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Age and sex differences in molt of the Montagu's Harrier Circus pygargus



Age and sex differences in the timing and pattern of feather molt in Montagu's harrier (Circuspygargus) was studied in 184 museum specimens and live birds collected throughout their breeding and wintering ranges. Molt in adults lasted 6-8 mo, starting in May-June in the breeding areas and finishing in January-February on the wintering grounds. Apparently, Montagu's harriers do not suspend molt during migration. Molt of primaries started earlier in adult females than in adult males, which was probably related to their different roles in feeding nestlings during the breeding season, but both sexes finished molt at about the same time. Yearlings started body molt on the wintering grounds and there was a considerable discrepancy in the extent of body molt of yearlings between data obtained from skins and those from live birds. Yearlings attained plumage similar to that of adults after the first complete molt. Timing of flight feather molt in yearlings and second-yr males was similar to that of adults even though they were not engaged in reproduction. Nevertheless, they finished molting earlier than older birds, but this difference was not significant.
J. Raptor Res. 30(4):224-233
¸ 1996 The Raptor Research Foundation, Inc.
B.E. APd•O¾O •'qt)J.R. KtNG
EGI, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, United Kingdom
ABSTRACT.--Age and sex differences in the timing and pattern of feather molt in Montagu's harrier
(Circus pygargus) was studied in 184 museum specimens and live birds collected throughout their breed-
ing and wintering ranges. Molt in adults lasted 6-8 mo, starting in May-June in the breeding areas and
finishing in January-February on the wintering grounds. Apparently, Montagu's harriers do not suspend
molt during migration. Molt of primaries started earlier in adult females than in adult males, which was
probably related to their different roles in feeding nestlings during the breeding season, but both sexes
finished molt at about the same time. Yearlings started body molt on the wintering grounds and there
was a considerable discrepancy in the extent of body molt of yearlings between data obtained from skins
and those from live birds. Yearlings attained plumage similar to that of adults after the first complete
molt. Timing of flight feather molt in yearlings and second-yr males was similar to that of adults even
though they were not engaged in reproduction. Nevertheless, they finished molting earlier than older
birds, but this difference was not significant.
K•Y WOP, DS: Age and sex differences; Circus pygargus; Montagu's harri• molt phenology; molt rate.
Diferencias en la muda del aguilucho cerizo con respecto a la edad y al sexo
RESUM}•2q.--Se ha esmdiado el patr6n de muda en 184 aguiluchos cenizos (Circus pygargus) (fundamen-
talmente ejemplares en museos, y algunos individuos vivos), procedentes de todo su rango de distri-
buci6n, y se analizan las diferencias en fenologia de muda debidas a la edad y al sexo. La muda en los
adultos dura 6-8 meses, comenzando en mayo-junio en las zonas de crfa, y terminando en enero-
febrero en los cuarteles de invernada. Los datos disponibles sugieren que los aguiluchos cenizos no
suspenden la muda durante la migracitn. La muda de las primarias comienza antes en las hembras
adultas queen los machos adultos, lo que probablemente estf relacionado con la diferente contribucitn
de los sexos a la alimentacitn de los pollos, pero ambos sexos terminan mrs o menos al mismo tiempo.
Los individuos de primer ado comienzan la muda corporal en las fireas de invernada. Encontramos
divergencias importantes en el grado de muda corporal de los individuos de primer ado, entre los datos
procedentes de ejemplares de museo y datos procedentes de observaciones de campo. Se discuten las
posibles razones de esta divergencia. A1 final de la primera muda completa, los individuos de primer
ado adquieren un plumaje similar al de los adultos. E1 comienzo de la muda en los individuos de primer
ado yen los machos de segundo ado es similar a la de los adultos, aunque estos grupos de edad no se
reproducen. Sin embargo, parecen terminar la muda antes que los individuos de mrs edad, aunque las
diferencias no son significativas.
[Traduccitn de author]
Feather molt, reproduction, and migration are
three of the more energetically cosfly components
of avian life history. Molt is usually timed to mini-
mize peaks in energy demands during either re-
production or migration, and the duration and ex-
tent of molt is constrained by the energy invested
in either of the latter two factors (Pietianen et al.
1984). If there are differences between the sexes
Present address: CNRS/CEBC, Beauvoir Sur Niort,
F79360 France.
in their relative contributions towards breeding,
they may be reflected in sexual differences in molt.
Likewise, age-related differences in molt may be
expected between individuals that have not yet en-
tered the breeding population, and this should be
especially prominent in long-lived species with de-
layed sexual maturation.
Relatively little is known about molt in diurnal
birds of prey, when compared with that of other
bird orders. Large raptors such as eagles and vul-
tures do not undergo a complete molt every year,
but have several foci in the primaries where pri-
maries are molted sequentially (Stresemann and
Stresemann 1966, Houston 1975, Edelstam 1984,
Bortolotti and Honeyman 1985). In so doing, they
avoid excessive raggedness in the wings, which
would impair the lift necessary for flight. In con-
trast, small accipitrids such as hawks usually have
one complete molt a year, replacing the primaries
from the innermost to the outermost feathers, and
the secondaries from three foci (Miller 1941,
Piechocki 1955, Stresemann and Stresemann
The Montagu's harrier (Circus pygargus) breeds
throughout Europe (wintering in Africa) and the
Asian/Russian steppes (in the Indian subconti-
nent). The molt of Montagu's harrier has never
been previously described in detail. Here, we use
museum skins (collected throughout its range)
and field data from Madrid (Spain) and S•n•gal
(Africa) to describe the patterns of molt in this
species, the differences in timing and duration of
molt between males and females, and the acquisi-
tion of adult body and flight feathers by yearlings.
Sources of Data. Data were recorded from 166 specimens
from the Natural History Museum, Tring, UK (28 female
yearlings, 31 male yearlings, 36 adult females, 71 adult
males), 10 skins from the Museum of Natural History,
Madrid (three adult females, seven adult males), three
skins of adult males from the Collection Hagi Botti, OR-
STOM Station, M'bour, S•n•gal, and five breeding birds
trapped alive in Madrid (two adult males, three adult fe-
Field observations of yearlings in breeding areas in
Spain were also used for comparison. All yearlings could
be identified by the presence of uniformly dark second-
aries characteristic of juvenile Montagu's harriers. Males
change their iris color from brown to uniformly pale yel-
low before they are 3-4 mo old. Females do not attain
yellow irides until they are 3-4 yr old, although they
might have various degrees of yellow spotting before that.
We used eye color to sex birds in juvenile plumage when
Molt Scores. Molt of individual remiges and rectrices
was scored from 0-5, following Ginn and Melville (1983),
where 0 represents an old feather, and 5 a fully-grown,
new feather. The intermediate values (1-4) represent
progressions of feather growth. Throughout this study,
primaries are numbered from 1 (the innermost) to 10
(the outermost), and the secondaries, including the ter-
tials, are numbered from 1-13 (1 being the outermost).
Molt for the vestigial outermost primary (Pll) was not
recorded. A Primary Molt Score (PMS) was calculated for
each bird as the sum of the molt scores of the 10 pri-
maries in one wing, and PMS ranged from 0-50 (Ginn
and Melville 1983). When the PMS differed between
wings of the same bird, an average of the two scores was
used in analyses. Similarly, we calculated a Secondary
Molt Score (SMS, range 0-65), and a Tail Molt Score
(TMS, range 0-30) for each bird.
We determined the order in which secondary feathers
or tail feathers were molted by calculating the accumu-
lated scores for each numbered feather and ranking each
according to its accumulated score. Variations between
wings in the order of secondary molt were recorded, but
the total score was not significantly different between
wings (Mann-Whitney; W•20.•20 = 14475, P = 0.97). Molt
of tail feathers was nearly always symmetrical. Thus, we
combined values from both sides for each individual. Pri-
maries were molted sequentially, so we did not calculate
accumulated scores for them.
The extent of body molt in yearlings was categorized
for each body tract (head, mantle, coverts, breast, and
belly) on a four-point scale: 0 (no molt), I (very few new
feathers present), 2 (mixed new and old feathers), and
3 (molt finished or nearly finished).
Raggedness Scores. An index of raggedness (Haukioja
1971) was calculated for each feather tract (primaries,
secondaries, and tail), using the sum of values in each
feather of the tract. Each feather was scored with values
ranging from 0-4 according to their molt score, where 0
indicated a feather of full length (whether old or new),
4 indicates a feather at the first stage of growth (molt
score 1), 3 a feather with molt score 2, 2 a feather with
molt score 3, and 1 a feather with molt score 4. For each
individual, the sum of raggedness scores from both sides
(left and right) of each tract was used as an overall index
of raggedness.
Data Analysis. Differences in the molt scores between
males and females and age classes were analyzed with
nonparametric statistics (Mann-Whitney test). Variations
in the timing of secondary molt related to primary molt
were analyzed with linear, parametric analysis.
Since several of the skins were dated only to month
and not to day of capture, we analyzed data by month
This helped to account for latitudinal differences in phe-
nology between birds coming from different regions be-
cause breeding phenology in the Montagu's harrier var-
ies on average less than a month between southern and
northern Europe (Arroyo 1995). Initially, data for birds
from Europe (west of Ural mountains) and data for birds
that had been collected in India (presumed to breed in
the central Asian steppes) were analyzed separately. This
avoided potential differences in phenology between both
areas, since no data on timing of breeding for the Asian
populations of Montagu's harrier were available. No sig-
nificant differences in molt scores were found between
birds from both regions for equivalent months (Mann-
Whitney; W21.10 = 319.0, P = 0.47), so they were com-
bined for subsequent analyses.
Molt in Adults. Adult males and females started
molting on the breeding grounds in May-June,
and finished molting on the wintering grounds in
January-February (Fig. 1). Primary molt was com-
pleted in 6-8 mo. Females started molting primar-
ies before males, but both sexes finished at about
226 B. ARroYo AND J. KING VOL. 30, NO. 4
ß female
40 [] male
a. 10
5 144 20
2 5
8 4 3 5 8
4 8 4 3
e 60
o 50
'• 40
'- 20
u• 10
5 144 20
2 4 9 5
2 5 4 8 4 3 5 8
5 14 4 20
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar
Figure 1. Molt phenology of flight feathers in adult Montagu's harriers. Histograms represent the mean (_SE)
primary, secondary and tail molt scores throughout the year for both males and females. Sample size (number of
individuals) is given above the histograms.
the same time (Fig. 1). The mean PMS for the
breeding months (April-July) was significantly low-
er in males than in females (Mann-Whitney; W,,4•
= 816.5, P = 0.0009), but the means were similar
in November-February (Mann-Whitney; W]],, =
185.5, P = 0.88). Accordingly, the slope of the re-
gression of PMS vs month was higher for males
(5.36) than for females (5.01).
Both males and females started molting second-
aries in August-September, and finished at the
same time that primary molt finished (Fig. 1).
Males started molting secondaries at a significantly
earlier stage of primary molt than females did
(GLM SMS = PMS + Sex; F],i• 7 = 1956.5, P =
0.0001 for PMS; F1,117 = 12.60, P = 0.001 for Sex;
Fig. 2). The pattern of secondary molt was highly
O 50
o 10
Males øø•øø
o oJoO
•o oo
•. rt . . , .
10 20 30 40
cma oo .9o
Males o•0
0 0 0
o o
i ß i , i ß i ß i
0 10 20 30 40 50
o 50
-. 40
13 20
o 10
Figure 2.
0 10 20 30 40 50
Primary Molt Scores
Onset of secondary molt in relation to molt
of primary feathers: correlation between secondary molt
scores and primary molt scores in adult male (N = 79)
and female (N = 40) Montagu's harriers.
30 Females
o 20 s s
_ 10
0 -- .......
i ß i
0 10 20 30 40 50
Primary Molt Scores
Figure 3. Onset of tail molt in relation to molt of pri-
mary feathers: correlation between tail molt scores and
primary molt scores in adult male (N = 79) and female
(N = 40) Montagu's harriers.
variable among individuals in relation to the order
in which feathers were replaced. The relative order
of the accumulated molt scores of each feather (all
birds combined) was as follows: for females (N =
40): S1 (accumulated score = 180), S5/Sll (165),
S2/S13 (160), S4 (155), S12 (150), S10 (146), S9
(130), S3 (123), S6 (120), S7 (115), S8 (108); for
males (N= 79): S1 (319), S5/S12 (315), Sll (308),
S13 (307), S2 (302), S10 (295), S9 (291), S4 (272),
S3 (250), S6 (242), S7 (220), S8 (218). These re-
suits indicated that two molt foci existed at S1 and
S5, and molt at both foci started nearly simulta-
neously. Another molt center was located in the
tertials (Sl1-13), which were molted at the time,
or shortly after the outer secondaries had started
to moltß Males seemed to molt the tertials as soon
as molt in the outer secondaries had started, while
females seemed to molt them more slowly. The or-
der in which tertials were molted did not seem to
follow a fixed pattern. In general, molt was ascen-
dant from S1 and S11 (the outermost tertial), and
appeared to be centrifugal from S5. Data from D.
Forsman (in litt.) suggests that molt from S5 is as-
cendant, given that he found S4 to be one of the
last secondaries to be molted. However, we found
S4 to be one of the earlier molted secondaries in
females and it was not the last secondary to be
molted by males.
Molt of tail feathers started about June and fin-
ished about January (Fig. 1). As with secondaries,
males started molting tail feathers at a significandy
earlier stage of primary molt than did females
(GLM TMS = PMS + Sex; F1,119 = 1268.6, P =
0.0001 for PMS; F1,119 = 8.15, P = 0.005 for Sex;
Fig. 3). The order in which feathers were molted
was, as with secondaries, variable but a general pat-
tern was clear from the accumulated molt scores.
Molt of tail feathers in females usually started with
228 B. ARROYO AND J. KING VOL. 30, NO. 4
the central pair (accumulated molt score of 175),
followed by the fourth (166), sixth (161), third
(156), fifth (148) and, finally, the second (132) (N
= 40). Males had a similar molt pattern, but the
sixth pair was the first one to be replaced. The
relative order was thus T6 (accumulated score
332), T1 (328), T4 (316), T3 (314), T5 (308) and
T2 (294). As in the primaries and secondaries, tail
molt seemed to occur more rapidly in males than
in females, and the variation in values of accumu-
lated TMS among feathers was lower in males than
females. The difference between the highest and
the lowest accumulated score was 12% in males
and 30% in females which suggested that males
molted a greater number of tail feathers at any one
time than females.
Primary feathers were most ragged between No-
vember-January among females and in June
among males. Degree of raggedness in secondaries
and tail feathers was generally low throughout the
molt cycle in both sexes (Fig. 4). On average, males
had higher values of raggedness for all flight feath-
er tracts than females, but differences between the
sexes were not statistically significant (Mann-Whit-
ney; W40.80 = 2598.5, 2365.0 and 2321.5, P = 0.82,
0.65 and 0.23 for primaries, secondaries, and tail
feathers, respectively). The accumulated score of
raggedness for all tracts combined peaked for
males at values ranging from 23-32 in September-
October, while values for females peaked at 25 in
November. However, no data existed for females
for September and October.
Molt in Yearlings. We recorded no body molt in
birds collected in October or November, although
sample size was small (N = 3). A small percentage
of birds of both sexes started body molt in Decem-
ber, usually in the region of the crop, and males
also on the rump (Table 1). Central tail feathers
were also replaced in about 25% of birds of both
sexes. However, the extent of molt completed be-
fore spring migration was limited in both sexes.
The proportion of yearlings with some degree of
body molt increased throughout the spring and
summer. Although a few yearlings had not started
molting any body feather tracts by July or August,
many started but had not completed molt in most
tracts by that time. Yearling males that attempted
to breed in Madrid were at the same stage of molt
as those that did not breed (B. Arroyo unpubl.
data). Furthermore, a yearling male that bred in
1994 had molted only the central tail feathers and
part of the feathers in the region of the crop when
it arrived on the breeding grounds in Spain. Body
molt of yearlings finished in October-November.
Males retained some juvenile feathers in the ear
coverts and in the nape, which allowed aging of
second-year individuals in the hand.
There was a considerable discrepancy between
the extent of body molt in yearlings recorded dur-
ing field observations and from museum speci-
mens. Detailed plumage observations of 30 year-
lings (21 males and nine females) were made in
breeding areas in Spain. Yearling males showed
more advanced body molt than did museum spec-
imens collected at the same time of year and for
birds observed in the field, the head, breast and
belly tracts, respectively, showed evidence of molt
in 91%, 62% and 62% of individuals, compared to
43%, 29% and 14% of museum specimens (Table
1). The extent of molt recorded was also greater
for individuals observed in the field. In contrast,
yearling females observed in the field showed less
advanced body molt than museum specimens.
Only 33% of the females observed in the field
showed molting in the crop, breast, and belly, com-
pared to over 63% of the museum specimens (Ta-
ble 1). This discrepancy might be an artifact of the
relatively small number of specimens of yearlings,
but it may well be a consequence of the relative
visibility of newly-molted, grey plumage in yearling
males, leading to overestimation of the extent of
molt when recorded in the field. In the latter case,
the similarity between old and new plumage in fe-
males would lead to an underestimation of the ex-
tent of body molt in that sex. Furthermore, year-
ling males showing little or no molt may have been
misidentified as females in the field, or more likely,
left unsexed due to apparently conflicting charac-
ters such as small size but no visible male plumage
Primary molt in yearlings apparently started at
the same time as that of adults (Fig. 5) and molt
scores were similar between both age groups in
April-July (Mann-Whitney; W90,12 = 194.5, P = 0.90
for females; W4•.•0 = 284.0, P = 0.64 for males).
The completion of primary molt in October-Feb-
ruary also appeared to be similar for yearlings and
adults, however sample sizes were very small for the
yearling group (Mann-Whitney; W22,4 = 57.5, P =
0.83 for males; insufficient data for females to al-
low statistical analysis).
Molt of secondary and tail feathers followed the
same pattern, and occurred at the same time in
relation to primary molt, as in adults. Juvenile
2 2
ß females
[] males
2 3
5 14 4 20 2 4
5 8
9 5 0 0 0 1 I 4 5 8
20 Tail
2 1
5 4 8 8
5 14 4 9 0 0 0 I•..•==T• 4 3 5,_•__,
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar
Figure 4. Degree of raggedness in flight feather tracts throughout the year in adult Montagu's harriers of both
sexes. Histograms represent the mean (-SE) raggedness scores for primaries, secondaries and tail feathers. Sample
s•ze (number of individuals) is given above the histograms.
feathers were replaced by feathers similar to those
of adults, only slightly darker. Thus, after the first
complete molt, yearlings were indistinguishable
from older birds in the field as confirmed by ob-
servations of dark-looking plumage in known-age
wing-tagged individuals ->3 years old.
Molt in Second Year Males. Fourteen males in
adult plumage could be identified as second-yr
males by the presence of a few unmolted, juvenile
feathers on the nape or ear coverts. Second-yr
males started molt at the same time as older males,
and from May-July the scores for the two age-class-
es were similar (Mann-Whitney; W•4,9 = 231.0, P --
0.13). However, in November-February, second-yr
males had higher PMS than older males, and the
difference approached significance (Mann-Whit-
ney; W16,4 ---• 60.5, P = 0.08). Similarly, second-yr
males had higher values of SMS than older males
230 B. ARROYO AND J. KING VOL. 30, NO. 4
Table 1. Percentage of yearling specimens of Montagu's harriers in which molted feathers were present in different
feather tracts. Figures in brackets show the mean score of the individuals in molt (1 [only a few feathers]-3 [molt
complete]). N = number of individuals; LC, MC, GC = lesser, middle, and greater coverts, respectively; CTF =
central tail feathers; OTF = other tail feathers.
Dec-Jan 9 0 33.3 22.2 0 11.1 0 0 33.3 11.1 0
(3.0) (2.0) (2.0) (3.0) (2.0)
Feb-Mar 3 33.3 66.6 0 0 0 0 0 66.6 0 0
(1.0) (1.0) (3.0)
Apr-May 7 42.8 85.7 28.6 14.3 42.8 14.3 28.6 57.1 57.1 14.3
(1.3) (1.8) (1.5) (2.0) (2.0) (1.0) (1.5) (2.0) (3.0) (2.0)
Jun-Jul 3 66.6 100 66.6 33.3 0 0 0 66.6 100 66.6
(1.0) (2.0) (2.0) (1.0) (1.5) (2.0) (2.0)
Aug 2 50.0 50.0 100 100 100 100 50.0 100 50.0 50.0
(1.0) (3.0) (2.5) (2.0) (1.5) (2.5) (1.0) (3.0) (3.0) (2.0)
Dec-Jan 6 0 16.7 16.7 16.7 0 0 0 0 50.0 0
(2.0) (2.0) (2.0) (2.7)
Feb-Mar 5 0 40.0 40.0 20.0 0 0 0 20.0 20.0 20.0
(1.0) (1.0) (1.0) (3.0) (3.0) (2.0)
Apr-May 8 37.5 87.5 62.5 62.5 25.0 0 0 25.0 37.5 0
(2.0) (1.8) (1.5) (2.2) (1.5) (1.0) (3.0)
Jun-Jul 3 66.6 100 66.6 66.6 66.6 33.3 0 66.6 100 100
(1.5) (2.6) (2.5) (2.5) (2.5) (3.O) (1.5) (2.0) (2.0)
during same period, although the difference was
not significant (Mann-Whitney; W•s,4 = 55.0, P =
0.14), perhaps due to small sample sizes.
Molt started in the breeding season (June-July)
and finished during the winter months approxi-
mately 6-8 mo later, indicating that the molt pe-
riod in Montagu's harrier is long compared to that
of nonmigratory raptors of comparable body size
such as northern goshawks (Accipiter gentilis; Read-
ing 1990), sparrowhawks (Accipiter nisus; Newton
and Marquiss 1982), and little banded goshawks
(Accipiter brevipes; Schmitt et al. 1981). We could
ß females
40' [] males
= 30
'= 10
I 2
1 3 4 0 0 0 0 0 0 0 0
Apr May Jun Jul A• Sep Oct Nov Dec Jan Veb Mar
Figure 5. Molt phenology of primary feathers in yearling Montagu's harriers. Histograms represent the mean
(_ SE) primary molt scores throughout the year for both males and females. Sample size (number of individuals)
is given above the histograms. Secondaries and tail feathers, as in adults, are molted in the interval when primaries
are molted.
not confirm that birds suspended molt during the
migration, as no birds were collected on the mi-
gratory route. However, molt scores of birds col-
lected in August-October in either breeding or
wintering areas together with high molt scores of
the birds collected in November on the wintering
grounds, suggested that at least some birds contin-
ued to molt while migrating. The duration of the
autumn migration is unknown, but may be in ex-
cess of 1 mo given the average dates of departure
from the breeding grounds (Studinka 1941, Petre-
ment 1968, Perez Chiscano and Fernandez Cruz
1971, Kjellen 1992, Martelli and Parodi 1992, Ar-
royo 1995), and dates of arrival in wintering areas
(Morel and Roux 1966). Migratory flight of harri-
ers seems to be slow and generally at relatively low
altitudes. They combine flapping and gliding and
hunting along the way (Brown 1976, Ali and Ripley
1978). This mode of migration may allow harriers
to continue molting slowly while migrating as they
are not soaring birds, for which gaps in the wings
would represent higher energetic costs. Piechocki
(1955) observed molt in a single captive pallid har-
rier (Circus macrourus). Each tail feather took 45 d
to grow completely. Presumably, the primaries
would take longer to grow especially from P5 on-
wards. If single primaries are not shed until growth
of the previous one is nearly finished, harriers may
keep molting at a slow rate during the migration
instead of suspending molt.
As in goshawks and sparrowhawks (Newton and
Marquiss 1982, Reading 1990), molt of primaries
in Montagu's harrier was spread throughout the
entire molt cycle. Secondaries were molted later in
the season, but finished at approximately the same
time as the primaries. In raptors, molt of second-
aries and especially of tail feathers, occurs more
quickly than that of primaries (Newton and Mar-
quiss 1985, Underhill 1986, Schmitt et al. 1987). In
the Montagu's harrier, molt of the secondaries ei-
ther did not start, or was not apparently advanced
before autumn migration. Secondaries provide
much of the lift required for flight (Newton and
Marquiss 1982), so it may be especially important
to minimize gaps in these feathers for long-dis-
tance migration. In accordance with this hypothe-
sis, raggedness scores in the secondaries were al-
ways low in comparison with those of primaries.
Males started molting later than females. A sim-
ilar temporal difference in the onset of molt be-
tween sexes has also been documented in sparrow-
hawks (Newton and Marquiss 1982) and northern
harriers (Circus cyaneus hudsonius; Schmutz and
Schmutz 1975). Breeding birds must divide their
energy between two energetically costly tasks:
breeding and molting. In Ural owls (Strix uralensis),
breeding delays molt and the number of molted
feathers is inversely related to energy expended
during the previous breeding season (Pietiainen et
al. 1984). Therefore, in species in which males do
most of the hunting in the breeding season, a sex-
ual dimorphism of molt initiation is expected. In
Montagu's harrier, females contributed less than
15% to the feeding of nestlings and fledglings (Ar-
royo 1995). Females can start molting during in-
cubation, when their energy expenditure is less
than that of hunting males. The onset of molt in
female sparrowhawks was related to the date they
started laying (Newton and Marquiss 1982), but
such a relationship was not found in northern har-
riers (Schmutz and Schmutz 1975) or Swainson's
( Buteo swainsoni) or ferruginous ( B. regalis) hawks
(Schmutz 1992). Most female Montagu's harriers
observed in Madrid appeared to have started molt
soon after incubation began (B. Arroyo unpubl.
data), but it is not known whether the timing of
molt was correlated with timing of egg laying.
As in the northern harriers (Schmutz and
Schmutz 1975), male Montagu's harriers appar-
ently molted at a faster rate than females. Female
feathers are longer (Nieboer 1973), and may take
slightly more time to reach their full length. How-
ever, in sparrowhawks in which sexual dimorphism
is much greater than in Montagu's harriers, differ-
ence between the sexes in the duration of molt is
related to the interval between shedding feathers,
not the time that each individual feather takes to
grow to its full extent (Newton and Marquiss
Both sexes differed slightly in the order which
secondary and tail feathers were molted. Females
molted secondaries 2 and 4 before finishing the
tertials, whereas molt of the tertials proceeded at
a faster rate in males. Additionally, males molted
the outermost tail feathers before any other. The
adaptive reasons (if any) for these different strat-
egies are unclear, and the apparent pattern might
be an artifact of sample size used in our study.
Yearlings do not usually have the same energetic
costs of adults, and their plumage is usually of
poorer quality. Thus, it would be expected that
they start replacing their flight feathers earlier than
adults. This has been found to be true of yearling
sparrowhawks (Newton and Marquiss 1982), cap-
232 B. ARROYO AND J. KiNG VOL. 30, No. 4
tive goshawks (Reading 1990), and pallid harriers
(Circus macrourus; Piechocki 1955). In this study,
yearlings did not appear to start molt significantly
earlier than adults. This again may have been due
to sample size limitations; however, younger birds
are less experienced hunters than adults, so they
may be under higher nutritional stress, especially
individuals that have undergone spring migration.
Some yearlings summer in Africa and apparently
they replace remiges faster than birds returning to
the breeding grounds (D. Forsman, in litt.). In this
study, the highest PMS among yearlings early in the
cycle were shown by females that bred (shot while
incubating), possibly because they were fed by
males, and were in better condition than other
yearlings (of either sex) that had to hunt for them-
selves. On the other hand, yearlings seemed to
complete molt before adults although the differ-
ences were not significant. Second-yr males, which
are probably nonbreeders (given that most males
do not start breeding until the third year; Cramp
and Simmons 1980), appeared to finish molt be-
fore older males. This suggests that nonbreeding
birds molted at a faster rate. Alternatively, different
age groups might have different migration strate-
gies, which in turn might influence molt if year-
lings migrate at a slower rate. If young birds make
more stopovers to feed, they might be able sustain
a higher rate of molt in flight feathers as has been
shown in steppe buzzards (Buteo buteo vulpinus;
Gorney and Yom-Tov 1994).
We are very grateful to Peter Colston (NHM, Tring),
Josefina Barreiro (MNCN, Madrid) and the ORSTOM
Station (S•n•gal) for access to their collections, and to
Dick Forsman, Josef Schmutz, Bruce MacWhirter and Ian
Newton for valuable comments on the manuscript.
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Received 29 November 1995; accepted 22 August 1996
... Honey Buzzard Pernis apivorus), although others continue moulting during migration (e.g. Montagu's Harriers Circus pygargus, Arroyo & King, 1996). All of them complete the moult in their winter quarters. ...
... They combine flapping and gliding with hunting along the way (Brown, 1976). This mode of migration may allow harriers to continue moulting slowly while migrating, as they are not obligate soaring birds, for which gaps in the wings would represent higher energetic costs (Arroyo & King, 1996). ...
Full-text available
Moult is one of three major events in the annual cycle of birds. However, in contrast to breeding and migration, relatively few studies have been carried out on this topic. This is particularly the case with the large group of birds of prey and is partly a consequence of a general lack of appreciation of the relevance of moult within the life cycle of species. This factor is exacerbated by the difficulty in obtaining large enough sample sizes in this group, since some species are scarce and birds of prey are almost always difficult to trap. Nevertheless, moult is an energy-demanding process that takes longer than the breeding cycle and, contrary to the latter, it occurs every year. We stress the importance of the annual moult process for providing a "fixed image" of an individual's biology and underline its utility in furthering knowledge of the life history of each species. In this review we first outline the basic definitions necessary for understanding the moult process, and discuss current information on the moult sequence of European birds of prey, as part of a comprehensive review of the mechanisms of the moult in each group. Secondly, we summarise the main methods used to study, analyse and understand the moult, and indicate how to use these to obtain relevant information. Thirdly, we explain the importance of the moult in the life cycle of birds of prey, and how we can use this information to advance our understanding of the ecology of each species. Finally, we include a view of possible strategies that may be used to improve future research, thanks to advances in knowledge of the moult process.
... Aunque con un pequeño desfase, la historia del estudio de la muda en España se ajusta al cronograma general esbozado arriba, donde el grueso de las contribuciones científicas se centran en descripciones de fenología, secuencia y extensión de la muda, y su aplicación potencial a la determinación de la edad de las aves (véase la ingente labor de Javier Blasco-Zumeta, En este sentido, se ha realizado una contribución muy significativa en lo que se refiere a especies de aves de distribución mediterránea [250][251][252] , (cuasi) endemismos ibéricos [253,254] y macaronésicos [255][256][257] , así como en aves rapaces [258][259][260][261][262] . Últimamente, además, se vienen desarrollando también trabajos de carácter más metodológico para describir procesos de muda poco habituales, como la muda suspendida en el caso del piquituerto común [263] . ...
... Tail raggedness was calculated as the sum of values for each tail feather, where 0 indicates a feather of full length (whether old or new), 4 indicates a feather at the first stage of growth (moult score 1), 3 a feather with moult score 2, 2 a feather with moult score 3, and 1 a feather with moult score 4 (cf. Arroyo and King 1996). Juvenile Amur Falcons undergo a partial body moult on the wintering grounds (Symes and Woodborne 2010), but the extent of new feathers was only readily apparent in males, which moult into the distinctive blue-grey adult male plumage. ...
... Tail raggedness was calculated as the sum of values for each tail feather, where 0 indicates a feather of full length (whether old or new), 4 indicates a feather at the first stage of growth (moult score 1), 3 a feather with moult score 2, 2 a feather with moult score 3, and 1 a feather with moult score 4 (cf. Arroyo and King 1996). Juvenile Amur Falcons undergo a partial body moult on the wintering grounds (Symes and Woodborne 2010), but the extent of new feathers was only readily apparent in males, which moult into the distinctive blue-grey adult male plumage. ...
Full-text available
Amur Falcons Falco amurensis undergo one of the most extreme migrations of any raptor, crossing the Indian Ocean between their Asian breeding grounds and non-breeding areas in southern Africa. Adults are thought to replace all their flight feathers on the wintering grounds, but juveniles only replace some tail feathers before migrating. We compare the extent and symmetry of flight feather moult in a large sample of Amur Falcons killed at communal roosts during two hailstorms in KwaZulu-Natal, South Africa in March 2019, shortly before their northward migration. Most adults had completed replacing their remiges, with only a few still growing 1–3 feathers (mainly secondaries), but most were still growing their tail feathers. Juveniles only replaced tail feathers. Moult typically was distal from the central rectrices, but 25% of adults and 1% of juveniles replaced the outer tail first, and a few individuals exhibited other moult patterns (simultaneous moult across the tail, or among the inner and outer feathers). These different moult strategies were independent of sex. Adults that replaced the outer tail first typically had replaced a greater proportion of the rectrices (mean ± SD; 0.81 ± 0.19) than adults starting from the central tail (0.17 ± 0.08). Proportionally fewer distal moulting adults were killed on 9 March than 21 March, resulting in the average proportion of rectrices replaced by adults decreasing between the two storm events from 0.52 ± 0.26 to 0.43 ± 0.23. By comparison, juvenile tail moult increased from 9 March (0.34 ± 0.18) to 21 March (0.40 ± 0.15). Overall, the probability of replacement for T1 was similar for adults (0.82) and juveniles (0.83), but adults were more likely to have replaced T2–6 (0.40–0.45) than juveniles (0.18 for T2 and 0.04–0.07 for T3–6). Asymmetry in tail moult was greater at T1 for adults (15%) than juveniles (10%), but asymmetry for T2 to T6 was greater in juveniles (3–10%) than adults (1–4%), especially given the greater probability of feather replacement in adults. Despite these differences, the degree of asymmetry was less than expected by random replacement across all rectrices in both age classes. Interestingly, moult tended to be more advanced on the left than right side of the tail. The extent of tail moult was correlated with body condition in adults and juveniles, suggesting that moult pattern might be used as an indicator of fitness in falcons.
... For example, it has been found that Palearctic Circus sp. moult during migration and eventually slow the moult- ing process (Arroyo and King 1996). Moreover, we can find the "suspended moult" strategy which is defined as a moult interrupted temporarily and subsequently resumed (Baker 2016). ...
Full-text available
Abstract Background Most of long-distance migratory raptors suspend moult during migration but detailed information is patchy for most of the Palearctic species. The aim of this research is to verify if active moulting in migrating Western Marsh Harriers occurs and to quantify the extent of moulting along the season focusing on primary feathers. Methods During a whole post-breeding migration at the Strait of Messina in Southern Italy, we gathered information about symmetrical flight feather moult from 221 adults by taking pictures of raptors passing at close range. Results We found active moulting primaries during autumn migration in 48.4% of our samples. Slight differences on the extension and timing among sex classes were recorded during the season, with adult females showing a more advanced moult stage than adult males. Conclusion The finding that the extension of the suspended moult was already defined in migratory individuals might be explained as an adaptation to minimize the energy required for moulting during migration.
... Overlap between primary moult and incubation is also observed in raptors, in females of species where the males provision food during the incubation period (e.g. sparrowhawk Accipiter nisus (Newton and Marquiss 1982) and Montagu's harrier Circus pygargus (Arroyo and King 1996)). ...
... For instance, in many wader species females depart soon after hatching of chicks, whereas males take care of brood rearing and can depart when chicks have grown older (Myers 1981b, Meissner & Ka-mont 2005, Meissner & Krupa 2009). In birds of prey, females can start moult of the flight feathers already during brood rearing while males, which provide food for the young only start moult later (Schmutz & Schmutz 1975, Newton & Marquiss 1982, Arroyo & King 1996. This can cause delayed autumn migration of adult males compared to females (Kjellén 1992). ...
Multiple studies have investigated differential migration of sexes during spring migration , while such differences during autumn migration are poorly studied. We tested several functional hypotheses explaining differences in autumn migration dates between sex and age classes and whether these patterns vary between short-and long-distance migrants (SDMs and LDMs, respectively). We used data of ringed birds from the Hanko and Lågskär Bird Observatory, Finland, North Europe. Altogether data for c. 200,000 ringed birds including 14 passerine species were used. Protogyny, with females migrating earlier than males, was common among young birds, and this difference was clearer in LDMs than in SDMs. However, in adults protogyny was not found, whereas protandy, males migrating earlier than females, was found in two species. Furthermore, species-specific sexual size dimorphism, SSD, was significantly connected with the time differences in migration between the sexes in SDMs, but not in LDMs. These results suggest that multiple factors are likely affecting differential timing of autumn migration in birds. It can be beneficial for males, especially young birds, to spend additional time at the breeding grounds to prospect for future nesting sites. The connection between SSD and autumn migration dates in SDM could be linked with the pattern where larger sized individuals can winter closer to the breeding grounds. In addition, later migration dates of adult females compared to adult males could suggest that larger reproductive investment by adult females on breeding may delay their autumn migration.
... Most birds passing through the central Mediterranean in autumn are juveniles and very few are adults, whereas in the Middle East there are many more adults, perhaps even a majority (Shirihai et al. 2000). Like adult Montagu's Harriers C. pygargus (Arroyo & King 1996), most adult Pallid Harriers are actively moulting during autumn migration (Forsman 1999; pers. obs.) so they would be expected to avoid long sea-crossings such as the Sicilian Channel where possible; perhaps most of them prefer to fly across the Middle East en route to the wintering grounds in East Africa, and from there some eventually move westwards (to wintering grounds from which they head north again in spring, crossing the central Mediterranean). ...
The Order Accipitriformes is said to have a standard sequential molt from innermost primary P1 to outermost P10. Recently, a new primary molt pattern has been reported for the Japanese Sparrowhawk Accipiter gularis, starting among the central primary feathers. However, this new sequence has not been fully clarified, and requires further elucidation. Furthermore, other aspects of the species’ molt are poorly known. Here we describe the form of immature Japanese Sparrowhawk molt, including molt sequence, molt timing, and sexual differences in molt. We collected data on the wing molt of immature Japanese Sparrowhawks by means of photographic analysis (146 samples), examination of museum specimens (56 samples), and observations of a captive bird. In order to describe the primary and secondary molt sequence, we used two different approaches: one based on the distribution of feather molt, and the other based on asymmetric molt position. In order to estimate sexual, temporal, and geographical variation in molt, we used statistical analysis. These analyses allowed us to discover several unique aspects of molt. Some individuals undergo divergent molt, in a sequence which begins with one of the middle primaries (P4–P6) and proceeds simultaneously in both directions towards the inner and outer primaries, and some molted while wintering. In addition, among divergent molting individuals, males molted earlier than females. Moreover, they molted secondary S2 before S1. Comparisons between ‘non-standard’ molt patterns (and related phenomena such as migration and molt timing) and ‘standard’ molt patterns, may further our understanding of the evolution of molt.
Events in the life cycle of migrant birds are generally time-constrained. Moult, together with breeding and migration, is the most energetically demanding annual cycle stages, but it is the only stage that can be scheduled at different times of the year. However, it is still not fully understood what factors determine this scheduling. We compare the timing of primary feather moult in relation to breeding and migration between two populations of Eurasian golden plover Pluvialis apricaria, the continental population breeding in Scandinavia and in N Russia that migrates to the Netherlands and southern Europe, and the Icelandic population that migrates mainly to Ireland and western UK. Moult was studied at the breeding grounds (N Sweden, N Russia, Iceland) and at stopover and wintering sites (S Sweden, the Netherlands). In both populations, primary moult overlapped with incubation and chick rearing, and females started on average 9 days later than males. Icelandic plovers overlapped moult with incubation to a larger extent and stayed in the breeding grounds until primary moult was completed. In contrast, continental birds only moulted the first 5-7 primaries at the breeding grounds and completed moult in stopover and wintering areas, such as S Sweden and the Netherlands. This overlap, although rare in birds, can be understood from an annual cycle perspective. Icelandic plovers presumably need to initiate moult early in the season to be able to complete it at the breeding grounds. The latter is not possible for continental plovers as their breeding season is much shorter due to a harsher climate. Additionally, for this population, moulting all the primaries at the stopover/wintering site is also not possible as too little time would remain to prepare for cold-spell movements. We conclude that environmental conditions and migration strategy affect the annual scheduling of primary feather moult in the Eurasian golden plover.
Full-text available
When the order in which feathers moult is not constant and the pattern is complex, it is difficult to discover the usual pattern simply by inspecting the data. This paper illustrates a method, the Gower analysis, that produces a graphical plot from which it is relatively easy to deduce the usual ordering.
Full-text available
The observed variation in the total number of feathers molted each year was due to annual differences in the number of tail and secondary feathers shed. With the exception of 1985, all alulas and primary feathers were replaced annually. The pattern of secondary and tail feather loss in any one year was largely dependent on the molt pattern of the previous year. Although the duration of the overall molt decreased from a maximum of 141 days in 1984 to a minimum of 98 days in 1988, the onset of each molt remained relatively constant between days 134 and 145. The length of 2nd and subsequent primaries was greater than that of the first primaries grown in the nest. A significant thickening of the rachis in some primaries was also found, indicating an increase in feather strength between juvenile and subsequent feathers.
Schmitt, M. B., Baur, S. & Von Maltitz, F. 1982. Mensural data, moult, and abundance of the Little Banded Goshawk in the Transvaal. Ostrich 53:74-78.During a five-year study 68 Little Banded Goshawks Accipiter badius were captured in the Transvaal, South Africa. Birds were significantly more common between March and July when moult was completed. Sexual dimorphism is indicated by eye colour and wing length. Mass is given and recaptures stated. The possibility of migration is discussed and compared with conditions in Nigeria.