J Ornithol (2009) 150:133–145
DOI 10.1007/s10336-008-0328-4
ORIGINAL ARTICLE
Mortality of Black-tailed Godwit Limosa limosa and Northern
Lapwing Vanellus vanellus chicks in wet grasslands: influence
of predation and agriculture
Hans Schekkerman Æ Wolf Teunissen Æ
Ernst Oosterveld
Received: 19 September 2007 / Revised: 6 May 2008 / Accepted: 19 June 2008 / Published online: 16 July 2008
Ó Dt. Ornithologen-Gesellschaft e.V. 2008
Abstract Grassland-breeding shorebirds show widespread declines due to a reduction in breeding productivity
following agricultural intensification. However, there is
also concern that increasing predation causes further
declines or precludes population recovery. Predation may
itself be enhanced by agriculture through changes in habitat
or food availability, but little is known about the mortality
of nidifugous shorebird chicks. We studied mortality by
radio-tagging 662 chicks of Black-tailed Godwit Limosa
limosa and Northern Lapwing Vanellus vanellus in 15
farmland sites in the Netherlands. Tagging and handling
had no effect on the condition and survival of godwit
chicks, but body condition was reduced by 6–11% in lapwing chicks wearing a tag for longer than 3 days. Fledging
Communicated by F. Bairlein.
H. Schekkerman (&)
Dutch Centre for Avian Migration and Demography,
Netherlands Institute of Ecology (NIOO-KNAW), P.O. Box 40,
6666 ZG Heteren, The Netherlands
e-mail: h.schekkerman@nioo.knaw.nl
H. Schekkerman
Alterra, Wageningen University and Research Centre,
Wageningen, The Netherlands
H. Schekkerman
Animal Ecology Group, Centre for Ecological and Evolutionary
Studies, University of Groningen, Groningen, The Netherlands
W. Teunissen
SOVON Dutch Centre for Field Ornithology,
Rijksstraatweg 178, 6573 DG Beek-Ubbergen,
The Netherlands
E. Oosterveld
Altenburg and Wymenga Ecological Consultants,
P.O. Box 32, 9269 ZR Veenwouden, The Netherlands
success was 0–24% in both species. Mortality was highest
in young chicks but remained considerable until after
fledging. Losses were traced mostly to predators (70–85%;
15 species, predominantly birds), but at least 5–10% were
due to mowing, and 10–20% were due to other causes,
including entrapment in ditches and starvation. Chicks
staying in fields that were cut before the next radio check
were found much more often as mowing victims and
somewhat more often as prey remains than chicks in fields
not cut, indicating that predation includes a limited amount
of scavenging. The predation hazard for godwit chicks was
higher in recently cut or grazed fields than in the tall, uncut
grasslands they preferred, while that for lapwing chicks
was lowest in grazed fields. In godwit chicks, poor body
condition increased mortality risk, not only from starvation
but also from other causes. Predation on godwit chicks was
thus enhanced by intensive farming through a decline in the
availability of cover, augmented by a reduced body condition, possibly due to food availability problems. Changes
in farming practice may therefore help reduce predation
pressure, though the observed interactions explained only
part of the high predation rate in godwits and none in
lapwings. Predator abundance has increased in Dutch wet
grassland regions, and chick predation has become a factor
that should be considered in planning the type and location
of conservation measures.
Keywords Agricultural intensification Chick survival
Condition Predation Shorebirds
Introduction
The study of processes affecting reproduction and mortality is important to gain an understanding of the population
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134
dynamics of animals and to identify appropriate conservation strategies for declining species (Green 2002). Most
shorebirds (Charadrii) breeding in wet grasslands have
shown severe population declines throughout western
Europe (Thorup 2006), and a reduction in breeding output
has been identified as the main driver of several of these
declines (Green 1988; Peach et al. 1994; Besbeas et al.
2002; Ottvall 2005; Schekkerman et al. 2008). There is
broad agreement that this reduced breeding productivity is
caused primarily by agricultural intensification, leading to
an increase in direct clutch and chick mortality and to food
availability problems (Beintema et al. 1997; Vickery et al.
2001; Wilson et al. 2004; Schekkerman and Beintema
2007). However, concerns have also been raised that predation causes population declines or precludes recovery in
response to conservation measures (Grant et al. 1999;
Langgemach and Bellebaum 2005; Bolton et al. 2007). A
complicating factor in the ensuing discussions about conservation strategies is the possibility that predation
eliminates mainly prey with already reduced survival
prospects (Swennen 1989) or interacts with agricultural
land use (Evans 2004). For example, changes in farming
practice may alter the amount of protective cover or, via
effects on food availability, the chicks’ risk-taking behaviour and escape response.
Chicks of most shorebird species are precocial and
feed themselves. The resulting high energy requirements
make them sensitive to foraging conditions (Schekkerman
and Visser 2001), while the associated activity and
movements may also render them vulnerable to predators
and fatal accidents. Because shorebirds often re-nest after
clutch failure but usually not after losing chicks (Cramp
1983), chick survival is a key component of breeding
productivity in this group, but the importance of different
loss factors is much less well known for chicks than for
eggs.
With the development of small radio transmitters that
can be attached to chicks, a practical method has become
available to investigate fledging success and the causes of
chick death in precocial birds. Radio-tagging has been used
to study chick mortality in ducks (Korschgen et al. 1996;
Pietz et al. 2003), gamebirds (Riley et al. 1998; Larson
et al. 2001), bustards (Combreau et al. 2002) and shorebirds (Miller and Knopf 1993; Grant et al. 1999; PearceHiggins and Yalden 2003; Ratcliffe et al. 2005; Bolton
et al. 2007). A potential drawback of radio-tagging is that
the transmitters may affect the chicks’ behaviour or physiology and reduce their survival prospects. Thus, it is
important to check whether such negative effects influence
the outcome of telemetry studies (Kenward et al. 1993;
Whittingham et al. 1999; Grant 2002; Krapu et al. 2006).
In the study reported here, we quantified the importance
of different mortality factors, including the roles of
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J Ornithol (2009) 150:133–145
predation and agricultural management, for chicks of the
two most abundant grassland shorebirds in The Netherlands, Northern Lapwing Vanellus vanellus and Blacktailed Godwit Limosa limosa. We describe causes of death,
identity of predators and associations between mortality
and chick age, body condition and agricultural field use.
We also investigated whether tagging and handling chicks
affected their growth and survival.
Methods
Study species and areas
Both of the species studied breed primarily in agricultural
grasslands in the Netherlands, but their chicks differ in
ecology. While Black-tailed Godwit chicks prefer tall,
structured swards and feed on invertebrates living in the
vegetation, their Northern Lapwing counterparts frequent
short grass, muddy ground and ditch edges, and take
invertebrates mainly from the soil surface (Beintema et al.
1991). Both species have declined in the Netherlands,
godwits much more strongly than lapwings (SOVON 2002;
Teunissen and Soldaat 2006).
The data were compiled from two studies conducted in
2003–2005, one into the effects of predation on meadow
bird populations (Teunissen et al. 2005, 2006), and the
other on the effectiveness of a new agri-environment
scheme (AES) for improving breeding success of godwits
(Schekkerman et al. 2005, 2008). Chicks were studied in 15
sites (lapwing, seven sites; godwit, 11 sites) scattered
through the Netherlands (Table 1). Godwits were studied
in grasslands used for dairy farming; lapwings in both
grasslands and sites with mixed arable and dairy farming.
One grassland site was managed entirely and one was
partly managed as a nature reserve. In four of the godwit
study sites, an experimental AES aimed at improving
breeding conditions for Black-tailed Godwits was implemented, with measures that included postponing grass
mowing and leaving refuge strips when cutting (Schekkerman et al. 2008). Although some sites in the predation
study were selected on the basis of above-average rates of
clutch predation, this does not imply that chick predation
was also above average, as predation rates on eggs and
chicks were not strongly correlated (rs = 0.37 in godwits,
0.44 in lapwings; Teunissen et al. 2005).
Radio-tagging and tracking chicks
A total of 297 lapwing and 365 Black-tailed Godwit chicks
were radio-tagged during the study period, 15–53 (godwit)
or 22–58 (lapwing) per site and year. Chicks were tagged
within a day after hatching (godwit 86%, lapwing 32%) or
J Ornithol (2009) 150:133–145
135
Table 1 Study sites with general characteristics, study year(s) and species
Site number
Sitea
Province
Habitat (soil)
1
Arkemheen
Gelderland
2
IJsseldelta
Overijssel
3
Soest
Utrecht
Management
Year
Species
Grassland (clay/peat)
Dairy farming + reserve
2003–2004
L,G
Grassland (clay)
Dairy farming, maize
2003
L,G
Grassland (clay/peat)
Dairy farming + maize
2003
L
4
Leende
Noord-Brabant
Mixed farmland (sand)
Arable + dairy farming
2004
L
5
Ruinen
Drenthe
Mixed farmland (sand)
Maize + dairy farming
2004
L
6
Texel
Noord-Holland
Mixed farmland (sand)
Arable + dairy farming
2004
L
7
Tijnje
Friesland
Grassland (peat)
Meadow bird reserve
2005
L,G
8
Gerkesklooster
Friesland
Grassland (clay)
Dairy farming with AES
2004
G
9
Grijpskerk
Groningen
Grassland (clay)
Dairy farming
2004
G
10
Oldeboorn A
Friesland
Grassland (peat)
Dairy farming with AES
2005
G
11
12
Oldeboorn B
Amstelveen
Friesland
Noord-Holland
Grassland (peat)
Grassland (clay/peat)
Dairy farming
Dairy farming with AES
2005
2004–2005
G
G
13
Mijdrecht
Utrecht
Grassland (clay/peat)
Dairy farming
2004–2005
G
14
Noordeloos
Zuid-Holland
Grassland (peat)
Dairy farming with AES
2005
G
15
Ottoland
Zuid-Holland
Grassland (peat)
Dairy farming
2005
G
L, Lapwing; G, godwit; AES, Agri-environment scheme aimed at improving godwit breeding success
a
Sites ranged in size from 117 to 493 ha [mean 268 ± 110 (SD) ha]
at older ages. We used small 153-MHz VHF transmitters
(type LB-2; Holohil, Canada, assembled by Microtes, the
Netherlands) weighing 1.0 g and measuring 5 9 10 9 3
mm equipped with a 12-cm whip antenna (battery life
C40 days). Signal range was usually 100–300 m, more
under some conditions (C1 km when up in a raptor nest),
and less in others (down to \50 m when in a ditch or
burrow). Transmitters were glued to a 1.5 9 1.5-cm piece
of cloth with superglue, and this was attached to the down
on the chick’s back, just outside the centre of the synsacrum, with latex-based glue retaining some flexibility
(Uhu-Creativ, Uhu, Germany). Chicks were recaptured
every 4–7 days to check and restore tag attachment, which
deteriorated over time due to breakage of down and growth
of underlying feathers. Two chicks were tagged in broods
of four, one or two in broods of three. All chicks were
ringed, and bill length and body mass were recorded at
each capture. Age at first capture of chicks not ringed at
hatching was estimated from bill length (Beintema and
Visser 1989). We calculated an index of condition at each
capture by dividing the observed body mass by the mass
predicted at the chick’s age from published growth curves
(Beintema and Visser 1989).
Tagged broods were relocated every 1–5 days (median
2 days) using hand-held receivers and antennas. The
presence of living chicks was deduced from their parents’
alarm behaviour and fluctuations in the strength of their
radio signals, indicating movement. Steady signals were
followed up to check whether chicks were alive. Missing
chicks were searched for throughout the study area and in
bushes and woodlots potentially containing predators’
haunts up to several kilometres away. Before the batteries
expired, most study areas were traversed completely on
foot to search for weak signals from transmitters in ditches
and burrows. We also searched for rings and transmitters
with a metal detector under nesting trees in Grey Heron
Ardea cinerea colonies and some known raptor nests up to
a distance of 10 km.
In 2005, in sites 10–15 only (Table 1), one of the parents
of the tagged godwit chicks was also fitted with a transmitter (Holohil type BD-2). This greatly facilitated
assessing the chicks’ fate, as the transmitter signals were
stronger, and adults could still be located and their
behaviour observed after their chicks’ signals were lost.
Adult behaviour reliably shows whether chicks are alive
until about 1 week after fledging (Schekkerman and Müskens 2000).
The cause of death of recovered chicks was deduced
from the state and location of the remains (Teunissen et al.
2008). Locations in particular were often informative (e.g.
in ditch, among recently cut grass, under raptor nest or
plucking tree, in stoat burrow), but the state of the carcass
and/or the transmitter (condition, bite or plucking marks)
also conveyed information. Nevertheless, several cases
were left as ‘unknown’, ‘eaten by bird’, ‘not eaten’, etc.
Field notes and photographs were re-examined after the
study to standardise between-observer interpretation and
utilise experience gained. Transmitters found detached
without traces of violence were considered to have fallen
off a live chick if tag attachment had last been checked
[5 days earlier; otherwise they were categorised as ‘chick
dead or transmitter lost’.
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136
The locations of radio-tagged broods were recorded on
maps. The agricultural status of all fields in the study areas
was mapped at least once but usually several times a week
(less often in sites 6, 8 and 9). Categories were based on
crop type (grass/arable), sward height and whether fields
had been cut or grazed (Table 3).
Survival analysis
Survival curves were derived according to Kaplan and
Meier (1958), including the staggered entry of chicks ringed at different ages and right-censoring. Observations on
tagged chicks could end in several ways: (1) the chick
survived until it lost the transmitter or observations were
stopped after fledging or at the end of the season (censored,
i.e. removed from the sample at this time); (2) the dead
chick or its ring were recovered; (3) its transmitter was
found and categorised as ‘chick dead or transmitter lost’;
(4) its signal was lost before the fledging age, but neither
chick nor transmitter were recovered (i.e. dead, tag failure
or moved beyond the search range). Minimum and maximum estimates of survival were calculated by treating
chicks from categories (3) and (4) as dead and censored,
respectively, from the day their signal was lost.
The effects of environmental covariates on the mortality
of unfledged chicks were explored with proportional hazard models (Cox 1972), using procedure RPHFIT in
GENSTAT (Payne 2005). Models were run for the overall
probability of a chick disappearing and for separate competing risks: missing (no remains recovered), predation
(total and by bird or mammal separately), agricultural, and
other losses. The models assume an unspecified baseline
hazard function (similar to the reciprocal of the Kaplan–
Meier survival curve) that is modified proportionally by
covariates which may vary in time but are assumed to be
constant during the intervals between consecutive localisations of the chick. Covariates examined were site/year
(always included to correct for differences in general
conditions, including landscape and predator abundance),
chick age (always included), type of field in which the
chick was observed at the start of the interval, agricultural
activity on this field during the interval and chick body
condition. Information was not available on all covariates
for each interval. For categorical covariates, a category
‘unknown’ was included to ensure that all intervals contributed to the baseline hazard and models could be fitted.
Because this ’unknown’ category affects the degrees of
freedom for the overall test of significance of the covariate,
effects were evaluated from the 95% confidence intervals
of the ratios between the mortality risk for each level of the
covariate and the baseline hazard [hazard ratios (HR);
interval including 1 or not].
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J Ornithol (2009) 150:133–145
Our analysis of the associations between mortality and
field characteristics was complicated by the fact that broods
often moved between fields during the interval between
radio checks. Godwit chicks changed fields in 59% of 860
intervals; lapwings were more sedentary and moved in
26% of 988 intervals. By selecting intervals lasting
B2 days for godwits and B3 days for lapwings, we minimised the probability that chicks changed field while still
retaining most of the data in the analysis [godwits: 55 vs.
74% moved in intervals of 0–2 and [2 days (n = 716 and
144), respectively; v21 = 17.6; P \ 0.001; lapwings: 25 vs.
42% moved in intervals of 0–3 and [3 days (n = 896 and
92), respectively; v21 = 17.6; P \ 0.001). Body condition
indices were used for intervals both following and preceding the measurement.
Evaluating effects of radio-tagging
Negative effects of radio-tagging on chicks may arise
through entanglement in vegetation (not observed) or by
chicks becoming more easily detectable to predators, either
because of the transmitters themselves or as a result of
handling (scent or behavioural changes). Transmitters may
also affect chick growth and condition by hampering
feeding or increasing energy expenditure, with possible
consequences for risk-taking behaviour and escape
response. We checked for such effects in three ways.
We examined the effect of tagging and handling on
growth rate using the fact that not all chicks were tagged at
the same age. If negative effects occur, the condition index
of chicks wearing a transmitter for some time should be
less than that of same-age chicks caught for the first time.
This was tested in a linear mixed model including site/year
and chick identity as random variables (accounting for
repeated measures on the same chicks), and chick age and
‘days tagged’ (tag worn 0, 1–3, or [3 days) as fixed variables. Observations on chicks \3 days old were excluded,
as the growth curve underestimates the mass of newborn
chicks, and effects are less likely to show up so early.
Short-term effects of handling on survival were examined by comparing, in a proportional hazard model,
mortality over intervals between radio localisations in
which chicks were handled (measured and weighed) at the
start with that over intervals in which their initial live status
was deduced from a distance by the radio signal. Finally, in
sites 10–15 in 2005, the survival of tagged godwit chicks
(n = 127) could be directly compared with that of their
tagless siblings (n = 100). In these broods, half of the
chicks and one of their parents were radio-tagged, and both
the number of chicks hatched and the number fledged
(tagged and total) were known from visual observations
made around the fledging age.
J Ornithol (2009) 150:133–145
137
Results
Chicks tagged for 1–3 days did not differ from tagless
chicks in terms of condition (P = 0.37).
Effects of radio-tagging on chicks
Overall survival and age
In godwit broods with a radio-tagged parent, five of the six
chicks surviving to fledging were tagged; for the sixth
chick, status was uncertain (possibly failed tag). Counting
this latter case half in both categories, survival was marginally higher in tagged chicks (GLM with binomial
distribution and logit link, F = 4.27, P = 0.04). Although
the number fledged is small, this does not indicate a lower
survival for tagged chicks.
The mortality of godwit chicks over intervals between
successive observations was not higher when they were
handled and measured at the start of the interval than when
they were located from a distance only (hazard ratio
HR = 1.05, F1 = 0.11, P = 0.74, n = 685 handled, 676
non-handled). The same result was obtained in lapwings
(hazard ratio 1.08, F1 = 0.20, P = 0.66, n = 642 handled,
734 non-handled).
The condition index of godwit chicks was 0.89 on
average (SD = 0.15, n = 391) and declined with age
(Wald test, W1 = 7.82, P = 0.005; Fig. 1a), indicating a
lower growth rate than that observed 25 years ago by
Beintema and Visser (1989). Tagging and handling did not
depress the growth rate in godwit chicks: inclusion of the
variable ‘days tagged’ (0, 1–3, or [3 days) did not significantly improve the model fit (W2 = 1.16, P = 0.56;
interaction age 9 ‘days tagged’ W2 = 4.62, P = 0.10). In
lapwings, condition indices were higher on average
(1.06 ± 0.19, n = 658) and did not decline with age
(W1 = 1.95, P = 0.16; Fig. 1b), but ‘days tagged’ had a
significant effect that increased with age (W2 = 35.7,
P \ 0.001; interaction age 9 ‘days tagged’ W2 = 7.97,
P = 0.019). Lapwings that had worn a tag for [3 days
were 6% lighter than tagless chicks when 5 days old; this
difference increased to 11% at 30 days of age (P \ 0.001).
Fig. 1 Condition index of
chicks in relation to age, at first
capture (without radio tag) and
at later captures (with tag)
The fate of 23% of all radio-tagged chicks remained
uncertain as no remains were found, and in a further 3% we
were unsure whether the chicks had lost their transmitter or
died (Table 2). Observations on godwit broods with a
tagged parent in 2005 (sites 10–15) showed that all 49
chicks that remained ‘missing’ had actually died; the parents stopped alarming before the chicks were 25 days old,
except in one case where a tagless sibling fledged. Hence,
true survival was very likely closer to the minimum than to
the maximum estimates.
Survival to fledging was low in both species (Fig. 2).
The minimum estimate varied between sites/years from 0
to 24% in godwits (mean 7%, SD = 7%, n = 14 sites) and
from 0 to 23% in lapwings (mean 14%, SD = 8%, n = 8).
In both species, survival was especially low in 2005 (mean
3 and 4%, respectively). Mortality was highest in the first
days after hatching, then it more or less stabilised before a
further decrease around the fledging age (Fig. 2). Appreciable mortality still occurred after fledging, especially in
lapwing chicks. The initial phase of high mortality lasted
longer and encompassed a larger proportion of the total
losses in lapwings than in godwits.
Causes of death
Of all chicks lost before fledging (dead or ‘missing’), the
cause of death remained unknown in 38%, 47% were found
to have been eaten by predators, 5% were victims of
agricultural activities and 9% succumbed to other causes
(Table 2). As part of the ‘missing’ chicks were probably
removed by predators (see Discussion), predation (including scavenging) was the primary cause of mortality.
northern lapwing
condition index
black-tailed godwit
1.50
1.50
1.25
1.25
1.00
1.00
0.75
0.75
0.50
0.50
0.25
without tag (N=217)
0.25
without tag (N=140)
with tag (N=441)
with tag (N=251)
0.00
0.00
0
5
10
15
chick age (days)
20
25
0
5
10
15
20
25
30
35
chick age (days)
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138
J Ornithol (2009) 150:133–145
Table 2 Summary of fates and causes of death of radio-tagged chicks, pooled over study sites and years
Fate
Total
Godwit
Number
Number of chicks tagged
Percentage
Number
662
Survived observation period
Loose transmitter: lost or dead
Differencea
Lapwing
Percentage
365
Number
Percentage
P value
297
119
18
49
13
70
24
23
3
6
2
17
6
Missing, no remains found
150
23
83
23
67
23
Dead, transmitter or chick found
370
56
227
62
143
48
Causes of death (% of lost chicks)
543
316
227
‘Missing’ + ’transmitter lost or dead’
Dead, cause unknown
173
35
32
6
89
28
28
9
84
7
37
3
3.24
6.84
0.072
0.009
Eaten by bird
0.24
155
29
83
26
72
32
1.38
Eaten by mammal
65
12
50
16
15
7
9.37
0.002
Eaten, predator unknown
35
6
16
5
19
8
2.24
0.13
Agricultural activity and trampling
26
5
22
7
4
2
7.46
0.006
Drowned/stuck in ditch/trench
29
5
15
5
14
6
0.50
0.48
Starvation/illness
13
2
9
3
4
2
0.65
0.42
Other causes
11
2
4
1
7
3
2.16
0.14
2
Difference gives v -test of differences in prevalence between godwits and lapwings
Northern Lapwing
Black-tailed Godwit
1.0
proportion surviving
1.0
proportion surviving
0.8
0.6
0.4
0.2
maximal
minimal
0
1.00
5
10
15
20
25
30
35
0.6
0.4
0.2
40
maximal
minimal
0
5
10 15 20 25 30 35 40 45
1.00
maximal
minimal
0.95
0.90
0.85
0.95
0.90
0.85
maximal
minimal
0.80
0.80
0
5
10
15
20
25
chick age (d)
Predation hazard declined with chick age by 7% per day
(proportional hazard model, godwits F1 = 10.9,
P \ 0.001; lapwings F1 = 16.6, P \ 0.001). Birds were
more often identified as chick predators than mammals,
particularly with respect to lapwings (Table 2). The four
species most frequently identified were Grey Heron Ardea
cinerea (18% of 255 chicks found predated), stoat/weasel
123
0.8
0.0
0.0
daily survival probability
Fig. 2 Survival curves (upper
panels) and daily survival rates
(lower panels, with smoothing
splines, df = 4) of Black-tailed
Godwit and Northern Lapwing
chicks, pooled over sites and
years. Maximum and minimum
estimates are based on different
assumptions about the fate of
‘missing’ chicks; minimum
values are more likely to be true
(see Methods). Grey areas
indicate ages at which chicks
had fledged. Godwit sample size
varied from 298 chicks at
hatching to 39 at fledging and 9
on day 40 (3,526 chick-days in
total); lapwing sample size
varied from 66 at hatching to
131 on day 4, 51 at fledging and
18 on day 45 (3,349 chick-days
in total)
daily survival probability
a
v21
30
35
40
0
5
10 15 20 25 30 35 40 45
chick age (d)
Mustela erminea/nivalis (15%), Common Buzzard Buteo
buteo (12%) and the Carrion Crow Corvus corone (6%); 11
other species made up B2% each (White Stork Ciconia
ciconia, Goshawk Accipiter gentilis, Sparrowhawk Accipiter nisus, Marsh Harrier Circus aeruginosus, Common
Kestrel Falco tinnunculus, Jackdaw Corvus monedula,
Lesser Black-Backed Gull Larus graelsii, Common Gull
J Ornithol (2009) 150:133–145
L. canus, Rat Rattus sp., Domestic Cat Felis catus and Red
Fox Vulpes vulpes). Lapwing chicks were taken more often
by herons than godwit chicks (11 vs. 26%, v21 = 7.90,
P = 0.005), while godwit chicks were taken more by stoats
(8 vs. 20%, v21 = 5.49, P = 0.02) and buzzards (6 vs.
17%, v21 = 6.30, P = 0.01); both species were taken
equally by crows (6 vs. 7%, v21 = 0.11, P = 0.74). Godwit
chicks preferred tall vegetations (94% of 1036 localisations
in fields with uncut or regrowing sward [15–20 cm high
vs. 37% of 1186 localisations of lapwing chicks in this
habitat) where they were proportionally more often taken
by mammals (mostly stoats) than in short swards (43 vs.
20% of predations by mammals, v21 = 4.62, P = 0.032;
Table 3).
All agricultural losses concerned chicks killed during
mowing and harvesting of grass, except for one newborn
lapwing trampled by cattle. More godwit than lapwing
chicks fell victim to mowing (Table 2) due to the godwits’
preference to forage in tall grassland ready to be mown. In
godwit chicks, the risk of mortality by mowing tended to
decline with age (-10%/day, F1 = 3.60, P = 0.058),
although chicks up to 23 days old were killed by machines.
Too few lapwings were killed by mowing to find an age
effect.
About 5% of lost chicks died in wet (both species) or dry
(lapwing only) ditches. Although chicks swim well, ditches
can form a trap when the sides are too steep to climb. The
risk of entrapment in ditches declined with age in lapwings
(-17%/day, F1 = 14.1, P \ 0.001) but not significantly in
godwits (-10%/day, F1 = 2.17, P = 0.14). Other causes
of death included starvation or illness (2%), acute exposure
to cold or rain (1%) and aggression by conspecifics (one
case).
Mortality and field use
Compared to the most-used field type (uncut grassland), the
risk that godwit chicks were taken by a predator was twice
as high as that in recently cut or grazed fields with a short
sward (Table 3). This effect was caused by avian predators
and translated into a 1.4-fold higher overall mortality. The
risk of predation by mammals was especially high in previously cut fields with a regrowing sward, but as mammal
predation was less frequent, this did not translate into a
higher overall mortality. The only significant effect of field
type on the mortality of lapwing chicks was a lower risk of
predation (by birds) in grazed fields. The predation hazard
for godwits was also low here, but not significantly different from that in uncut grassland (Table 3).
Godwit chicks located in fields with a tall (uncut or
regrowing) sward were 13 times more likely to be found as
a mowing victim when the field was cut during the subsequent interval than when it was not, but the associated
139
50% increase in overall mortality was not significant
(Table 3). No lapwing chicks were killed by mowing in
intervals B3 days. Because avian predators and scavengers
are often attracted to mowing activity, it was of interest to
know whether other risks increased when the field was cut.
Hazard ratios for predation (particularly by birds) were
greater than 1, but the effect was not significant (godwit
P = 0.15, lapwing P = 0.10). It was significant in godwits
when only 1-day intervals were considered (HR = 22,
P = 0.07). The probability that chicks went ‘missing’ was
not associated with cutting of the field (Table 3).
Mortality and condition
A low body condition index greatly increased the risk of
dying by ‘starvation or illness’ in both species (Table 4),
which is expected as the diagnosis was based on a lack of
injuries combined with a poor condition. The mean condition index of chicks considered to have died from
starvation was 0.48 (SD = 0.06, range 0.41–0.56, n = 10).
Condition affected the overall risk of mortality in godwits
but not in lapwings, and there was also a near-significant
tendency for godwit chicks in poor condition to be lost to
causes other than starvation or illness. This was not so
much due to predation (except by mammals) as to a higher
probability to end up ‘missing’ (Table 4). In lapwings, no
condition effects were observed on risks other than
starvation.
Discussion
Radio-tagging as a method to study chick mortality
Our study did not reveal any negative effects of radiotagging and handling on the condition and survival of
Black-tailed Godwit chicks. Northern Lapwing that had
worn a tag longer than 3 days were 6–11% lighter than
same-age chicks captured for the first time. This result
suggests that tags induced negative effects on growth by
reducing the chicks’ insulation, increasing energy costs of
locomotion or impairing foraging success. Lapwing chicks
are smaller than their godwit counterparts and, therefore,
the transmitter adds proportionally more to their body mass
(5.7 vs. 3.5% at hatching, decreasing to 0.5% at fledging),
which may help explain why we did not observe a condition effect in godwits. We found no association between
condition and the overall survival of lapwing chicks
(Table 4), but condition indices B0.6, associated with a
strongly increased risk to be found starved, were only
observed in tagged lapwings (Fig. 1). Since no adult lapwings were radio-tagged, we were unable to directly
compare the survival of tagged chicks with that of tagless
123
140
123
Table 3 Proportional hazard analysis for different causes of death in relation to the field type where chicks stayed at the start of observation intervals and (for chicks in uncut or regrowing
grassland) to whether the field was cut during the interval
Field type
n
Lost (dead/missing)
Predation total
Predation by birds
Predation by mammals
‘Missing’ (not recovered)
Mowing and trampling
HR
HR
HR
HR
95% CI
HR
HR
95% CI
95% CI
95% CI
95% CI
95% CI
Black-tailed Godwit
Uncut grassa
638
1.0
1.0
1.0
Regrowing grassb
70
1.1
0.6–2.2
1.9
0.6–5.7
0.5
0.0–4.2
1.0
5.2*
1.2–23.8
0.7
1.0
0.2–2.5
0.4
1.0
Refuge stripsc
36
2.0
1.0–4.1
1.9
0.7–5.4
1.0
0.3–3.4
0.0
0–[100
1.4
0.3–6.5
9.7
0.7–122
Grazed grassd
Short grasse
105
135
0.7
1.4
0.4–1.3
0.9–2.2
0.6
2.0*
0.2–1.8
1.1–3.7
0.8
3.2**
0.2–2.8
1.5–6.7
0.6
1.0
0.1–4.5
0.3–3.4
1.1
1.1
0.5–2.2
0.6–2.4
0.0
0.0
0–[100
0–[100
Field not cut in interval
601
1.0
Field cut in interval
46
1.5
0.2–2.6
13.4**
0.0–4.3
In uncut and regrowing grass
1.0
0.8–2.8
1.8
1.0
0.6–5.6
2.8
1.0
0.7–11.0
1.3
1.0
0.1–13.8
0.8
1.0
3.2–56
Northern Lapwing
Uncut grassa
175
1.0
Regrowing grassb
303
0.9
0.4–1.6
0.7
0.3–1.6
1.0
0.3–3.0
0.1
0.0–1.3
1.6
0.5–5.1
–
–
Grazed grassd
349
0.5*
0.3–0.9
0.2**
0.1–0.6
0.2*
0.1–0.7
0.5
0.1–3.1
1.4
0.5–4.3
–
–
Short grasse
87
1.1
0.5–2.4
0.8
0.3–2.8
2.1
0.5–9.7
0.0
0–[100
2.7
0.8–9.0
–
–
Arablef
104
0.9
0.4–2.3
0.3
0.1–1.3
0.7
0.1–4.5
0.1
0.0–2.9
3.0
0.7–12.6
–
–
Field not cut in interval
450
1.0
–
–
Field cut in interval
28
1.1
–
–
1.0
1.0
1.0
1.0
No fit
In uncut and regrowing grass
1.0
0.3–3.8
1.6
1.0
0.3–8.3
4.6
1.0
0.8–27.8
0.1
1.0
0–[100
1.4
0.2–12.9
P \ 0.10, *P \ 0.05, **P \ 0.01
Hazard ratios (HR) are given relative to the site- and age-specific mortality risk in uncut grasslands (baseline hazard), with 95% confidence limits (CI). Ratios significantly different from 1 are
given in bold. n is the total number of intervals (B2 days in godwits, B3 days in lapwings) for each type
a
Grassland not yet cut or grazed in the present year; vegetation usually C15–20 cm high
Grassland previously cut or grazed; vegetation regrown to C15–20 cm
c
Recently (B3 weeks ago) cut grassland with strips of tall vegetation; usually 2- to 10-m-wide strip left uncut
d
Currently grazed grassland; vegetation height dependent on time grazed
e
Recently cut or grazed grassland where regrowing vegetation has not yet reached 15–20 cm
f
Diverse arable crops, including maize fields within grassland areas
J Ornithol (2009) 150:133–145
b
J Ornithol (2009) 150:133–145
141
Table 4 Hazard ratios for different causes of death, for a reduction in the body condition index of chicks from 1 (baseline hazard) to 0.6 (a very
poor condition; Fig. 1)
Hazard type
Black-tailed Godwit (n = 554)
HR
95% CI
Northern Lapwing (n = 825)
P value
HR
95% CI
P value
All mortality (dead and missing)
3.0
1.6–5.6
\0.01
1.1
0.7–1.6
0.67
All except starvation/illness
1.9
1.0–3.8
0.06
1.0
0.7–1.5
0.99
Predation (total)
Predation (bird)
1.0
0.8
0.4–2.7
0.2–3.5
0.99
0.82
0.9
0.7
0.5–1.7
0.3–1.5
0.86
0.40
Predation (mammal)
2.7
0.4–16.0
0.29
1.4
0.2–8.1
0.70
‘Missing’
Other (including starvation)
2.1
219.6
0.7–6.2
12.5–3872
0.17
0.8
0.5–1.5
0.56
\0.01
20.5
3.6–116
\0.01
Hazard ratios (HR) are given with 95% confidence limits (CI) and P values indicating whether they differ significantly from 1. Models for
agricultural losses did not converge
siblings, as in the godwits, but a study in the UK did
observe that poor condition induced by repeated handling
reduced the survival of lapwing chicks (F. Sharpe et al.,
unpublished). Other studies have generally not found
adverse effects of back- or leg-mounted tags in chicks of
shorebirds and gamebirds (Kenward et al. 1993; Whittingham et al. 1999; Grant 2002). Nevertheless, there may
be effects of telemetry studies that cannot be detected by
within-brood comparisons. For example, the effects of
repeated disturbance during the tracking of broods will
affect both tagged and tagless chicks equally.
Notwithstanding these potential problems, radio-tagging
provides the only feasible method to study causes of death
of chicks. In our study, 6–9% of observed deaths could not
be attributed to a cause, and 23% of all chicks disappeared
without a trace. The contribution of different mortality
factors may well differ between ‘missing’ chicks and those
found dead. Indeed, it is even uncertain whether ‘missing’
chicks died or survived with a failed tag or after moving
out of the search area. The resulting minimum and maximum estimates of chick survival lay so far apart (Fig. 2)
that estimating reproductive success from tagging chicks
would be problematic without additional information. In
the Black-tailed Godwit and in Eurasian Curlew Numenius
arquata studied by Grant (2002), the additional tagging of
one parent greatly enhanced the interpretation of the
chicks’ fate and the precision of survival estimates. None
of the ‘missing’ chicks of tagged godwits survived to
fledging. As tagged parents are easier to relocate than
tagged chicks and as mortality seemed especially high in
2005, this result may not hold for all missing chicks, but it
is very likely that most signal losses reflected chick deaths.
Transmitters may have been destroyed by mowing and
harvesting machinery or buried in silage stacks (signals are
lost when buried [1.5 m deep, unpublished observations),
but as the probability that a chick ended up ‘missing’ was
not higher if the field in which it stayed was cut during the
observation interval, this is unlikely to have occurred frequently. Some signals may have been lost when chicks
drowned, but as most ditches in our study areas were
shallow, this was probably not a major cause of signal loss
either. It is highly probable that most ‘missing’ transmitters
were destroyed by predators or carried out of the search
range to distant sites or deep burrows.
In bird species where parents stay with the brood until
fledging, we recommend tagging both chicks and parents to
study details of chick mortality, including causes of death.
If the primary aim is to quantify breeding success or brood
movements, we prefer tagging parents only as it makes
tracking less time-consuming and minimises negative
effects on chicks.
Identifying predators
No less than 15 species were identified as chick predators
in this study, with Common Buzzard, Grey Heron and
stoat/weasel recorded the most frequently. The large fraction of unidentified causes of death calls for caution in
interpreting the importance of different species, as some
may leave more readily identifiable remains than others.
Red foxes might be particularly likely to bury or destroy
transmitters, but in two of our study sites where foxes were
known to be absent neither the fraction lost to unknown
causes (30 vs. 39%, v21 = 0.92, P = 0.34) nor the share of
mammals in known predations (41 vs. 26%, v21 = 2.61,
P = 0.11) were lower than in the eight sites where foxes
were present. This makes it unlikely that foxes were
responsible for the majority of unexplained losses. Stoats
and weasels also bury chicks underground (often in European Mole Talpa europaea tunnels), but in several cases
where we located such caches, we could pick up the signals
from distances up to 50–100 m. Nevertheless, buried tags
are less easily located than tags under raptor nests or
plucking trees, and we may have missed those buried deep.
123
142
However, such bias would have to be strong to fully
explain the large share of avian predators in chick predation. This contrasts with predation on shorebird eggs,
where mammals, particularly Red Foxes, usually take a
larger share (Langgemach and Bellebaum 2005; Bolton
et al. 2007; Teunissen et al. 2006).
The greater contribution of stoats and smaller share of
herons in the predation of godwit than lapwing chicks
(Table 3) is probably associated with vegetation preferences. Stoats do not usually hunt and are less likely to
approach chicks unnoticed in the short (cut and grazed)
swards preferred by lapwing chicks, but herons often forage in short (cut) grassland swards (unpublished
observation).
Disentangling the roles of predation and agriculture
Chick survival rates observed in this study are low compared to previously published estimates, both for Blacktailed Godwits (7 vs. 9–46%; Beintema 1995; Ratcliffe
et al. 2005; Schekkerman et al. 2008) and Northern Lapwings (14 vs. 7–50%; Galbraith 1988; Baines 1990;
Beintema 1995). Predation was the most frequent direct
cause of death; we estimate that 70–85% of all lost chicks
were taken by predators, 5–10% were mowing victims and
10–20% died of other causes. However, mowing losses
may have been underrecorded, as in 2004 the first grass cut
was already underway in some sites before we tagged most
chicks. Also, in the four godwit AES sites, grassland use
included measures aimed at avoiding chick losses. Mowing
losses tended to be lower in AES sites than in controls, but
the difference was not significant (5 vs. 11%, Schekkerman
et al. 2008). Finally, mowing victims may have been
removed by scavengers, and as we could rarely deduce this
from the remains, these would have been recorded as
predated. Avian predators regularly foraged among the cut
grass on recently mown fields. If many dead or injured
chicks were taken here, not only a chick’s probability to be
found as a mowing victim but also its probability to be
found ‘predated’ should be higher if its field of residence
was cut during the interval between observations than if it
was not. Although predation hazard ratios tended to be
greater than 1 if the field was cut, the effect was significant
only for 1-day intervals in godwits. Scavenging probably
occurs mainly on the first day after mowing, and its effect
may be diluted by ‘true’ predation over longer observation
intervals. Although some chicks may thus have been
removed from cut fields by predators, their number was
probably smaller than that of identified mowing victims,
otherwise a clearer effect on predation hazard would be
expected.
Predation may be enhanced by farming practice through
changes in the vulnerability of prey to predators. Godwit
123
J Ornithol (2009) 150:133–145
chicks were two- to threefold more likely to be killed by a
(avian) predator if they stayed in recently cut or grazed
fields than in uncut grasslands, which form their preferred
habitat (Table 3). The small chicks are less visible here,
while the detection of predators is taken care of by the
larger parents. Hence, godwit broods that are forced to
forage in or frequently travel through cut fields due to a
scarcity of uncut grassland are more likely to suffer predation losses. By multiplying the observed average daily
survival rate with the field-type-specific hazard ratio divided by the average of the hazard ratios for all field types
weighted according to their frequency of use (Table 3),
2.7% of chicks instead of the observed 7.2% are predicted
to fledge if broods had to stay in short-sward grasslands
throughout. Survival would increase to 8.7% if broods
stayed in uncut fields continuously. Field use can thus
induce a more than threefold change in predation rate, but
this interaction effect does not explain why godwit chicks
survived poorly in all field types in our study. Nevertheless,
overall survival of Black-tailed Godwit chicks increases
with the availability of tall (not cut or grazed or sufficiently
regrown) grassland swards during their prefledging period
(Schekkerman and Müskens 2000; Schekkerman et al.
2008).
Predation on lapwing chicks was not reduced on uncut
fields. Their earlier hatch dates, (causing fields to have
shorter swards when visited by lapwings than by godwits)
and the fact that within uncut fields lapwing chicks will
feed in patches with less vegetation may contribute to this.
Lapwings prefer short swards, including grazed fields
where they ran a significantly lower predation risk that was
also observed (but was not significant) in godwits. It is
possible that some predator species, including herons and
stoats, avoid foraging in fields with livestock.
The importance of predation may be overestimated if it
selectively affects individuals with already reduced survival prospects (e.g. Swennen 1989). Might predation
represent the final elimination of shorebird chicks that lag
behind in growth because of suboptimal feeding conditions? Godwit growth in our study was retarded in
comparison to measurements from the 1980s (Beintema
and Visser 1989), and this may reflect a deteriorated food
supply due to agricultural intensification (Schekkerman
and Beintema 2007). In both godwit and Lapwing chicks,
the likelihood to be found starved increased with declining
body condition, but other hazards increased only in godwit
chicks, where the probability to end up ‘missing’ was
elevated rather than that of predation. This is unexpected
given our interpretation that most ‘missing’ chicks were
depredated. Modifying daily survival rates by the estimated
hazard ratio predicts that average chick survival would
increase to just 11% in godwits at a mean body condition of
1 instead of the observed 0.89. In lapwing chicks, the
J Ornithol (2009) 150:133–145
condition had no significant effect on deaths other than by
starvation and did not affect overall mortality rate. This
suggests that predators did not strongly select chicks in
poor condition, but it cannot be excluded that chicks
experiencing food shortage extend their foraging activity or
take more risks and are eliminated even before their condition is visibly affected. A poor condition did increase
predation on lapwing chicks in a similar study in the UK
(F. Sharpe et al., unpublished).
Conservation implications
Our results indicate that predation on godwit chicks is
increased up to threefold by intensive agricultural grassland use through a reduced availability of fields with
protective cover, and possibly also by a reduction of food
availability, leading to poor body condition or risky foraging behaviour. Cutting fewer grasslands early will
therefore reduce predation losses in addition to direct losses due to mowing and starvation. However, these
interactions between predation and agriculture explained
only part of the high predation rate observed in godwit
chicks and none in lapwing chicks. Some frequent chick
predators have increased notably in numbers in the wet
grassland regions of the Netherlands. Common Buzzards
were absent here until the 1980s, but they now occur nearly
everywhere (SOVON 2002). Goshawk, sparrowhawk,
White stork, Carrion crow, gulls and Red Fox have also
increased. These increases partly represent a return to
natural population levels after historical reduction by
human persecution and pollution, but it is greatly enhanced
by man-made changes opening up formerly unsuitable
landscapes to several of these species. Though Grey Heron
has been common throughout and stoat and weasel have
declined, overall predator abundance has probably
increased. Simultaneously, changes in farming practice
have made grassland birds more vulnerable to predation
through the interactions described here and by reducing—
via breeding density—the ability of meadowbirds to
cooperatively evict potential predators (Green et al. 1990).
Over the past 20 years, prefledging survival of Black-tailed
Godwit chicks has declined significantly in the Netherlands
(Schekkerman et al. 2008). Little historical data are
available on the mortality of lapwing chicks, which are less
sensitive to changes in grassland cutting regimes. Between
1990 and 2000, lapwings declined less rapidly in the
Netherlands than Black-tailed Godwits (-0.5 vs. -1.9%
per year), but since 2000 the population has shown an
annual 3.4% decline that approaches that of godwits
(-5.6%; Teunissen 2007). It is possible that the recent
acceleration in the declines of both species shows the
additive effect of increased predation on top of that of
ongoing agricultural intensification.
143
An increasing predation pressure makes conservation
measures to counteract negative effects of modern farming
even more urgent than before. ‘Shallow’ measures that
worked 30 years ago may no longer suffice to raise
meadowbird breeding productivity to a level that can sustain the population. Control of predation is a complex
matter scientifically, ethically and practically, and requires
careful consideration of all available options (Bolton et al.
2007). There is much to gain by considering effects on
predation risk in the development of practical conservation
measures and in concentrating these in areas with optimal
external preconditions, including a landscape structure that
does not sustain high predator densities or in areas where
such conditions can be created.
Zusammenfassung
Kükensterblichkeit von Uferschnepfe Limosa limosa
und Kiebitz Vanellus vanellus im Feuchtgrünland:
Einfluss von Prädation und Landwirtschaft
Watvögel, die im Grünland brüten, nehmen aufgrund eines
Rückgangs im Reproduktionserfolg nach Intensivierung
der Landwirtschaft weiträumige ab. Außerdem besteht
Grund zur Annahme, dass ein Anstieg in der Prädation zu
einer weiteren Abnahme führt, oder eine Bestandserholung
unmöglich macht. Prädation selbst wiederum könnte von
der Landwirtschaft durch Veränderungen in Lebensraum
oder Nahrungsverfügbarkeit verstärkt werden, aber über
die Mortalität nestflüchtender Watvogelküken ist nur
wenig bekannt. Wir untersuchten die Mortalität von
Uferschnepfen- und Kiebitzküken, indem wir 662 Küken
an 15 landwirtschaftlich genutzten Standorten in Holland
telemetrierten. Bearbeitung und Besenderung hatten keinen
Einfluss auf die Kondition und die Überlebensrate der
Uferschnepfenküken, aber die Körperkondition der Kiebitzküken, die einen Sender länger als drei Tage trugen,
war um 6–11% verringert. Bei beiden Arten betrug die
Ausfliegerate 0–24%. Bei jungen Küken war die Mortalität
am höchsten, blieb aber bis nach dem Ausfliegen erheblich.
70–85% aller Verluste gingen auf das Konto von Beutegreifern (15 Arten, überwiegend Vögel), mindestens 5–
10% waren auf eine Mahd zurückzuführen und 10–20%
hatten andere Ursachen, wie Einschluss in Gräben und
Verhungern. Küken auf Flächen, die vor der nächsten
telemetrischen Erfassung gemäht worden waren, wurden
viel häufiger als Mahdopfer wieder gefunden und etwas
häufiger in der Form von Beuteresten, als Küken auf ungemähten Flächen. Demnach finden sich unter den
Prädationsereignissen in geringem Maße auch Fälle, in
denen bereits tote Küken gefressen wurden. Für
Uferschnepfenküken war die Prädationsgefahr auf kurz
123
144
zuvor gemähten oder beweideten Flächen höher, als im
bevorzugten hohen, ungeschnittenen Grasland. Für Kiebitzküken war das Prädationsrisiko auf beweideten Flächen
am geringsten. Eine schlechte körperliche Verfassung erhöhte bei den Uferschnepfen die Sterblichkeit nicht nur
durch Verhungern, sondern auch durch andere Ursachen.
Demnach erhöhte eine intensivierte Landwirtschaft die
Prädation der Uferschnepfenküken aufgrund einer Abnahme an verfügbarer Deckung, verstärkt durch eine
verschlechterte Körperkondition, möglicherweise aufgrund
von Problemen in der Nahrungsverfügbarkeit. Änderungen
in der Bewirtschaftungspraktik könnten folglich dazu beitragen, den Prädationsdruck zu verringern, wenngleich die
festgestellten Zusammenhänge die hohe Prädationsrate bei
den Uferschnepfen nur zum Teil und bei den Kiebitzen
überhaupt nicht erklären. In Holland hat die Häufigkeit von
Beutegreifern in Gebieten mit Feuchtgrünland zugenommen, und die Kükenprädation ist zu einem wichtigen
Faktor geworden, der bei der Planung von Schutzmaßnahmen bezüglich Art und Ort berücksichtigt werden
sollte.
Acknowledgments This work was supported by Natuurmonumenten, Staatsbosbeheer, the Union of Dutch Landscapes, Birdlife
Netherlands, Landschapsbeheer Nederland, the provinces of Drenthe,
Flevoland, Fryslan, Gelderland, Noord-Holland, Noord-Brabant,
Overijssel and Zeeland, the Ministry of Agriculture, Nature
Management and Food Quality and the Postcode Loterij. Fieldwork
was conducted by L. Beskers, K. Bouwman, I. Geelen, P. Heemskerk,
Y. van der Heide, B. Henstra, H. de Jong, M. de Jong, A. van Kleunen, M. Kuiper, F. Majoor, G. Müskens, W. Nell, R. Oosterhuis,
H.-J. Ottens, K.-P. Plas, T. Meijer, E. Vromans, F. Weijdema and F.
Willems. Many farmers and volunteers provided assistance by
establishing contacts, providing access to land, finding and monitoring nests and many other ways. Patrick Jansen and Hans-Peter
Koelewijn advised on the survival analysis. Rudi Drent and Piet
Heemskerk commented on drafts of this paper. Radio-tagging of
chicks complied with Dutch law on animal experiments.
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