ORIGINAL ARTICLE
How many juvenile Teal Anas crecca reach the winteringgrounds? Flyway-scale survival rate inferred from wing age-ratios
Matthieu Guillemain Æ Jean-Maxime Bertout ÆThomas Kjær Christensen Æ Hannu Poysa Æ Veli-Matti Vaananen ÆPatrick Triplet Æ Vincent Schricke Æ Anthony D. Fox
Received: 25 August 2008 / Revised: 9 April 2009 / Accepted: 25 May 2009 / Published online: 23 June 2009
� Dt. Ornithologen-Gesellschaft e.V. 2009
Abstract Autumn postnuptial migration is critical in the
dabbling duck annual cycle, when first-year birds in par-
ticular suffer high losses to natural and hunting mortality.
Mortality rates in this age-class are generally unknown in
Europe where winter ringing predominates. We used data
from large-scale wing collections from hunters in Finland,
Denmark and France to test the prediction that juvenile
proportions among killed Teal (Anas crecca) would decline
with distance along the flyway. As expected, this propor-
tion decreased from 89% in Northern Finland to 58% in
Western France. Potential biases linked with age determi-
nation from the wings, differential migration of age-clas-
ses, relative susceptibility to different forms hunting and
gradual improvement of juvenile survival as they learn to
avoid hunters could not explain the observed decline of
juveniles in the shot population. This pattern was therefore
considered to be genuine, the result of the cumulative
depletion of first-years along the flyway, likely through
hunting. On this assumption, combined with known adult
monthly survival rates during August–November (94.2%),
monthly juvenile survival rate was estimated at 52.8%, i.e.
14.7% (range 13.9–15.4% based on extreme values of adult
survival) amongst Scandinavian juveniles reaching win-
tering quarters in Western France. Despite lack of precision
in such estimates based on relative proportions, there is
little doubt about the magnitude of autumn juvenile
mortality and its consequences for the population dynamics
of Teal. Lack of correlations between annual proportions of
juveniles in the hunting bag and an index of Teal breeding
success in Finland may result from such high and variable
inter-annual mortality.
Keywords Wing examination � Hunting bag � Age-ratio �Survival � Autumn
Introduction
Management and conservation of quarry species require
good knowledge of their population dynamics to ensure
sustainable exploitation and provide a warning system to
prevent over-exploitation (e.g. Williams et al. 2002;
Communicated by F. Bairlein.
M. Guillemain (&)
Office National de la Chasse et de la Faune Sauvage,
CNERA Avifaune Migratrice,
La Tour du Valat, Le Sambuc 13200 Arles, France
e-mail: [email protected]
J.-M. Bertout
Avifauna, 2 rue Germain Delhaye, 59710 Pont a Marcq, France
T. K. Christensen � A. D. Fox
Department of Wildlife Ecology and Biodiversity,
National Environmental Research Institute,
University of Aarhus, Kalø, Grenavej 14, 8410 Rønde, Denmark
H. Poysa
Finnish Game and Fisheries Research Institute,
Joensuu Game and Fisheries Research, Yliopistokatu 6,
80100 Joensuu, Finland
V.-M. Vaananen
Department of Forest Ecology, University of Helsinki,
P.O. Box 27, Helsinki 00014, Finland
P. Triplet
SMBS, Maison Ramsar, 1 Place de l’Amiral Courbet,
80100 Abbeville, France
V. Schricke
Office National de la Chasse et de la Faune Sauvage,
CNERA Avifaune Migratrice, 39 Bld Einstein, CS42355,
44323 Nantes cedex 3, France
123
J Ornithol (2010) 151:51–60
DOI 10.1007/s10336-009-0425-z
Elmberg et al. 2006; Baldassarre and Bolen 2006). Dab-
bling ducks (Anas spp.) are popular quarry (e.g. Mondain-
Monval and Girard 2000; Hirschfeld and Heyd 2005; Mooij
2005), yet their demographic parameters are still poorly
estimated, especially in Europe (review in Devineau 2007).
Juvenile dabbling duck survival rates from fledging to
arrival at the wintering grounds (hereafter autumn survival)
are difficult to quantify and generally unknown (Owen and
Black 1990), because most large-scale duck ringing oper-
ations historically occurred on the wintering grounds post-
autumn migration. The notable exception of data from the
massive ringing operations on Engure Lake in Latvia (e.g.,
Blums et al. 1996) have never been analysed to estimate
autumn survival rates. This is unfortunate, because natural
mortality during this first migratory journey is probably
high in juveniles (as in other non-passerines, cf. Owen and
Black 1991), not least because first-year duck hunting
mortality is very high at the start of the autumn hunting
season where this has been quantified (e.g. Hickey 1952;
Parker 1991). Many first-year dabbling ducks may never
reach the wintering quarters, so differences between post-
breeding population size inferred from breeding success
indices and true winter population size may be substantial.
Hunting mortality of ducks differs between age-classes
(Bellrose and Chase 1950; Bellrose et al. 1961; Harradine
and Clausager 1990), so it is prudent not to estimate autumn
mortality of first-year birds from that of adults, though this
is so far the only option available to model growth rates
(e.g. Devineau 2007).
Ringing on breeding areas may offer a solution, but low
breeding densities of dabbling ducks restrict sample size.
An alternative solution is to assess the proportion of juve-
niles in the population, and to monitor this rate along the
migration journey, down to the wintering areas where birds
eventually get ringed. The gradual depletion of the juvenile
element of the population (Harradine and Clausager 1990
for Europe, also evident through continental North America;
J. Leafloor, personal communication), combined with
known adult survival rate, can potentially allow estimation
of autumn survival rate in first-year birds. Only Eurasian
Wigeon Anas penelope can be reliably assigned to age-class
on conspicuous plumage characteristics in the field (e.g.
Guillemain et al. 2003; Mitchell et al. 2008), while age
determination is only possible with confidence in the hand
for most other dabbling ducks (Boyd et al. 1975; Carney
1992). Plumage characteristics of shot birds (i.e. tail or wing
feathers) offer a means of sampling adults to juvenile ratios
along the flyway. New wing feathers are produced only at
birth (first-year birds), breeding (adult females) or specific
moulting areas (adult males; Cramp and Simmons 1977),
so individual age can be determined with confidence
throughout the year. Assuming that wings from shot birds
provide a sample of population structure that is subject to
constant bias (though not a direct measurement of age-ratio
in the live population, see below), one can potentially
monitor changes in age-ratio along the flyway by collecting
wings from hunters in different countries.
Here, we analyse Teal (Anas crecca) age-ratios amongst
over 30,000 wings collected by hunters in Finland,
Denmark and France during 2002–2008 (2005–2008 for
Finland), to estimate first-year autumn survival rate. These
three countries are directly connected and make a contri-
bution to the flyway for Teal wintering in Western Europe
(Scott and Rose 1996). Teal have been intensively ringed
in winter in western Europe since the 1950s, especially in
UK and France (e.g. Ogilvie 2002; Guillemain et al. 2005),
providing fairly robust adult survival rate estimates from
which to calculate juvenile autumn survival rates. This is
important from a management and conservation point of
view, since Teal is the most heavily hunted wild dabbling
duck in Europe (the bag for Mallard Anas platyrhynchos is
actually larger, but a large proportion of these are released
hand-reared individuals; Mondain-Monval and Girard
2000), and a large part of the hunting pressure occurs
during autumn and early winter in Europe.
In addition, we relate bag age-ratios to annual indices of
Teal breeding success in Finland to determine if the former
may be used as an estimate of the latter. Teal autumn age-
ratios have been used as a proxy for breeding success in an
earlier study (Guillemain et al. 2008) as for other Anatidae
(e.g. Mitchell et al. 2008), but the validity of this index for
Teal remains to be tested.
Methods
Wing collection
Teal wings were provided on a voluntary basis by Finnish,
Danish and French hunters to the Finnish Game and
Fisheries Research Institute, The National Environmental
Research Institute, Denmark, and Avifauna (a French
hunter NGO), respectively. The flyway was divided
into seven geographic areas of comparable size (Fig. 1):
Finnish areas were determined after the boundaries
of game management districts, i.e., Area 1 (extreme NE
Finland; 38,517 km2) = Keski-Lappi (no data were
available from Yla-Lappi district in the far north); Area 2
(NE Finland; 83,710 km2) = Kainuu, Ala-Lappi, Oulu
etelainen and Oulu pohjoinen; Area 3 (Central Finland;
107,964 km2) = Etela-Savo, Keski-Suomi, Pohjanmaa,
Pohjois-Karjala, Pohjois-Savo and Ruotsink-Pohjanmaa;
Area 4 (SW Finland; 63,930 km2) = Etela-Hame, Kymi,
Pohjois-Hame, Satakunta, Uusimaa, Varsinais-Suomi.
Denmark is Area 5 (43,094 km2). French areas were
determined after the boundaries of coastal departments,
52 J Ornithol (2010) 151:51–60
123
i.e., Area 6 (N France; 62,774 km2) = Nord, Pas-de-
Calais, Somme, Seine-Maritime, Eure, Calvados, Manche,
Ille-et-Vilaine, Cotes-d’Armor and Finistere; Area 7
(W France; 46,465 km2) = Morbihan, Loire-Atlantique,
Vendee, Charente-Maritime, Gironde and Landes. Wings
from inland France and for the Mediterranean were not
used here, because although Teal wintering in Western
Europe potentially belong to one single panmixic popu-
lation (Guillemain et al. 2005), many Teal from these two
areas originate from Russia (Guillemain et al. 2009b) and
so are less directly connected to the Finnish breeding
grounds. The French data for the first collection season
(winter 2002–2003) were not included in the analyses
because of small samples (Table 1). A total of 31,593
wings were included in the present analysis (Table 1). A
‘‘season’’ hereafter refers to a general annual hunting
season, August (September in Denmark) to the following
January inclusive, defined by the August of that season
(i.e. 2002 for the August 2002–January 2003 hunting
season).
Age-ratio along the flyway
We first tested if and how the proportion of juvenile
wings differed between areas and seasons. The proportion
of juveniles in the wing sample was therefore computed
for each case (28 different cases, see Table 1). The dis-
tribution of these data did not depart significantly from a
normal distribution (Kolmogorov–Smirnov: d = 0.107,
P = 0.1839), so general linear modelling was applied
using straight (i.e. untransformed) juvenile proportion as
the dependent variable, and area and season as indepen-
dent factors.
There are potential biases associated with the use of age-
ratios from hunted birds (e.g. Caughley 1974; J. Leafloor,
personal communication), although that of gradual plum-
age change affecting age determination amongst dabbling
ducks in autumn and winter is not amongst these (see
above). Firstly, juvenile proportion may differ between
geographic areas if the two age-classes show differential
migration, i.e. do not migrate the same distances from the
Fig. 1 Geographic zones from
which Teal (Anas crecca) wings
were provided by hunters
between 2002 and 2007 (2005–
2007 in Finland)
J Ornithol (2010) 151:51–60 53
123
breeding grounds (e.g. if larger or dominant adults winter
further north of smaller subordinate juveniles; Cristol et al.
1999). This is unlikely here, since the average age-ratio of
ca. 45,000 Teal ringed in Camargue, Southern France and
at Abberton Reservoir, Essex, UK, did not differ signifi-
cantly (Guillemain et al. 2009a), though the numerous
exchanges of birds between these two areas suggest they
belong to the same vast population (Guillemain et al.
2005). Secondly, changes in age-ratios along the flyway
may not reflect the gradual depletion of the juvenile part of
the population if survival of these first-year birds gradually
improves, especially if they learn to avoid hunters. To test
this hypothesis, we also assessed changes in juvenile pro-
portions in wing samples over time for area 5 (Denmark), 6
(N France) and 7 (W France) against the expectation of a
gradual decrease within each area if young birds learnt to
avoid hunters. In each area, the proportion of juvenile
wings was computed for 15-day periods from 1 August
each season, these annual values then being used as rep-
licates in a polynomial regression per site (preliminary
analyses having shown a non-linear pattern over time, and
that other types of non-linear relationships did not provide
a significant better fit to the data than a second order
polynomial regression). Only the September–December
period could be included for Denmark, because hunting
only starts in September and data were too scarce in the
following time interval. Thirdly, the proportion of juveniles
may differ between geographic areas if the two age-classes
are not equally sensitive to different hunting methods, and
some of which are known to be more frequent in some
geographic areas. The major difference in hunting methods
along the flyway is that nocturnal hunting is allowed in
some coastal Departements of France (all of those in area 6
and all those in area 7 except Morbihan, Loire-Atlantique
and Vendee, which represent 44% of the total area of area
7). Although used throughout the 24-h period, nocturnal
hunting is only practised with live decoys on managed
foraging ponds, a method potentially more likely to attract
less fit or subdominant individuals (i.e. likely first-year
birds). To ensure differences in the juvenile proportions
between geographic areas were not due to this practice in
France, we compared by mean of v2 tests the relative share
of adult and juvenile wings provided from daytime (0600–
2000 hours) versus nocturnal hunting, in areas 6 and 7.
Juvenile survival along the flyway
The annual adult survival rate of Teal based on ringing in
Western Europe was estimated to be 0.50–0.59 in the UK
(Gitay et al. 1990), 0.36–0.45 in other European countries
(review in Bell and Mitchell 1996) and 0.38–0.50 for
Green-winged Teal (Anas crecca carolinensis) in North
America (review in Johnson 1995). Here, we used the
value of 0.49 (averaged between the two sexes; Devineau
2007), since it is the most recent one derived for birds
ringed in France (although from the Camargue, in the south
of the country). We assumed that birds departed the
breeding grounds in early August, and that the wintering
population was in Western France by early November (e.g.
Lebret 1947), constituting a 3-month migration period.
Under the assumption that monthly adult survival rate is
even throughout the year, survival rates of adults over this
period would therefore be equal to 0.4853/12 = 0.835.
Juvenile survival would be equal to that of adults if age-
ratio remains constant along the flyway while assuming
adult survival does not vary. Any trend in age-ratio, com-
pared to the above adult survival, thus enables the calcu-
lation of a relative juvenile autumn survival rate. For
example, if the juvenile proportions decreased by, e.g.,
50% from Northern Finland to Western France, then
Table 1 Juvenile Teal (Anas crecca) proportions in each geographic area along the flyway (see detailed map in Fig. 1) and per season (2002–
2007 except for Finland: 2005–2007)
Seasons
2002 2003 2004 2005 2006 2007
Area 1: extreme NE Finland 0.92 (25) 0.80 (10) 0.94 (17)
Area 2: NE Finland 0.79 (80) 0.90 (63) 0.70 (89)
Area 3: Central Finland 0.80 (201) 0.75 (136) 0.78 (100)
Area 4: SW Finland 0.85 (109) 0.86 (73) 0.80 (55)
Area 5: Denmark 0.74 (2,038) 0.76 (4,884) 0.77 (6,215) 0.69 (2,448) 0.68 (2,312) 0.73 (5,274)
Area 6: N France 0. 36 (103)a 0.67 (763) 0.65 (1,361) 0.62 (1,295) 0.47 (649) 0.56 (1,025)
Area 7: W France 0 (1)a 0.76 (76) 0.51 (571) 0.51 (547) 0.55 (431) 0.56 (746)
The total number of Teal wings examined in each case is given in parentheses
Note that the 2002 data were discarded for France because one single bird was examined in area 7, and the proportion of juveniles in area 6
differed markedly from that of other yearsa Not included in analyses because of small samples
54 J Ornithol (2010) 151:51–60
123
juvenile autumn survival would be half that of adults.
Adult survival is likely to vary throughout the year and
between years in wild animal populations (e.g. Clobert
et al. 1985; Lebreton et al. 1993), including Teal (Devineau
2007). No estimate of Teal adult survival was available for
the study period. However, the value we used here is
derived from a long-term (20 years) study, and was con-
sidered to be robust enough to encompass potential varia-
tion between years. Likewise, no estimate of adult survival
is available along the European flyway for the autumn
season itself, so that the extent to which the value calcu-
lated from the annual average adequately represents reality
cannot be addressed. In order to provide some range esti-
mate for juvenile autumn survival, and to account for the
fact that there are uncertainties over the extent to which
0.835 is a valuable estimate of adult Teal survival over the
autumn, the same calculation above was also made for the
two extreme annual adult survival rates recorded in Europe,
i.e. 0.38 and 0.59, corresponding to a 0.785–0.876 adult
survival over the 3 months of autumn migration.
Age-ratios as indices of breeding success
Although age-ratios from shot duck wings may not provide
a direct measure of breeding success, if one of the age-
classes is more susceptible to hunting, changes in age-
ratios over time may be used as an index of breeding
success, as in North America where the ‘‘Waterfowl Parts
Collection Survey’’ has long been used as a check of
annual breeding output (e.g. Boyd et al. 1976). In order to
assess if the same could be done in Europe, we computed
Pearson correlations between the annual proportion of
juvenile wings in Denmark and the two French areas (there
were not enough years of bag statistic data from Finland),
and a composite measure of Teal breeding success in
Finland: the ratio of Teal production index to the Teal
breeding population index (normality criteria were asses-
sed for these latter data). The breeding population index is
based on the number of breeding pairs seen in May or early
June pair surveys in Finland, and the Teal production index
is based on the number of broods seen in brood surveys and
on the number of ducklings in broods in age-class IIa or
older (i.e. about 2.5 weeks or older ducklings; Gollop and
Marshall 1954; Pirkola and Hogmander 1974) provided by
nationwide brood counts done once in late June or early
July. Both indices thus come from long-term monitoring
time series in which the long-term mean is set at 100.0 (see
Poysa et al. 1993).
Results
Age-ratio along the flyway
The proportion of juveniles in the bag differed between
geographic areas but not between seasons (complete
model: F11,16 = 7.54, P = 0.0002; season: partial
F = 1.89, df = 5, P = 0.1528; area: partial F = 13.04,
df = 6, P \ 0.0001). The proportion of juveniles averaged
over all available seasons decreased significantly along the
flyway, from area 1 (88.7% ± 4.4 SE, n = 6) to area 7
(58.0% ± 4.7 SE, n = 5; Spearman rank correlation:
rS = -0.89, df = 5, P \ 0.01; Fig. 2).
In Denmark (area 5), the proportion of juveniles did not
show a significant trend over time, expressed as 15-day
periods (F2,45 = 1.13, r2 = 0.05, P = 0.3372). Conversely,
the proportion of juveniles was significantly fitted by a type
2 polynomial regression both in Northern France (area 6:
F2,44 = 8.47, r2 = 0.28, P = 0.0008; Y = 0.368 ? 0.062X
- 0.004X2) and in Western France (area 7: F2,42 = 3.71,
r2 = 0.15, P = 0.0329; Y = 0.429 ? 0.052X - 0.004X2)
(Fig. 3).
0.5
0.6
0.7
0.8
0.9
1.0
Ext NEFinland (3)
NE Finland(3)
CentralFinland (3)
SW Finland(3)
Denmark(6)
N France(5)
W France(5)
Pro
po
rtio
n o
f F
irst
-yea
r b
ird
s
Fig. 2 Average juvenile
proportions in the bag of shot
Teal per geographic area.
Sample size (i.e. number of
hunting seasons over which
wings were collected) is
indicated in brackets in each
case. Vertical bars show
standard errors. See text for
statistics
J Ornithol (2010) 151:51–60 55
123
The number of first-year and adult wings provided for
nocturnal and for daylight hunting in Northern France (area
6) and Western France (area 7) are given in Table 2. The
share of juveniles in area 6 was 61.8% by day and 60.4%
by night, while corresponding values were 53.9 and 54.1%
in area 7. More juveniles were thus harvested in area 6 than
in area 7 either by day (v2 = 19.41, P \ 0.0001) or by
night (v2 = 12.11, P = 0.0005).
Juvenile survival along the flyway
The above juvenile proportions suggest that among 1,000
Teal shot in NE Finland, 887 would be juveniles, thus 113
would be adults. Given the French survival rate for adults
along the 3 months of the migration period (0.835), 94 of
these adults would reach the Western coast of France.
Adults representing 1 - 0.58 = 42% of the population in
Western France, the total population reaching this area
would be 224 individuals, of which 130 would therefore be
juveniles. The number of juveniles in this model population
would thus have decreased from 887 to 130 individuals in
3 months, suggesting that they had a 0.147 survival rate
over the period, or a 0.528 monthly survival rate.
If the two extreme known adult annual survival rates are
considered instead of the above single French value (i.e.
0.38–0.59 annual survival, or 0.785–0.876 survival over
the 3 months of autumn migration), the number of
remaining adults after the migration journey would vary
between 89 and 99 individuals, so that the total population
alive would be 212–236 individuals, of which the 58%
juveniles would represent 123–137 birds. Juvenile autumn
survival would thus range from 0.139 to 0.154, or a 0.518–
0.536 monthly survival rate.
Age-ratios as indices of breeding success
The composite index of Teal breeding success varied
markedly across seasons of the study period, reaching a
maximum in 2006 and being lowest in 2003 (Fig. 4). There
was no significant correlation between the proportion of
juveniles in the Teal bag and the index of Teal breeding
success in Finland (all r values negative and\-0.86 while
sample size was 5 or 6 depending on geographic areas, all
P [ 0.05).
Discussion
Age-ratio along the flyway
The proportion of juveniles among the Teal wings provided
by hunters gradually decreased along the Western
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12 14
15-day periods since 1st August
Pro
po
rtio
n o
f fi
rst-
year
bir
ds
Fig. 3 Average juvenile proportions in the bag of shot Teal in
Denmark (area 5; black dots) and France (areas 6 and 7, white circlesand grey triangles, respectively) over time, expressed as 15-day
periods starting 1 August. Vertical bars show standard errors,
computed after six annual averages (2002–2007) for Denmark, 5
for France (2003–2007). The regression curve is shown where
significant (N and W France). See text for statistics
Table 2 Number of first-year and adult Teal wings provided by
hunters in Northern France (area 6) and in Western France (area 7) for
daylight and for nocturnal hunting over the 2003–2004 to 2007–2008
hunting seasons
Hunting period No. of first-year
birds
No. of adults v2 P value
Northern France (Area 6)
Day 1,236 765 0.82 0.3653
Night 1,485 972
Western France (Area 7)
Day 671 573 0.01 0.9344
Night 566 480
0
0.4
0.8
1.2
1.6
2
2001 2002 2003 2004 2005 2006 2007
Seasons
Tea
l bre
edin
g s
ucc
ess
ind
ex
Fig. 4 Teal annual breeding success index in Finland over the study
period. This composite index is the ratio of the Teal production index
(based on the number of broods and ducklings recorded in brood
surveys) on the Teal breeding population index (based on the number
of breeding pairs in pair surveys)
56 J Ornithol (2010) 151:51–60
123
European flyway, from 89% in the North-East of Finland to
58% in Western France. The proportion of juveniles among
UK shot Teal wings (58.6%; Harradine and Clausager
1990) fits remarkably well with our picture, being similar
to the percentages recorded for France (59.4 and 58.8% in
Northern and Western France, respectively). Similarly,
data from Sweden (73.3%), Denmark (64.2%) and France
(59.9%; Boyd et al. 1975) show the same pattern over the
flyway, though absolute values for Denmark were lower
than at the present time. Dabbling ducks do not moult wing
feathers between August and November (e.g. Boyd et al.
1975), so declines cannot be attributed to gradual moult of
wing feathers in first-years. Differential migration of the
age-classes (sensu Cristol et al. 1999) cannot be responsi-
ble, because analysis of ringing data detected no such
difference in average wintering latitude between adults
and first-year birds (despite differences between sexes;
Guillemain et al. 2009a). Age-ratios can also be biased if
one of the age-classes gradually becomes less likely to be
represented in the sample. This could particularly be the
case here if first-year birds gradually learn to avoid hunters.
This is apparently not the case here, since juvenile pro-
portions did not decrease over the season in any of the three
areas where this could be tested: no significant trend could
be detected over time in Denmark, and a slightly dome-
shaped pattern was recorded in the two French areas,
though less than 30% of the variance could be explained
by time in these two cases. A similar dome-shape pattern
was recorded amongst Danish and British Wigeon wings
by Mitchell et al. (2008), attributed to earlier migration
of adult males. It is also possible that earlier migration of
adult male Teal is responsible for the within-season pattern
we recorded in France, since adult males are known to
migrate ahead of females and juveniles (review in Lebret
1950). Changes in age-ratios over time suggest that young
Teal do not learn to avoid hunters, so this potential source
of bias is unlikely in the present dataset. Similarly, daylight
and nocturnal hunting yielded similar age-ratios in the two
French areas (while differences in the sex composition of
the bag has been reported in the past for nocturnal hunting,
especially for Teal; Schricke 1983). It is therefore unlikely
that the pattern of age-ratio change along the flyway was
due to differences in hunting practices, which would
unequally select for different age-classes. Conversely,
more juveniles were harvested in Northern than in Western
France here, whatever the time of day considered.
The juvenile proportions in the bag should not be con-
sidered as a direct measurement of the age-ratio in the
actual population in the wild. For instance, 89% of juve-
niles, even in post-breeding population in Northern Scan-
dinavia, is very unlikely: based on the Finnish duck
monitoring data the average number of fledged birds (i.e.
age-class IIa or older) per brood is 5.2 in northern Finland
between 2005 and 2007 (H. Poysa, unpublished data).
Thus, 100 breeding pairs would produce maximally (i.e. all
pairs successfully produce juveniles) 520 juveniles, and the
proportion of juveniles in the post-breeding population
would be 72.2%. The proportion of juveniles in the hunt-
ers’ bag is thus only used as an index here, whose biases
have been examined and then considered to be constant
along the flyway (as age-ratios in bags of shot birds or
catches of birds for ringing generally are; see also Mitchell
et al. 2008), so that changes in this index can be used as a
proxy for depletion of the juvenile proportions.
Autumn survival of juvenile Teal
The above results suggest that the observed change in age-
ratio along the flyway was not an effect of bias in sampling
procedures. Rather, it is apparently representative of a
gradual depletion of the juvenile part of the population as
migration progresses. This is consistent with the higher
mortality expected amongst naıve young birds during their
first migration episode (Owen and Black 1991), and higher
hunting mortality (as is typical amongst less fit or sub-
dominant individuals generally amongst dabbling ducks;
e.g., Heitmeyer et al. 1993; Guillemain et al. 2007; see also
Harradine and Clausager 1990).
Based on the above depletion of the juvenile propor-
tions along the flyway and the 0.485 annual survival rate
of adult Teal ringed in France, only 14.7% of juveniles
would survive the 3-month autumn migration period, or a
0.528 monthly survival rate. In order to include uncer-
tainty in these analyses, we considered the extreme
published rates of adult Teal survival, but these only
marginally affected this result, which predicted that
between 13.9 and 15.4% of juveniles would reach western
France, representing a 0.518–0.536 monthly survival rate.
For comparison, the monthly survival rate of adults rin-
ged in France would be 0.4851/12 = 0.942 after Devineau
(2007), confirming major differences in survival rates for
adults and juveniles during the autumn migration period.
The above calculations are based on even monthly adult
survival rates, which are unlikely (e.g. Clobert et al.
1985; Lebreton et al. 1993). Conversely, it is possible that
adults too suffer a higher mortality in autumn, when
being confronted with hunting anew and migrating fol-
lowing a costly breeding episode, than during other
periods of the year. This approach could therefore be
refined by computing an autumn survival rate for the
August–November period in this age-class, data permit-
ting. This, however, would likely only marginally change
the results, insofar as juvenile mortality is clearly sub-
stantially higher during their first migration episode, so
that ca. 85% of these never reach the wintering grounds
in South Western Europe. If anything, adult autumn
J Ornithol (2010) 151:51–60 57
123
survival is probably lower than the annual average, so
that the calculated survival rate of juveniles too would be
estimated to lower values. There are unfortunately no
data available from closely related species for comparison
with our results, since the lack of capture–recapture
information from fledging to the arrival at the wintering
grounds is a general pattern in these species (see
‘‘Introduction’’).
Age-ratios as an index of annual breeding output
The correlations between the proportion of juveniles in a
given area and Teal breeding output in Finland provided
unconvincing results. Indeed, in no single tested case did
the proportion of juveniles increase with the index of Teal
breeding success in Finland. The most likely explanation
for this result is the limited sample size (a maximum of six
annual values in Denmark). Mitchell et al. (2008) recently
validated a positive relationship between the proportion of
juveniles in the bag of birds shot in the UK or Denmark and
favourable weather condition at the breeding grounds, but
over twice as many years. Harradine and Clausager (1990),
as well as Boyd et al. (1975), argued that age-ratios should
reflect breeding success more closely in northern European
countries, closer to the breeding grounds. There were not
enough data to run powerful Spearman rank correlations in
our case. The massive mortality of juvenile Teal recorded
here along the flyway may prevent the relatively few
juveniles remaining in wintering areas to be informative
about population breeding output. Lastly, Finland repre-
sents only a fraction of the breeding grounds for Teal that
winter in Western Europe, since the bulk of the population
originates from Russia (Scott and Rose 1996). It is thus
possible that Teal age-ratios in Western Europe are related
to breeding success at a broader geographic scale than
Finland. International collaboration at even wider scale
than during the present study would be necessary to test
this hypothesis.
To conclude, this paper is to our knowledge the first to
provide an estimate for autumn survival in juvenile dab-
bling ducks in Europe, although it is based on an indirect
calculation. It suggests that using adult survival rate value
as a proxy for juvenile survival in demographic studies
(e.g., Devineau 2007) may be strongly misleading, as the
former is almost twice as high as the latter. Although we
lack precision in the estimation of autumn juvenile mor-
tality rates based on relative proportions, there is little
doubt about its magnitude, which suggests that the vast
majority of juvenile Teal die before reaching their win-
tering grounds. This means that the observed positive
trend in numbers of individuals within the Teal population
in Western Europe (Delany and Scott 2006) is largely
based on a very high annual breeding output. Though a
better knowledge of the ecology of Teal and other dab-
bling ducks on their autumn migratory stopovers would
be welcome (as was the case for spring staging areas;
Arzel et al. 2006), one main conclusion of this study is
therefore that any change in first autumn Teal survival
rates may very quickly lead to major consequences at the
population level.
Zusammenfassung
Wie viele junge Krickenten Anas crecca erreichen die
Uberwinterungsplatze? Uberlebensrate auf dem
Zugweg abgeleitet aus Flugelalter-Verhaltnissen
Der postnuptiale Herbstzug ist entscheidend im Jahreszyk-
lus von Grundelenten, da hier insbesondere einjahrige
Vogel hohe Verluste aufgrund von naturlicher Mortalitat
und Bejagung erleiden. Die Sterblichkeitsraten in dieser
Altersklasse sind im Allgemeinen unbekannt in Europa, wo
die Beringung im Winter dominiert. Wir haben Daten aus
groß angelegten Flugelsammlungen von Jagern in Finnland,
Danemark und Frankreich verwendet, um die Vorhersage
zu testen, dass der Anteil der Jungtiere an getoteten
Krickenten (Anas crecca) mit zunehmender Distanz entlang
des Zugweges abnehmen wurde. Wie erwartet verringerte
sich dieser Anteil von 89% in Nordfinnland auf 58% in
Westfrankreich. Mogliche systematische Abweichungen
verbunden mit einer Altersbestimmung anhand der Flugel,
einer relativen Anfalligkeit fur unterschiedliche Jagdfor-
men und einer graduellen Verbesserung des Uberlebens von
Jungtieren, weil sie lernen, Jager zu meiden, konnten den
beobachteten Ruckgang von Jungtieren in der ,,geschosse-
nen Population’’ nicht erklaren. Dieses Muster wurde daher
als echt angesehen und als Ergebnis des zunehmenden
Schwundes einjahriger Tiere entlang des Zugweges,
wahrscheinlich durch Bejagung. Basierend auf dieser
Annahme und in Kombination mit bekannten monatlichen
Uberlebensraten erwachsener Tiere von August bis
November (94.2%) wurde die monatliche Uberlebensrate
von Jungtieren auf 52.8% geschatzt, d.h. 14.7% (Span-
nweite 13.9–15.4% basierend auf extremen Uberleben-
swerten erwachsener Tiere) der skandinavischen Jungtiere
erreichten die Uberwinterungsplatze in Westfrankreich.
Trotz der Ungenauigkeit solcher Schatzwerte, die auf
relativen Anteilen beruhen, besteht wenig Zweifel an dem
Ausmaß der Sterblichkeit von Jungtieren im Herbst und den
Folgen fur die Populationsdynamik von Krickenten. Auf
eine derart hohe und variable interannuale Sterblichkeit
konnte es zuruckzufuhren sein, dass keine Korrelationen
zwischen den jahrlichen Anteilen von Jungtieren in der
Jagdbeute und einem Index des Bruterfolgs der Krickente in
Finnland gefunden wurden.
58 J Ornithol (2010) 151:51–60
123
Acknowledgments We are deeply grateful to the thousands of
hunters who have provided the Teal wings in France, Denmark and
Finland over the 6 years of this study, as well as the experts who
examined these in each region. The late Reynald Vanlerberghe
greatly contributed to the set-up of duck wing collection in France.
We would like to thank Peter Blums for valuable information con-
cerning the ringing of ducklings in Latvia, together with Bob Clark,
Jim Leafloor and Jean-Marie Boutin for discussion and advice con-
cerning the use of age-ratios in duck populations, and finally the
editor and two anonymous referees for their help in improving an
earlier version.
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