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Bird Study
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Mass-change during moult in the
Great Tit Parus maj or
A. G. Gosler
a
a
Depart ment of Zoology, Edward Grey Inst it ut e of Field
Ornit hology, Sout h Parks Road, Oxf ord, OX1 3PS, UK
Published online: 25 Jun 2009.
To cite this article: A. G. Gosler (1994) Mass-change during moult in t he Great Tit Parus
maj or , Bird St udy, 41: 2, 146-154
To link to this article: ht t p: / / dx. doi. org/ 10. 1080/ 00063659409477211
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Bird Study (1994) 41, 146-154
Mass-change during moult in the Great Tit Parus major
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A.G. GOSLER Edward Grey Institute of Field Ornithology, Department of
Zoology, South Parks Road, Oxford OX 1 3PS, UK
Changes in mass during moult were investigated by analysis of data recorded
on standard moult cards by members of the British Trust for Ornithology
Ringing Scheme. In the analysis of post juvenile moult, all available cards that
presented wing length, mass and time of capture were used. The analysis of
adult post-nuptial moult was restricted to cards from a single year. Juvenile
mass correlated strongly with body size, time of day and the number of greater
coverts being replaced. A fter controlling for the effects of size and time of day,
mass was found to increase with date except in those individuals in which all
juvenile greater coverts were retained. In these birds residual mass declined
with date. In adult males, mass increased with both date and moult scores of
primary, secondary and tertial remiges and of rectrices but because of the strong
correlations between moult scores and date, the true relationship was not clear.
In adult females, mass declined with date before increasing again. Unlike the
males, female moult scores did not predict body mass. However, the timing of
moult was a strong predictor of body mass. The differences between age and sex
groups are discussed in relation to the nutritional demands of moult.
he moult period can be a stressful time for
many birds. This is not only because it is
nutritionally demanding for both energy and
proteins providing specific amino acids" but
also because it may compete in time with other
activities such as breeding and migration so
that its specific food requirements may not be
at their most abundant at the time that moult
4-7
To overcome this, many species have
occurs
evolved variable moult strategies that relieve
the individual from the commitment of a complete moult each year. For example, many
migratory passerines arrest remige moult
before migrating in Autumn rather than
delaying their departure until the moult is
completed; 7' 8 many others delay moult entirely
until they are within the winter quarters, 9 or, in
far-northern populations, may attempt to
moult so rapidly that they become temporarily
flightless. 10 Furthermore, variation in moult
within a species may reflect differences in the
costs of moult between various sections of the
population. Hence, while adult passerines generally have one complete moult each year the
post-juvenile (PJ) moult is frequently reduced
in extent (usually excluding the remiges) and
may vary between individuals in its complete-
T
ness. Common examples are the incomplete
and variable moult in juveniles of greater
coverts, tertials, alula and in some species also
the tail. 5,9 Age differences in the extent of
moult may reflect the poorer foraging skills of
juveniles, so adding to the relative cost to them
of a complete moult. 11
Such a situation is found in Great Tit
populations in northern Europe. Here, adults
undergo a complete moult after breeding (postnuptial or PN moult). This usually commences
with the loss of the first (innermost) primary
remex. At about the time that the third to fifth
remex is dropped, secondary, tertial and tail
moult all commence. Several studies give
detailed descriptions of the moult sequence
and the nutritional conflict between breeding,
moult and territoriality in this species. 5 ' 6,12-15
In contrast, the PJ moult is partial and never
includes remiges. It usually includes all body
feathers and rectrices but the moult of the
greater coverts is often incomplete, as is the
replacement of the alula. The proportion of
juveniles which fail to complete greater covert
moult varies greatly between populations,
between years within populations, between
sexes and in relation to the date of fledging.12
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Condition and moult of Great Tits
The evidence is now considerable that the
extent of PJ moult in the Great Tit is related to
the availability of resources, and subsequently
the physiological condition of the individual, at
the time of moult. For example, first-year Great
Tits trapped in winter in Wytham Woods,
Oxford that had incomplete greater covert
moult were in significantly poorer condition
in terms of a pectoral muscle score (a measure
of protein reserves) than those that had completed moult of this tract. It was suggested that
both measures were indicators of an individual's ability to obtain food. 16 Hence, a bird
that had poor access to resources during winter
may have been similarly limited during moult.
Furthermore, the muscle score correlated negatively with the number (rather than simply the
presence) of unmoulted greater coverts, suggesting a graded response to stress induced by
resource-limitation.
While a relationship has been described between the extent of moult and muscle condition some months after moult, 16 it is not clear
whether this is causal or whether both are
merely correlates of resource availability. This
paper considers this relationship further by
examining variation in mass during the moult
period in both juveniles and adults. This is the
first time that mass change during moult has
been described in this species. In addition, the
sequence of PJ moult is described, since little
has been published on this and existing studies
have differed in their conclusions. 14
METHODS
This study is based chiefly on the analysis of
data collected by members of the British Trust
for Ornithology Ringing Scheme and contributed to the BTO Moult Enquiry between 1973
and 1989. 5,17,18 Data were recorded in a standardized way on printed recording cards supplied to ringers by the BTO. 18 ' 5 Cards were
only completed for birds that were undergoing
moult so that no biometric data (e.g. mass) are
available from the moult period for birds that
were not moulting.
Prior to 1971, biometrics other than moult
details were not requested by the Moult Enquiry, so that no cards returned before that
year could be used here (see below for biometrics). Only data referring to the primary,
secondary and tertial remiges, rectrices, greater
(secondary) coverts and alula have been
147
used, 18 although the actual set of tracts used
must depend upon whether the analysis refers
to PJ or PN moult. The state of replacement of
each feather was scored by the observer on a 6point scale where 0 represents an old feather
and 5 a fully grown new feather. The state of
moult in any tract was summarized by summing these scores across all feathers in the
tract. Hence, a maximum score of 50 (5 x 10
feathers) can be attained for the primaries of
one wing, 45 for the greater coverts, 60 for the
whole tail and so on.5'14'19 During PJ moult, the
greater coverts are generally dropped and
replaced simultaneously as a block such that
any retained juvenile coverts are the outermost.
Therefore, in these analyses I have used the
number of greater coverts replaced in addition
to the combined moult score of feathers in the
greater covert tract. Observation dates have
been taken as the number of days after 1 June.
Wing-length was measured by the maximum chord method. 9 Recent studies (to be
published elsewhere) suggest that this measurement is highly repeatable between British
ringers. In the analysis of PJ moult this has
been taken as a measure of body size. However, as the wing point is formed by the
seventh primary in the Great Tit, wing-lengths
of birds that had already dropped this feather
(in practice, those with primary scores greater
than 30) could not be used in the equivalent
analysis of PN moult. This might introduce a
bias in the analysis of size-corrected mass in
adults since later-moulting birds must be
excluded from any analysis. Mass was recorded to 0.1 g with a x 50 g 'Pesola' spring
balance, and the time of weighing was
recorded as the last full hour passed.
The data on PJ moult (181 cards) span the
full period 1973-89. The present analysis of PN
moult, however, is based on 179 cards (106
male, 73 female) from 1975. As the data are not
complete on all cards, sample sizes varied
between analyses, but this is indicated where
appropriate. No significant differences between years were found in wing-length, mass
or the relationship between moult scores, hence
between-year effects have been ignored
throughout in the analysis of PJ moult. Statistical analyses were carried out by the use of
the correlation, stepwise multiple regression
and general linear modelling (GLM) procedures available on Minitab Release 8. 20 Because
of the problems inherent in the regression of
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148 A .G. Gosler
moult scores on date °,13,2122 and hence the
unreliability of residual moult scores derived
from such regression, to control for date, moult
scores were expressed as deviations from the
mean moult score for the date on which they
were recorded (by day or, in the analysis of
female PN moult where sample size was usually too small on any one day, by the week).
This avoids the need to fit a pre-determined ad
hoc model. A negative value indicates a bird
with a lower than average moult score for the
day (or week) on which it was captured.
Finally, new data on the relationship between fat reserves and mass are presented to
allow some interpretation of mass changes
during moult. These data were collected during routine trapping (in winter, October-March
inclusive and when feeding well-grown young,
May-June) as part of the Edward Grey
Institute's long term population study of
the Great Tit in Wytham Woods, near
Oxford. 12 ' 23 ' 24 Since May 1982, I have recorded
a score of the visible tracheal pit fat of each bird
as follows: 0: no visible fat; 1: a trace of fat; 2:
base of tracheal pit obscured by fat, but less
than half full; 3: pit at least half full but still
below the level of the clavicle; 4: fat filling the
tracheal pit but not bulging; 5: fat filling
tracheal pit, bulging and overlying pectoral
muscle. Additional half score values (e.g. 2.5,
3.5 etc.) were also interpolated. This system
was first described by McCabe 25 and is now
widely used to give an indication of total body
fat. 26-28 Mass was recorded as above.
tail score of 24 (out of 60) was reached
(Fig. 1).
Moult of the alula is undertaken less often
than that of the greater coverts. When it does
occur, however, its replacement is co-ordinated
with the greater covert moult. Of 136 birds in
which a score for the alula was recorded, 56
(41%) were replacing part or all of the alula.
This is greater than the 8% described in Hertfordshire, 13 but is the same as that described
for the Belgian population. 14 Of the 56 birds
showing alula moult, only 2 were moulting
fewer than all the greater coverts (significantly
fewer than random x 2 = 25.7, df = 1, P <0.001).
Changes in mass through the moult period
Mass correlated significantly with wing length
(r = 0.585, df = 179, P <0.001), indicating that
the latter may be used as an indicator of size,
and time of capture (r = 0.324, df = 179,
P <0.001) indicating that birds were heavier
later in the day. This increase in mass with time
of day has been noted many times in Great Tit
50
NM 40* 40 00
30 -
RESULTS
Post-juvenile moult
Sequence of moult
Flegg & Cox 13 stated that PI moult started with
the dropping of the central pair of rectrices.
Dhondt 14 disputed this, saying that tail moult
did not commence until some 10 days into
moult which usually started with the loss of
some feathers on the breast and then humerus
(secondary coverts) and back. Although the
analysis of body moult is not included here the
present data support Dhondt's argument, since
greater covert moult is under way by the start
of tail moult, and of those birds in which
greater covert replacement occurred only 10%
had failed to complete this by the time that a
30 40
50 60 70
Tail moult score
Figure 1. The relationship between greater covert
and tail moult scores for 145 juvenile Great Tits. The
broken vertical line marks a tail moult score of 24
(completed moult scores 60) by which time 90% have
already completed replacement of the greater
coverts.
Condition and moult of Great Tits
samples measured in winter and while breeding. Gosler 16 showed that this component of
variation in mass could be explained largely in
terms of daily changes in the amount of stored
fat. This is the first time that the same daily
mass cycle has been described for Great Tits
250
240
230
220
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210
200
190
180
170
160
150
40 50 60 70 80 90 100 110 120 130
Day (June 1=1)
5.00
4.00
3.00
149
undergoing moult. The high masses observed
in some individuals (Fig. 2A) can best be
explained in terms of considerable fat storage
(see below and Fig.5). In a stepwise multiple
regression in which wing-length and time were
entered as predictors they explained 40.9% of
the variance in mass.
Overall, no significant change in mass was
recorded in relation to date through the moult
period. If the variance in mass due to winglength and time of day was first removed by
regression, the date explained 2.8% of the
residual variance in mass and this was statistically significant (r = 0.168, df = 178, P =
0.024) suggesting a weak improvement in body
condition through the moult. In contrast to this
general trend (and even to birds replacing just
2, 3 or 4 coverts), the 26 birds in which no
greater covert moult was recorded showed a
strongly significant decline in mass with date
(r = — 0.539, df = 24, P = 0.005, Fig. 2A) although when corrected for wing-length and
time of day, their mass showed no significant
relationship with date. These results, however,
may have resulted from sampling bias. A
sample of birds showing no greater covert
loss, trapped early in the season, may include
some individuals that will moult but which
have not yet started. Since females are
significantly more likely than males to retain
juvenile greater coverts, 14• 16 it is possible that a
juvenile sample from early in the season may
include some males, while later in the season
most (if not all) birds lacking greater covert
moult would be female. Since males are
significantly larger and heavier than females
(usually some 4% larger 12 ) such a bias could
lead to the apparent change in mass (or lack of
2.00
0.00
-1.00
-2.00
-3.00
60
70
80 90 100 110 120 130
Day (June 1=1)
Figure 2. Mass change during moult in juvenile Great
Tits. (A) The relationship between mass and date. A ,
birds undergoing greater covert moult. (- - -), regression of mass on date: Y = 182+0.124X, F = 3.59, df =
1,153, n.s.•, birds which did not replace greater
coverts. (—), significant decline in mass with date
in these birds: Y = 226 — 0.414X, F = 9.69, df = 1,24, P=
0.005. (B) The relationship between mass standardised for wing-length and time of day with date.
Only birds trapped after day 60 were used to remove
sex bias (see text). Symbols are as (A). Regression
statistics are for moulting birds: Y = — 1.18+0.0143X,
F = 5.98, df = 1,141, P = 0.016; for non-moulting birds:
Y = 1.95-0.0235X, F = 5.21, df = 1,23, P = 0.032).
150
A .G. Gosler
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change in the residual) observed here, although
overall no significant correlation was found
between wing-length and the number of
juvenile greater coverts moulted. To test for
such a bias, I looked at the correlation between
residual mass (after correction for wing and
time as above) and date in only those birds
trapped after day 60 (30 July) by which time
moult should have been apparent in all those
juveniles that would moult. This residual mass
still declined significantly with date in the
non-moulting group and differed significantly from the birds that underwent moult
(interaction: F = 5.75, df = 1,143, P = 0.018)
which showed a more strongly significant
increase in mass with date (see Fig. 2B). This
suggests that those juveniles that failed to
moult any greater coverts may have done so
because they were, in some way, in poorer
3.0
condition and even declined in condition
0
during the moult period.
5.0
4.0
-
3.0
-
2.0 -
o^
-1.0
-2.0 v
i
^
1
2
3
4
6
57
8
No greater coverts replaced
Changes in mass in relation to moult score
Once initiated, moult of any tract proceeds in a
more or less determinate fashion so that moult
scores are strongly correlated with date, therefore the analysis of mass in relation to moult
was carried out on the date-standardized
moult scores. Mass also was expressed as a
residual, as described above. This residual
mass was entered as the dependent variable
in a stepwise multiple regression with the standardized moult scores for greater coverts, tail
and alula as predictors. Only the standardized
tail score correlated significantly with the
residual variance in mass (r = 0.217, df = 119,
P = 0.017). There is, therefore, some evidence
that tits that were more advanced in tail moult
than average for a given date were relatively
heavy for their size.
Figure 3 shows the relationship between residual mass and the number of greater coverts
being replaced. Birds undergoing replacement
of a few greater coverts were significantly
lighter than those undergoing either no moult
or a full moult of the greater covert tract. A
quadratic regression fitted to these data
explained 8.5% of the variance in residual
mass and was strongly significant (F = 7.23, df
= 2,156, P = 0.001). This was still true if observations made before day 60 (F = 8.32, df =
2,144, P <0.001, r 2 = 10.4%), or even 75 (F = 3.99,
Figure 3. The relationship between residual mass
(mass corrected for body size and time of day by
regression) and greater covert moult in 160 juvenile
Great Tits. Note that birds undergoing a partial
moult of the greater coverts were significantly lighter
than those that completed moult of the tract (Y =
—0.182-0.391X+0.047X2 , F = 7.23, df = 2,156, P =
0.001). They were also lighter on average than birds
which moulted none of the coverts, suggesting that
the latter group benefited somewhat from the
strategy, but see text and Fig. 2.
df = 2,106, P = 0.021, r 2 = 7%) were omitted to
eliminate possible biases described earlier.
Post-nuptial moult
Unlike the data on PJ moult all of the cards
used for PN moult included the bird's sex,
therefore analyses were carried out separately
for each sex. However, the fact that wing
length is unreliable as a measure of size in
more than half the birds sampled (102 birds
had primary moult scores greater than 30)
means that mass cannot be controlled for variation in body size within sexes. Although this
may be a serious drawback, there is no a priori
reason to expect sampling to have been biased
in respect of body size. As the sequence of
feather replacement observed in the present
data set agreed well with that already
described elsewhere, 5 ' 12-15 it will not be
described further here.
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Condition and moult of Great Tits
Mass change through the moult period
240
Male mass increased significantly through the
moult (r = 0.331, df = 96, P <0.001, Fig. 4).
Unlike juveniles, adult male mass was not
significantly correlated with time of day.
Female mass, in contrast, shows a quadratic
relationship to date, with mass dipping before
recovering again (F = 3.5, df = 2,65, P = 0.035,
Fig. 4). Like the males, mass was not significantly correlated with time of day. The high
masses observed in both sexes (in females
especially near the start of moult) suggest that
birds maintained high fat levels through the
moult. However, it is not possible to distinguish mass changes due to changes in fat
reserves from those due to changes in muscle
condition in these data. 28 ' 16
Some information relating to this question is
provided by the relationship between body
mass and fat score recorded at Wytham
between May 1982 and December 1993. Figure
5A shows this relationship for 2246 Great Tits
trapped during the winter; Fig. 5B shows the
same relationship for 1907 trapped during the
breeding season. In each sex and season the
relationship was essentially linear. In winter,
males were consistently about 1 g heavier than
females for a given fat score. This sex difference
was reduced to less than 0.5 g when breeding.
Although, within a season (breeding or
winter), the regression slopes did not differ
significantly between sexes (Fig. 5), for each sex
the slope was significantly greater in winter
than when breeding (males: F = 24.07, df =
1,2166, P = 0.0000; females: F = 4.79, df =
1,1981, P = 0.029). While one cannot standardize the masses from the BTO moult
dataset for variance due to body size or fat
scores, the Wytham data indicate that, irrespective of seasonal differences in the details of
the relationship, any Great Tit under 17.5 g is
likely to have very reduced fat reserves while
any birds over 20 g are likely to be carrying a
substantial fat store. Such variation is too great
to be explained by changes in muscle mass
alone. 16 Hence, at least a part of the mass
variation observed in these moulting birds
must be due to considerable variation in fat
reserves.
230
Mass change in relation to moult score
Male mass correlated significantly with all of
151
220
210
200
190
180
170
160
150
140
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Day (June 1st = I )
Figure 4. Mass change in relation to date in adult
Great Tits in 1975. Males (0) showed a significant
increase in mass through the moult period (Y =
189+0.149X, F = 11.81, df = 1,96 P = 0.001) while
females (A) showed a significant decline in mass
after the start of moult followed by a recovery after
the end of July (Y = 196 — 0.460X+0.004X 2 , F = 3.5, df
= 2,65, P = 0.035).
the moult score data (primaries: r = 0.326, P
<0.001, secondaries: r = 0.226, P = 0.028,
tertials: r = 0.211, P = 0.041, tail: r = 0.294 P =
0.006, df = 96 throughout). However, because
of the strong correlations which inevitably exist
between moult scores of the different tracts and
date, these results are confounded by date. No
significant correlation was found between
mass and any of these moult scores after they
were transformed to date-standardized scores
as described above. Nor was there any significant relationship between these scores and
residual mass (controlling wing-length as
above) in those males with primary moult
scores of less than 30 (n = 18). In females, mass
was not correlated significantly with any of the
moult scores. However, a strongly significant
quadratic relationship existed between mass
and the standardised primary moult score (Fig.
6). Females that were advanced or retarded in
their primary moult relative to the week in
which they were measured were significantly
lighter in mass than birds that showed the
average state of moult. No such relationship
152
A .G. Gosler
was found between mass and standardized
moult scores of any other tract or in males.
DISCUSSION
Of the many published studies of passerine
moult, remarkably few comment on other
205
o Male
• Female
200
195
190
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185
170
165
160
2
155
5A
150
0
2
4
3
5
6
Fat score class
2
3
Fat score class
4
5
changes in body condition suffered by the
birds at that time, and even fewer consider
which tissues might be involved in such
changes. Across different species, both
'
increases 19 3o,31 and decreases followed by
2 ' 29 ' 32 ' 33
in mass have been noted in
recovery
adults undergoing primary moult. Where
reported, loss of mass during moult has been
attributed to negative protein balance rather
than a loss of fat 4'29'33 The increase in mass in
adult Bullfinches Pyrhula pyrhula during moult
is attributable largely to an increase in protein
mass, but also to increased fat and water. 34 The
importance of protein reserves to the moult
cycle is also implied by observations of
arrested moult. For example, juvenile Starlings
Sturnus vulgaris (which normally have a
complete PJ moult) may arrest primary moult
if the food supply is poor, and this is associated
especially with low muscle condition. This
occurs less often among adults. 35
The present study also found evidence for
the involvement of protein stress in determining the character of moult in the Great Tit
(especially in juveniles). As in the Starling, the
degree to which individuals were affected
depended on their social status (age and sex)
since adult males are usually dominant over
females and, at the time of moult, both are
dominant over juveniles. 12 While juveniles
showed both variation in the extent of moult
and relationships between this and mass, and
adult females showed some effect of moult on
mass, adult males appeared comparatively
immune to the demands of moult.
Juveniles which moulted a few greater
coverts were significantly lighter than those
Figure 5. The relationship between body mass and
tracheal pit fat score class in Great Tits trapped at
Wytham, Oxford. (A) Winter (October—March inclusive). (B) When feeding large young (May June). All
data were collected by the author. Separate points are
shown for each sex but each point indicates the
mean+1 se mass for the fat score class concerned;
sample sizes are also indicated. Regression lines are
indicated for each sex (unbroken: male, broken:
female) in each season. All were significant at P
<0.0001. Winter: male mass = 181.4+4.17 fat, F =
365.4, df = 1,1251, female mass = 170.1+3.67 fat, F =
236.3, df = 1,992; breeding: male mass = 187.5+2.16
fat, F = 41.27, df = 1,915, female mass = 183.4+2.79
fat, F = 72.1, df = 1,989. See text for further
explanation.
Condition and moult of Great Tits 153
220
210
200
p 190
ISO
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170
160
15 0
—15
—10
—5
0
5
10
15
20
Standardized primary score
Figure 6. Change in mass in relation to primary
moult score corrected for date (week) in adult female
Great Tits. Birds moulting close to the mean date for
the population were significantly heavier than those
that were either advanced or retarded in timing of
their moult (Y = 188 — 0.087X — 0.066X 2 , F = 4.65, df =
2,65, P = 0.013). These results suggest that the timing
of moult is critical for these birds. No such relationship was observed in males.
that completed this moult. As this observation
still held after removing both a size-dependent
(wing-length) and the time-dependent component of mass which reflects daily changes (but
not seasonal changes) in fat reserves, 16 this
suggests that these birds were probably lighter
partly because they had poorer protein
reserves. It is interesting that birds which
moulted no greater coverts had a slightly
greater residual mass than those that moulted
just a few coverts, suggesting that they may
have benefited a little from their lack of moult.
However, that their residual mass declined
steadily through the moult period while the
moulting birds showed a significant increase
may indicate that they were, nevertheless,
suffering greater resource stress since a
reduction in muscle mass at this time is unlikely to reflect an adaptive mass reduction. 36
Juveniles that were advanced in their tail
moult were also relatively heavy and again,
the fact that the analysis was conducted after
the prior removal of the time-dependent
component of mass suggests that protein
mass is probably involved. The presence of a
relationship between mass and the number of
greater coverts replaced in juveniles strongly
supports my earlier contention that the
retention of unmoulted juvenile greater coverts is a badge of poor condition during
moult. 16 • 12
I have shown elsewhere that adult male
mass may increase during the nestling stage of
their brood due to an increase in pectoral
muscle condition although, compared with
earlier breeders, later breeding males tended
to have poorer muscle scores. These observations contrasted with the mass and muscle
scores of their mates which declined through
the breeding attempt. 16 The present data suggest that these patterns of mass change with
time are continued into the moult, with males
increasing, and females continuing to decline
in mass before finally recovering their losses
after the end of July (Fig. 4). It is not clear why
these differences exist between sexes during
the moult, as it was not clear why they occurred during the breeding season, although they
suggest that, at least during the early stages,
moult may be more stressful for females than
for males. Some further support for this argument is provided by the observation that the
timing of primary moult appears to have a
critical effect on the condition of females (Fig.
6) while it is apparently irrelevent to males.
A.A. Dhondt (pers. comm.) suggests that the
difference between sexes may also relate to the
overlap between moult and the onset of autumn territorial behaviour in males. 15'31
This study indicates the great potential for
biometric studies of data collected routinely by
members of the British Ringing Scheme. While
detailed observations of fat and muscle condition of birds undergoing moult are still required to identify more precisely which tissues
are involved in mass change, the Moult
Enquiry data clearly represent a valuable resource for initial analysis in this area. For
example, in the analysis of PN moult I have
considered a single year. Equivalent data are
available for the whole period 1973-89 so that
the analysis of between-year variation in moult
and condition could be considered in future in
relation to differences in environmental conditions, as might comparative studies, for example, of migratory and resident species.
154 A .G. Gosler
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ACKNOWLEDGEMENTS
I am grateful to the British Trust for Ornithology for permission to analyse these data and to
Drs S.R. Baillie and P.C. Lack for their time and
assistance in making the data available to me. I
thank Professor A.A. Dhondt, Professor C.M.
Perrins and Dr S.R. Baillie for their constructive
comments on an earlier draft of the manuscript. Finally, I should like to thank the many
ringers that contributed data to the Moult
Enquiry since it too often goes unrecognized
that such analyses are possible only after
thousands of hours of fieldwork by others.
15.
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(MS received 1 September 1993; revised MS accepted 17 December 1993)