & Rippe H. 1997. Ernährungsbedingungen
für Kiebitzregenpfeifer Pluvialis squatarola und
Knutts Calidris canutus im Windwatt bei Hiddensee,
deutsche Ostseeküste. Vogelwelt 118: 269–275.
Górecki D. 2003. Behaviour of the Curlew Sandpiper
Calidris ferruginea (Ponn. 1763) during autumn
migration in the Gulf of Gda´nsk. PhD thesis. Warmia
and Masuria University, Olsztyn. (in Polish)
Gromadzka J. 1992. Knots on the Polish Baltic coast.
Wader Study Group Bull. 64 Suppl.: 161–166.
Gromadzka J. 1998. Wader ringing at the Vistula mouth
(Baltic coast, Poland) – a summary of the long-term
studies. Ring 20: 5–20.
Meissner: STOPOVER STRATEGY OF KNOTS
103
Hel
seth A., Lindström Å., Stervander M. 2005. Southward
migration and fule deposition of Red Knots Calidris
canutus. Ardea 93: 213–224.
Kautsky L. 1998. Monitoring eutrophication and pollu-
tion in estuarine environments – focusing on the use
of benthic communities. Pure Appl. Chem. 70:
2313–2318.
Kube J. 1994. Aspekte der Nahrungsökologie ziehender
Limikolen an der südlichen Ostseeküste. Corax 15,
Sonderheft 2: 57–72.
Meissner W. 1992. Knots’ autumn migration in the west-
ern part of the Gulf of Gda´nsk, Poland: preliminary
results. Wader Study Group Bull. 64 Suppl.:
167–171.
Meissner W. 1998. Some notes on using walk-in traps.
Wader Study Group Bull. 86: 33–35.
Meissner W. 2000. The wader station. In: Busse P. (ed)
Bird Station Manual: 98–102. University of Gda´nsk,
Gda´nsk.
Meissner W. 2005a. Variation in timing of the Siberian
Knot Calidris c. canutus autumn migration in the
Puck Bay region (southern Baltic). Acta Ornithol. 40:
95–101.
Meissner W. 2005b. Autumn migration of the Broad-
billed Sandpiper Limicola falcinellus on the southern
Baltic Coast. Ring. Migrat. 22: 171–176.
Meissner W. & Górecki D. 2006. Biometrics and body
mass variation of Curlew Sandpiper Calidris ferrug-
inea caught on the Puck Bay coast, Poland, during
southward migration. Internat. Wader Studies 19:
125–129.
Meissner W. & Kamont P. 2005. Seasonal changes in body
size and mass of Red Knots Calidris canutus during
autumn migration through southern Baltic. Ornis
Svecica 15: 97–104.
Meissner W. & Koziróg L. 2001. Biometrics of Turnstone
Arenaria interpres migrating in autumn through Gulf
of Gda´nsk region. Ornis Svecica 11: 181–188.
Meissner W. & Sikora A. 1995. Spring and autumn migra-
tion of waders (Charadrii) on the Hel Peninsula.
Not. Orn. 36: 205–239. (In Polish with English sum-
mary)
Meissner W. & ´
Sciborski M. 2002. Autumn migration of
the Bar-tailed Godwit (Limosa lapponica) in the Gulf
of Gda´nsk region. Ring 24: 3–15.
Nebel S., Piersma T., van Gils J., Dekinga A. & Spaans B.
2000. Length of stopover, fuel storage and sex-bias in
the occurrence of Red Knots Calidris c. canutus and C.
c. islandica in the Wadden Sea during southward
migration. Ardea 88: 165–176.
Pehrsson O. 1987. Effects of body condition on molting in
Mallards. Condor 89: 329–339.
Pienkowski M.W. & Minton C.D.T. 1973. Wing length
changes of the Knot with age and time Since Mount.
Bird Study 20: 63–68.
Piersma T., Davidson N.1992. The migrations and annual
cycles of five subspecies of Knots in perspective.
Wader Study Group Bull. 64 Suppl.: 187–197.
Piersma T., Dekinga A., van Gils J.A., Achterkamp B. &
Visser G.H. 2003. Cost-benefit analysis of mollusc-
eating in a shorebird. I. Foraging and processing costs
estimated by the doubly labeled water method. J.
Exp. Biol. 206: 3361–3368.
Piersma T., Hoekstra R., Dekinga A., Koolhaas A., Wolf P.,
Battley P. & Wiersma P. 1993. Scale and intensity of
intertidal habitat use by Knots Calidris canutus in the
western Wadden Sea in relation to food, friends and
foes. Neth. J. Sea Res. 31: 331–357.
Piersma T., Prokosch P. & Bredin D. 1992. The migration
system of Afro-Siberian Knots Calidris canutus canu-
tus. Wader Study Group Bull. 64 Suppl.: 52–63.
Prater A.J., Marchant J.H. & Vuorinen J. 1977. Guide to
the identification and ageing of Holarctic waders.
BTO, Tring.
Remisiewicz M. 1996. Influence of weather conditions on
the autumn migration of Dunlin (Calidris alpina) at
the southern Baltic. Ring 18: 73–88.
Schaub M., Pradel R., Jenni L. & Lebreton J.-D. 2001.
Migrating birds stop over longer than usually
thought: an improved capture-recapture analysis.
Ecology 82: 852–859.
StatSoft Inc. 2001. STATISTICA (data analysis software
system), version 6. www.statsoft.com.
Stawarczyk T. 1984. Aggression and its suppression in
mixed-species wader flocks. Ornis Scand. 15: 23–57.
van Gils J.A., Battley P.F., Piersma T. & Drent R. 2005.
Reinterpretation of gizzard sizes of red knots world-
wide emphasis overriding importance of prey quality
at migratory stopover sites. Proc. R. Soc. Lond. B.
272: 2609–2618.
van Gils J.A., Piersma T., Dekinga A. & Dietz M.W. 2003.
Cost-benefit analysis of mollusc-eating in a shorebird.
II. Optimizing gizzard size in the face of seasonal
demands. J. Exp. Biol. 206: 3369–3380.
Weber T. P. & Hedenström A. 2000. Optimal stopover
decisions under wind influence: the effects of corre-
lated winds. J. Theor. Biol. 205: 95–104.
Wlodarczak-Komosi´nska A. 2004. Feeding ecology of
Dunlin Calidris alpina (L. 1758) and Little Stint
Calidris minuta (Leisler 1822) during autumn migra-
tion in the Gulf of Gda´nsk. PhD thesis. Warmia and
Masuria University. Olsztyn. (In Polish)
Za
r J.H. 1996. Biostatistical analysis. Third edition.
Prentice
-Hall, London.
104
ARDEA 95(1), 2007
SAMENVATTING
Voor Kanoeten Calidris c. canutus op najaarstrek lijkt het
zuiden van de Oostzee op het eerste gezicht een matig
tussenstopgebied. De zandige kust herbergt amper
schelpdieren en de voedselomstandigheden zijn onvoor-
spelbaar vanwege de windrichting en windkracht, die de
waterstand en daarmee de mogelijkheid om voedsel te
zoeken bepalen. Om te onderzoeken of deze onvoorspel-
bare voedselomstandigheden een weerslag hebben op de
snelheid waarmee Kanoeten lichaamsreserves aanleggen,
werden vogels gevangen en gewogen in de Puck Baai in
Polen. In dit gebied aten Kanoeten vooral Zeeduizend-
poot Nereis sp., Vlokreeft Gammarus sp., Wadslakje
Hydrobia sp. en kleine insecten die ze van het zand
oppikten. Er werden aanzienlijk meer jonge (1071) dan
oude vogels (410) gevangen. Op basis van terugvangsten
in hetzelfde seizoen werd geschat dat de verblijfsduur
van jonge vogels 9,4 dagen bedroeg, van oude vogels
slechts 3,0 dagen. Het kwam als een verrassing dat de
gewichtstoename per dag (adulte vogels 2,4 g, jonge
vogels 2,7 g) vergelijkbaar was met de toename in de
Nederlandse Waddenzee en de Oostzeekust van ZO-
Zweden. Er wordt verondersteld dat het lage voedselaan-
bod gecompenseerd wordt door het ontbreken van getij
waardoor de vogels permanent kunnen eten. Ook zou het
ontbreken van de noodzaak de maag te vergroten, zoals
schelpeters dat doen, een extra energetisch voordeel kun-
nen opleveren. (JP)
Corresponding editor: Jouke Prop
Received 30 June 2006; accepted 16 March 2007
... However, in this study, we found no relationship between a proxy of body condition (scaled mass index) and the index of primary moult asymmetry. It should be borne in mind that sandy coasts of the southern Baltic do not offer good feeding conditions for waders due to the lack of tides (Kube 1994;Meissner 2007) and that the vast majority of non-juvenile arctic waders stopping here arrive with low energetic reserves, usually remain for a few days only and reveal only a slight body mass increase during this time (Meissner 1998(Meissner , 2007Meissner and Górecki 2006). Moreover, very few individuals are retrapped in subsequent migratory seasons (Waterbird Research Group KULING, unpublished data), which suggests that the Gulf of Gdańsk is an emergency feeding site for these birds rather than a regular stopover. ...
... However, in this study, we found no relationship between a proxy of body condition (scaled mass index) and the index of primary moult asymmetry. It should be borne in mind that sandy coasts of the southern Baltic do not offer good feeding conditions for waders due to the lack of tides (Kube 1994;Meissner 2007) and that the vast majority of non-juvenile arctic waders stopping here arrive with low energetic reserves, usually remain for a few days only and reveal only a slight body mass increase during this time (Meissner 1998(Meissner , 2007Meissner and Górecki 2006). Moreover, very few individuals are retrapped in subsequent migratory seasons (Waterbird Research Group KULING, unpublished data), which suggests that the Gulf of Gdańsk is an emergency feeding site for these birds rather than a regular stopover. ...
Article
Full-text available
The Dunlin is one of very few wader species that moults primaries when migrating to its wintering grounds. In our study, a total of 68.2% of immatures and 26.6% of adults underwent their primary moult when passing through the southern Baltic in autumn. More than 30% of moulting birds revealed differences in moult scores of left and right wings. However, 13% of Dunlins showed differences between the left and right wing not greater than 1%, and 50% of individuals showed differences between the two wings that were lower than 3% of the total mass of all primaries. The probability of asymmetry during the primary moult increased over time and decreased with the advancement of the primary moult. Sex and age of birds had no significant effect on the occurrence of moult asymmetry. The level of asymmetry in the primary moult increased in the following days of autumn migration and with the advancement of the primary moult. The mean index of primary moult asymmetry (the absolute value of the difference in moult advancement between the left and right wing) was lower in immature Dunlins than in adults, and in females compared to males. Hence, sex (males) and age classes (adults) that spend a longer time on the breeding grounds revealed a higher index of primary moult asymmetry, most probably as a result of higher levels of physiological stress. However, the low proportion of birds showing large asymmetry suggests that this is strongly constrained by selection for aerodynamic efficiency, as asymmetry in primaries affects aerodynamic stability, take-off costs, manoeuvrability and agility in birds with flapping flight.
... In addition, there are indications that juvenile shorebirds use more, and different, stopovers than adults (Davidson and Wilson 1992, Lindström et al. 2002, Meissner 2007. Because sex and age affect the timing of migration and may affect the migration route, we also investigated effects of age and sex on the southward migration strategy of C. c. islandica. ...
... Our most remarkable finding is the evidence that juveniles of C. c. islandica may also fly nonstop from their natal tundra to their wintering grounds in the Dutch Wadden Sea (Fig. 2), as it contradicts the expectation that during southward migration young shorebirds make more stopovers than adults (Davidson and Wilson 1992, Lindström et al. 2002, Meissner 2007. That juveniles of C. c. islandica wintering in the Netherlands fly nonstop from the natal site to the winter range implies that juveniles may be able to fuel up to a level similar to that of adults. ...
Article
Full-text available
Subspecies Calidris canutus islandica of the Red Knot breeds on the arctic tundra of northeastern Canada and northern Greenland and winters along the coasts of northwestern Europe. During northward migration, it stops over in either Iceland or northern Norway. It has been assumed that it does the same during southward migration. Using ratios of stable carbon isotopes (δ 13 C) in whole blood, blood cells, and plasma, we investigated evidence for a stopover in Iceland en route from the breeding grounds to the Dutch Wadden Sea. With the expected diet (shellfish) and stopover duration at Iceland (12-15 days, maximum 17 days) and the turnover rates of blood cells (15.1 days) and plasma (6.0 days), Red Knots that stopped in Iceland should arrive with a blood (cell) δ 13 C midway between a tundra (-24.7[per thousand]) and a marine value (-14.0[per thousand]) and a plasma δ13 C approaching the marine value (-15.3[per thousand]). However, many adults arriving at the Wadden Sea had δ13 C ratios in blood (cells) and plasma below these levels, and some arrived with clear tundra signals in blood cells, suggesting that they skipped Iceland during southward migration. Surprisingly, available data suggest this also to be true for juveniles during their first southward migration. The δ 13 C signature of second-year birds confirmed that they oversummered in the Wadden Sea. Our findings contradict the largely untested idea that juvenile shorebirds make more stopovers than adults as well as the idea that the migration between the Nearctic and Europe is necessarily a two-leg process.
... In addition, there are indications that juvenile shorebirds use more, and different, stopovers than adults (Davidson and Wilson 1992, Lindström et al. 2002, Meissner 2007. Because sex and age affect the timing of migration and may affect the migration route, we also investigated effects of age and sex on the southward migration strategy of C. c. islandica. ...
... Our most remarkable finding is the evidence that juveniles of C. c. islandica may also fly nonstop from their natal tundra to their wintering grounds in the Dutch Wadden Sea (Fig. 2), as it contradicts the expectation that during southward migration young shorebirds make more stopovers than adults (Davidson and Wilson 1992, Lindström et al. 2002, Meissner 2007. That juveniles of C. c. islandica wintering in the Netherlands fly nonstop from the natal site to the winter range implies that juveniles may be able to fuel up to a level similar to that of adults. ...
Article
Full-text available
Subspecies Calidris canutus islandica of the Red Knot breeds on the arctic tundra of northeastern Canada and northern Greenland and winters along the coasts of northwestern Europe. During northward migration, it stops over in either Iceland or northern Norway. It has been assumed that it does the same during southward migration. Using ratios of stable carbon isotopes (δ13C) in whole blood, blood cells, and plasma, we investigated evidence for a stopover in Iceland en route from the breeding grounds to the Dutch Wadden Sea. With the expected diet (shellfish) and stopover duration at Iceland (12–15 days, maximum 17 days) and the turnover rates of blood cells (15.1 days) and plasma (6.0 days), Red Knots that stopped in Iceland should arrive with a blood (cell) δ13C midway between a tundra (−24.7‰) and a marine value (−14.0‰) and a plasma δ13C approaching the marine value (−15.3‰). However, many adults arriving at the Wadden Sea had δ13C ratios in blood (cells) and plasma below these levels, and some arrived with clear tundra signals in blood cells, suggesting that they skipped Iceland during southward migration. Surprisingly, available data suggest this also to be true for juveniles during their first southward migration. The δ13C signature of second-year birds confirmed that they oversummered in the Wadden Sea. Our findings contradict the largely untested idea that juvenile shorebirds make more stopovers than adults as well as the idea that the migration between the Nearctic and Europe is necessarily a two-leg process. Calidris canutus islandica se reproduce en la tundra ártica en el noreste de Canadá y norte de Groenlandia y pasa el invierno a lo largo de la costa del noroeste de Europa. Durante la migración hacia el norte, los individuos de esta subespecie hacen paradas migratorias en Islandia o en el norte de Noruega. Para la migración hacia el sur, se ha asumido el mismo patrón de paradas migratorias. Utilizando cocientes de isótopos estables de carbono (δ13C) en la sangre, células sanguíneas y plasma sanguíneo investigamos si existe evidencia a favor de paradas en Islandia durante la ruta desde las áreas de cría hacia el Mar de Wadden en Holanda. Con base en la dieta esperada (mariscos) y en la duración esperada de la parada en Islandia (12–15 días, máximo 17 días) y en las tasas de recambio de las células sanguíneas (15.1 días) y plasmáticas (6.0 días), los individuos de C. c. islandica que habrían parado en Islandia deberían llegar con un δ13C en la sangre (células) con un valor medio entre tundra (−24.7 ‰) y marino (−14.0 ‰) y con δ13C plasmático cercano al valor marino (−15.3‰). Sin embargo, muchos adultos que llegaron al Mar de Wadden tuvieron un cociente de δ13C en la sangre (células) y en el plasma menor al de los niveles esperados, y algunos llegaron con señales claras de tundra en las células sanguíneas, lo que sugiere que estos individuos se saltaron la parada en Islandia durante la ruta migratoria hacia el sur. Sorpresivamente, los datos disponibles también surgieren este mismo patrón para los juveniles durante su primera migración hacia el sur. La señal del δ13C de los individuos en su segundo año confirmó que estos prolongaron su estadía durante el verano en el Mar de Wadden. Nuestros resultados contradicen la idea no corroborada de que los juveniles de aves costeras hacen más paradas que los adultos, como también la idea de que la migración entre el Neártico y Europa es un proceso que necesita dos etapas.
... Common Ringed Plovers spent more time foraging in the regulated section of the river, probably because of the lower abundance of invertebrates at Lisewo. Shorebirds can compensate for lower quality foraging sites by increasing the time they spend feeding to satisfy their energy requirements (Dierschke 1994;Meissner 2007). In this study we showed that juvenile birds spent more time foraging, whereas adults spent similar time feeding in the unregulated and the regulated sections of the river. ...
Article
Full-text available
River development and dam construction alter ecological conditions by changing geomorphologic processes affecting the nutrient and biogeochemical cycles in riverine ecosystems. Changes to the food chain might thus harm the quality of migratory shorebirds’ stopover sites. In this study we compare the abundance of potential prey and the foraging behaviour of migrating adult and juvenile Common Ringed Plovers in autumn at unregulated and regulated stretches of the Vistula River, an important stopover site for migrant shorebirds. We collected invertebrates from the substrate’s surface and the top 3 cm. We also observed the birds’ foraging behaviour, noting characteristic feeding techniques, such as pecking, probing and foot trembling; foraging intensity; distance between foraging individuals, and the number of aggressive interactions. The lower abundance of invertebrates in the regulated section below the Włocławek Dam extended the plovers’ foraging times and required more pecks per minute for them to meet their energy requirements. The confined foraging habitat of the regulated river forced the migrants to gather closer than on the unregulated section, causing more intraspecific aggression. Nevertheless, worse foraging conditions in the Vistula’s regulated section affected only the inexperienced juvenile Ringed Plovers migrating for the first time. Our study shows that the river below the dam offers less potential prey for migrating shorebirds than the semi-natural section above the dam. Lower quality stopover sites may reduce juvenile plovers’ prospects of successful migration, because the rate of fat deposition in migratory shorebirds is related to conditions at stopover habitats, which also affect their speed of migration.
... In autumn, most islandica stop in the British Isles and overwinter there (Davidson & Piersma 2009). The rest of the islandica population arrives in the eastern Wadden Sea and a few of them spread along the coasts of the Baltic (Nehls et al. 1987, Meissner 2004, 2007). Then, they can stage for moulting alongside canutus for several weeks, but almost exclusively in the western part of the Wadden Sea (Koopman 2002, Nebel et al. 2000, van Roomen et al. 2005). ...
Article
Wader Study Group Bulletin 119 (2012): 17-25 The two subspecies of the Red Knot Calidris canutus that occur in Europe during northward and southward migration, islandica and canutus, are only observed simultaneously at a few sites such as the Wadden Sea. Mostly islandica winters on estuarine bays in NW Europe, while canutus go to wintering grounds in W or S Africa. The coasts of France have been described as the main southern limit of the winter distribution of islandica and as providing stopover sites for canutus migrating between the W African coast and breeding grounds in Siberia. Nevertheless, the role and the importance of French sites remain unclear for both subspecies, especially during southward migration. This study updates information on the numbers and the distribution of Red Knots staging or wintering along the coasts of France using International Waterbird Census (IWC) data (counts carried out in Jan, 1976–2010, organised by Wetlands International) and synchronized monthly counts carried out in France’s National Nature Reserves during 2000–2010. In recent years, France has supported around 9% (c. 35,000 individuals) of the estimated population of islandica in mid-winter. Ninety percent of these birds are concentrated in just six bays, two along the Channel coast and four along coasts of Vendée and Charente-maritime. As intertidal areas are limited along the Mediterranean shore, it does not support Red Knots in winter. Numbers of islandica peak in mid-winter, but significant passage of canutus occurs in May on the central Atlantic coast. Patterns of autumn migration remain unclear and information on occurrence of both subspecies is lacking. Long term trends in site use differ from place to place; this is probably an indication that they are used by birds of different origin and age.
... Age-related differences in timing of migration at stopover sites are common in many bird species (e.g. Restani 2000;Ueta and Higuchi 2002;Hake et al. 2003;Meissner 2007;Lee et al. 2008). In our case, a preliminary study addressed the fact that the majority of adult spoonbills (ca. ...
Article
Full-text available
Long-distance migration is widespread among birds, connecting breeding and wintering areas through a set of stopover localities where individuals refuel and/or rest. The extent of the stopover is critical in determining the migratory strategy of a bird. Here, we examined the relationship between minimum length of stay of PVC-ringed birds in a major stopover site and the remaining flight distance to the overwintering area in the Eurasian spoonbill (Platalea l. leucorodia) during four consecutive autumn migrations. We also analysed the potential effect of timing (arrival date), as well as the role of experience in explaining stopover duration of spoonbills. Overall, birds wintering in Africa, and facing long-distance travel from the stopover site (ca. 3,000km) stay for longer (2.7±0.4days) than Iberian winterers (1.5±0.2days) that perform a much shorter migration (ca. 800km). These differences were consistent between years. Stopover duration was not significantly affected by the age of the bird. However, there was a significant reduction as migration advanced. Our results suggest that spoonbills develop different stopover strategies depending on the expected distance to the wintering grounds. Adults, especially long-distance migratory ones, could reduce the potential negative effects of density-dependence processes by avoiding stopover at the end of the migration period. These findings are of significant relevance for understanding differences in migratory behaviour within single populations, especially for declining waterbirds, as well as stress the relevance of preserving stopover localities for the conservation of intraspecific diversity in migratory species. KeywordsExperience-Intraspecific competition-Migratory strategies-Spoonbill-Stopover duration-Timing
Article
Full-text available
Reductions in body size are increasingly being identified as a response to climate warming. Here we present evidence for a case of such body shrinkage, potentially due to malnutrition in early life. We show that an avian long-distance migrant (red knot, Calidris canutus canutus), which is experiencing globally unrivaled warming rates at its high-Arctic breeding grounds, produces smaller offspring with shorter bills during summers with early snowmelt. This has consequences half a world away at their tropical wintering grounds, where shorter-billed individuals have reduced survival rates. This is associated with these molluscivores eating fewer deeply buried bivalve prey and more shallowly buried seagrass rhizomes. We suggest that seasonal migrants can experience reduced fitness at one end of their range as a result of a changing climate at the other end.
Article
Estimates of refuelling rates in migrating waders are best based on intra-seasonal recaptures of individually marked birds. This method, however, has methodological problems associated with capture effects and difficulties in attaining sufficient sample sizes. An alternative method had been proposed whereby refuelling rates are approximated by the body mass increment from the slope of the regression between body masses of all birds caught at a site and date. We investigated the appropriateness of this indicator with a simulation study in non-synchronized migratory species (i.e. arrivals and departures of individuals at the stopover site are not synchronized). Simulation results indicated that the mass increment across the population may be used as an approximation of refuelling rate only in populations with low turnover rates (percentage of birds arriving at/departing from stopover site per day <2%) and ideally with constant numbers of staging birds. The mass increment of non-synchronized populations with moderate or high turnover rates (higher than 5%) depends mainly on body masses of arriving birds and is not indicative of the individual rate of refuelling. The results of the simulation study were confirmed with empirical data gathered from Wood Sandpiper Tringa glareola and Common Snipe Gallinago gallinago during their autumn migration at a stopover site in Poland. The population mass increment methods considerably underestimated refuelling rate obtained from the recapture-based approach of individual birds in both species. As a consequence, we suggest that population mass increment should not be used as an indicator of refuelling rates in non-synchronized stopover populations of migrating waders.
Article
Many migratory waterbird populations are declining and wetland connectivity is a major conservation challenge. The importance of stopover sites has been typically assessed by peak counts of birds, which underestimate the total number of individuals using the site over a migratory season, especially in small wetlands. We analysed the accuracy of different daily count schemes to estimate the total number of Eurasian Spoonbill that stop over at two tidal wetlands during their autumn migration and compared them with the birds observed leaving the area each day. Total number of birds was obtained by combining numbers of each flock of birds leaving during the season. We obtained different accurate predictors for birds departing daily from each stopover area. Daily low-tide counts were the best predictor of the daily number of birds that stopover in a tidal wetland mainly used to refuel (staging site), whereas daily high-tide counts were best at a wetland mainly used to rest (stopover site). Each measure also accurately predicted annual trends for each area, respectively. Daily low-tide counts could be used as an easy census method to estimate the daily number of individuals using a staging site consistently during the entire migratory season, as well as indicating trends, without the necessity of estimating turnover rates. By contrast, daily high-tide counts would be especially suitable for determining the minimum relevance and the population trends of other tidal wetlands (especially the smaller ones), which regularly support moderate numbers of spoonbills during migration where birds use to stop over for less than one day. This method developed for the spoonbill, a flagship and umbrella species, could represent a first step in improving the conservation of other endangered migratory waterbird populations.
Article
In western Europe, the coasts of Vendée and Charente-Maritime on the central Atlantic shoreline of France constitute the most southern wintering site for the Red Knot Calidris canutus islandica, welcoming around 37,000 individuals each winter, and also represent a key stopover area for C. c. canutus (up to 60,000 individuals) between Africa and the Wadden Sea in spring. Nevertheless, the origin of the birds in France arriving in the autumn is unclear, considering that the first islandica start to appear for wintering while canutus could use the same sites as a stopover on their migration route to Africa. We used biometric data from birds caught between 1983 and 2008 to assess the origin of Red Knot staging and wintering in France, and we also investigated the age structure of Red Knot groups during three distinctive periods of their biological cycle: autumn migration, wintering and spring migration. This first assessment was completed by data analysis of ring recoveries over a period of 44 years and isotopic ratio signatures of feathers. The obtained results were in conformity with the general patterns of distribution throughout the year and the timing of migration of both subspecies in Europe as described in previous reviews. Nevertheless, this study highlights two unknown features for both subspecies in this part of their European area distributions. First, a very large predominance of juveniles was recorded at the expense of an extreme rarity of adults in autumn and early winter. We prove the presence of canutus-knots among these juveniles, even with staging later in the autumn and the possibility of wintering there for some of them. Although the Atlantic coast of France welcomes only 9% of the population of islandica-knots wintering in Europe, this network of estuarine bays could represent a crucial strategic area for juveniles. The place could constitute a liberated area for islandica juveniles coming later than adults already settled in northern sites. It offers the possibility for canutus to migrate south on distance-limited stages in order to experiment with the flyway and refuel to join the traditional African wintering grounds.
Article
Full-text available
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.
Article
Full-text available
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
Article
Full-text available
Body size and mass of 1458 juvenile and 558 adult Knots were measured during autumn migration in Puck Bay, Poland, between 1983 and 1999. The measurements fit well with those gathered along the migration route of Siberian Knots Calidris c. canutus. All linear measurements of adults, except body mass adjusted for size, were significantly higher than those of juveniles. Shorter wing in juveniles than in adults seems to be a general rule regardless of geographical region. Adult females migrated ahead of males, confirmed by decreasing mean measurements. Juveniles carried larger relative energy stores than adults. Puck Bay may be an emergency feeding place for adult Knots, whereas juveniles may use it as one of many stop-over sites. Juveniles were significantly smaller, but had higher adjusted body mass, in late September than earlier, indicating a behavioural difference between late and early birds during preceding stages of migration. Late juveniles probably follow a time-minimising strategy with larger fuel stores and fewer stop-over sites, whereas birds passing in August migrate with very small energetic reserves, making only short flights and stops.
Article
Full-text available
In this study we analyse biometrical data of 69 adult and 389 juvenile Turnstones Arenaria interpres caught during autumn migration when passing through the Gulf of Gdańsk in the period 1983-1999. Adults had significantly longer wings than juveniles. Mean values of other measurements did not differ significantly between the age classes. Adults migrating early were larger than those passing the study area later. These earlier and larger migrants are regarded as females, which leave nesting areas before the somewhat smaller males. Data obtained from 67 juvenile Turnstones caught at least twice in the same season showed that juvenile Turnstones may have more than one migration strategy when departing from the Gulf of Gdańsk. Some of them behave as energy minimising migrants and migrate with low fat reserves in small steps. The others stay longer, build up large energy reserves (up to 50% of their initial body mass) and are probably able to reach West Africa in one flight.
Article
Wind is of foremost importance to affect migrating birds' optimal flying speed and altitude, and their optimal migratory routes. This paper contains two main sections, wherein (1) the selective influence of wind on the compensatory behaviour of migrating birds is evaluated, and (2) a first attempt is made to examine the complex interplay between different selection factors governing the relationship between winds and bird migratory activity. Wind conditions determine the advantages and disadvantages of wind compensation and drift, respectively, in migrating birds. Complete compensation for wind drift is the optimal behaviour under constant winds, but with varying winds the optimal behaviour is partial and flexible compensation, i.e. the migrants should allow extensive drift far away from the goal and gradually compensate to a higher and higher degree on approaching the goal, finally compensating completely during the last flight to the goal. Different species of migrating birds are expected to be selective of favourable winds for their migratory flights to different degrees. The relationship between costs for fuel transportation and benefit by being highly selective of winds will determine optimal wind selectivity and fuel ratio in migrating birds. Optimal wind selectivity will be expected to be high with, i.a., widely chinging winds along the migratory route, a short distance and long duration of migration, and long durations of migratory flights. It is furthermore expected to be higher in adult than in juvenile birds, and in migrants recently having experienced successful flights as compared to migrants being delayed.
Article
The effects of high and low protein food on molting in penned, wild-strain Mallards (Anas platyrhynchos) from southwestern Sweden were studied in 1979, 1980, and 1985. Body condition, as indicated primarily by lipid deposits, declined during the flightless period. This decline was probably more the result of a strategy to recover flight capability rapidly than the result of stress. By metabolizing stored lipids, birds may be able to seek refuge in habitats offering protection from predators, even if food in such habitats is scarce. During the flightless period, food quality did not affect the growth rate of the primary feathers. But when exposed to low protein food, even after recovering flight capability, birds, especially males, developed shorter wing feathers; this feather shortening may reduce their condition capacity, i.e., their capacity to increase body condition without reducing their flight capacity in late autumn. A lowered condition capacity could lead to decreased survival of the ducks when exposed to severe winter conditions. Female Mallards demonstrated a higher condition capacity than males, which may contribute to the more rapid recovery of female primaries. Since the final length of the primaries is affected by food quality during molting, wing length is probably not a variable suitable for use in a condition index.