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: matthieu.guillemain@oncfs.gouv.fr

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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