Pattern and Chronology of Prebasic Molt for the Wood Thrush and Its Relation to Reproduction and Migration Departure

9 Pages·1998·4.5 MB·English

by  J H Vega Rivera

n
Wilson Bull., 110(3), 1998, pp. 384-392
PATTERN AND CHRONOLOGY OF PREBASIC MOLT FOR THE
WOOD THRUSH AND ITS RELATION TO REPRODUCTION AND
MIGRATION DEPARTURE
J. H. VEGA RIVERA,1-3 W. J. MCSHEA,2 J. H. RAPPOLE,2 AND C. A. HAAS1
—
ABSTRACT. Documentation of the schedule and pattern of molt and their relation to reproduction and
migration departure are important, but often neglected, areas of knowledge. We radio-tagged Wood Thrushes
(Hylocichla mustelina), and monitored their movements and behavior on the U.S. Marine Corps Base, Quantico,
Virginia (38° 40' N, 77° 30' W) from May-Oct. of 1993-1995. The molt period in adults extended from late
July to early October. Molt of flight feathers lasted an average of 38 days ( = 17 birds) and there was no
significant difference in duration between sexes. In 21 observed and captured individuals, all the rectrices were
lost simultaneously or nearly so, and some individuals dropped several primaries over a few days. Extensive
molt in Wood Thrushes apparently impaired flight efficiency, and birds at this stage were remarkably cautious
and difficult to capture and observe. All breeding individuals were observed molting 1—4 days after fledgling
independence or last-clutch predation, except for one pair that began molt while still caring for fledglings. Our
data indicate that energetics or flight efficiency constraints may dictate a separation of molt and migration. We
did not observe Wood Thrushes leaving the Marine Base before completion of flight-feather molt. Departure of
individuals with molt in body and head, however, was common. We caution against interpreting the lack of
observations or captures of molting individuals on breeding sites as evidence that birds actually have left the
area. We argue also that current reports on overlapping of molt and migration based on observations of molting
individuals out of the breeding range could be misleading because some individuals may leave the breeding
area to molt in other places before starting a true migration. Received 30 June 1997, accepted 18 March 1998.
The Wood Thrush (Hylocichla mustelina) is slow, diurnal migration with frequent feeding
one of the most studied avian species in North stops (e.g., hirundinids; Niles 1972).
America, and details about its natural history The scarcity of information about timing
are relatively well known (Roth et al. 1996 and patterns of molt in Wood Thrushes and
and citations therein). Despite this, informa- many other species is not unexpected. During
tion about its molt is limited and details on the molt period, birds become secretive and
how molt is integrated with reproduction and less likely to be observed or caught, making
migration departure are lacking. Separation of documentation of activities during this period
reproduction, molt, and migration within the difficult (Ginn and Melville 1983). Because
annual cycle is considered an evolutionary individuals of some species move away from
strategy to reduce energetic stress (Payne their breeding territory for molting (Nolan
1972, Ginn and Melville 1983), but excep- 1978, Cherry 1985, Rappole and Ballard
tions exist. Overlap of reproduction with 1987) following these individuals can be dif-
flight- feather and extensive body molt is ap- ficult. Consequently, molt data often come
parently common in tropical species (Payne from observations and/or recoveries of indi-
1972, Foster 1974). In temperate regions, its viduals of unknown breeding history or when
occurrence has been reported in some species known, these relationships have not yet been
that breed at high latitudes where food re- investigated. In recent years there has been an
sources are abundant only briefly (Payne increased interest in use of radiotelemetry for
1972). Joint occurrence of molt and migration the study of small birds, mainly because of
is less common, except for species with a the improvement in transmitter technology
(i.e., smaller size). Transmitters are still rela-
tively expensive and of limited range and life,
1 Virginia Polytechnic Institute and State Univ., Dept, but radiotelemetry-based studies can provide
VA
of Fisheries and Wildlife Sciences, Blacksburg, information that otherwise would be difficult
24061-0321.
to obtain.
2 Conservation and Research Center, 1550 Remount
VA The objectives of the study were (1) to pro-
Rd., Front Royal, 22630.
vide information on the patterns and chronol-
3 Corresponding author;
E-mail: [email protected] ogy of the prebasic molt in the Wood Thrush,
384
Vega Rivera el al. • WOOD THRUSH MOLT 385
and (2) to discuss the interaction between re- receiver (Telonics Communications, Inc.). When a
production, molt, and migration departure. bird’s signal could not be found at its previous loca-
For the Wood Thrush, a long-distance mi- tion, we searched first along roads using a I-element
antenna mounted on the roof of a truck. If loss ofcon-
grant of conservation concern (Hunter 1992),
tact lasted more than 4 days, we also performed aerial
identification of the schedule of molt and its radiotracking with a Cessna 172 at 350-500 m above
relation to reproduction and timing for migra- ground in an area roughly 7 km in radius centered on
tion are important for a better understanding a bird’s last known location. We assumed that a bird
of its biological cycle. Also, identification on had left the study area if we could not detect its signal
how the molt process may modify the behav- during the aerial search. Signal loss also could have
been due to transmitter failure, but ourexperience with
ior of individuals and their vulnerability to
these transmitters indicates that transmitter failure was
predation may be critical to the conservation
rare.
of this species. For all captured individuals, we recorded the stage
(old, missing, in pin, emerging, full grown/new) of
STUDY AREA AND METHODS
remiges and rectrices and the condition of body molt.
BasTeheinstQuudayntwiacso,coVnidrugcitniead,atwhtihechU.isS.loMcaartiende5C6orkpms wAwdhediictrhieocnnoaruldmlebyd,ertihne108leionnfgtWthhoeoofd18fTlahidgruhutlsthsfeeacstahp(et1rus0r.tehdPrveiinsmtairmgioileatsl,,,
southwest of Washington, D.C. on the eastern edge of
not considered), were numbered one to nine distally
the Piedmont Plateau physiographic region, along the
from the carpal joint. The secondaries were numbered
west bank ofthe Potomac River (Hunt 1967). The Ma-
rine Base covers an area of 24,364 ha of which 75% one to nine from the carpal joint inward to the body,
is forested. The most common vegetation types in- ninucmlbuedrinegdtoheneterttoialssixS7d-isSt9a.llyThferosimx trhecetrciexntpearirsofwetrhee
cluded American beech Fagus grandifolia), yellow
( tail. Molt on four regions ofthe body was scored using
poplar (Liriodendron tulipifera), oak {Quercus spp.) the terminology of Niles (1972).
(47.5% of the forested area); Virginia pine (Pinus vir- We pinpointed locations by approaching each bird
ginFiranoums,Ma16y.2t%o),OcatnodbeVrir1g9in9i3a—1p9i9n5e,/owake(c1a5p.t2u%r)e.d, ra- ounbtsielrviattmioonvsedo.n Wthheenbirpdo’sssibbelhea,viwoer umpadteo 3c0onmtiinnuutoeuss
dio-tagged, and monitored the movements and behav-
ior of 42 juvenile and 61 adult Wood Thrushes as a after locating it. Specifically, we noted the type of ac-
tivity (foraging or perching), substrate (ground, bush,
part of a larger study on the behavioral ecology and
tree), and interactions with conspecifics. Direct obser-
habitat use of this species during the breeding and
vation of radio-tagged birds allowed us to record if a
postbreeding periods (Vega Rivera 1997). During the bird was involved with breeding activities (either in-
breeding season, we captured and radio-tagged females
cubating, brooding nestlings, or caring for fledglings)
and males positively identified as reproducing (i.e., or molting. To obtain data on onset of molt and date
those whose nests were found or who were associated of departure from the study area, we relied on radio-
with fledglings). Juveniles were captured during the tracking data. Incipient molt was difficult to observe
nesting period by locating active nests, and during the from a distance; therefore, for most birds we approx-
postfledging period by mist-netting. imated the beginning of the molting period by back-
Because some adult birds were killed by predators, dating 15 days from the time we observed the birds in
lost their transmitter, or left the study area before ini- heavy molt. We based this approach on the observation
tiating molt, this analysis was based on observations that 13-16 days elapsed for four individuals captured
of 30 radio-tagged individuals. Five of these birds before they started molting and later captured the first
were recaptured when molting. Additionally, we cap- time they were observed in heavy molt. Additionally,
tured 13 molting adults for which we did not have for eight birds that changed locality after reproducing,
previous data on reproductive activities. Information a similar period elapsed between the day they moved
on juvenile molt was obtained from the capture of 23 to the new locality and the first day on which they
molting individuals; 21 of these were captured after were observed in heavy molt.
they had dispersed. Throughout the paper, individual Wood Thrushes are
Adult and fledgling Wood Thrushes were fitted with referred to by a specific letter prefix followed by three
radio transmitters attached using a backpack harness numbers. The letter prefix refers to the individual's
(Rappole and Tipton 1991). Each transmitter weighed age/sex category: M = adult male. F = adult female,
2.1 g (Holohil Systems Ltd., Woodlawn, Ontario, Can- J = juvenile, and U = adult of unknown sex. Values
ada) and made up 5-6% of the bird’s body weight presented are means ± SE. In all statistical tests a
when attached. We detected no effect on the behavior probability of 0. 1 or less was accepted as significant.
of radio-tagged birds as compared with those without All statistical analyses were performed using SAS
transmitters. Transmitters had an average life ofat least (version 6.03 for PC) software.
120 days, and could be detected at distances of 400- RESULTS
1000 m on the ground, and 1-3 km from the air.
Radio-tagged birds were located on foot on alternate Most Wood Thrushes arrived at the study
days using a 4-element Yagi antenna and a 12-channel area in late April and early May. Nestlings
n
386 THE WILSON BULLETIN • Vol. 110, No. 3, September 1998
from first and second clutches fledged in late birds. Molt of secondaries, S1-S6, started
May-early June and late July-early August, when replacement of the primaries was al-
respectively. For most pairs, timing of clutch- ready underway; shedding of SI occurred as
es subsequent to the first was associated with P5 was out of the pin. Simultaneous replace-
the success or failure of previous clutches. ment of two or three secondary feathers was
The latest record of an adult attending fledg- observed in three individuals.
lings was 14 September, but most adults Twenty-one individuals lost all rectrices at
(90%) in the population completed their once. They were observed without tail feath-
breeding activities by mid-August. ers or with growing rectrices of the same size.
Of 61 adult birds radio tagged during the Shedding of the rectrices occurred shortly af-
study, six birds were killed, seven lost their ter P5 was replaced, and finished with the re-
transmitters during nesting, and 18 (15 F and placement of P9 or shortly after.
3 M) left the study area after nest failure or Body molt started before that of remiges
fledgling independence. Of these, 13 females and rectrices. Two males were captured show-
and 1 male left in June and early July, and 2 ing body molt while still possessing all of
females and 2 males left in late July-August. their old flight feathers. By the onset of molt
Thirty (10 F and 20 M) adults stayed at the in remiges and rectrices, body molt was ex-
study area for molting. Of these, 1 1 (6 F and tensive, and continued at least 13 ± 1.4 days
5 M) molted in the same area where they nest- (n — 12) after molt of flight feathers was ter-
ed; 4 males moved to deciduous sapling minated or in its final growth stages. We base
stands adjacent to their nesting territories and this statement on our observation of adults in
15 (4 F and 11 M) moved 2015 ± 520 m full-new plumage still showing molt on the
(range 545—7291 m) from the mature decid- head the day before they left. —
uous or deciduous-pine forest where they had Molt chronology in adults. The earliest
nested to Virginia pine forest, young decidu- record of a molting adult was of a male cap-
ous stands, or forest edges whe—re they molted. tured on 27 July with moderate molt on breast
Pattern of molt in adults. A composite and sides. The earliest and latest records of
pattern of molt in adult Wood Thrushes is in- adults observed having completed molt were
ferred from 18 captured individuals for which 16 September and 14 October, respectively.
we recorded the molt stage. In eight of these For most adults, the molt period extended
birds, we measured remiges and rectrices (Fig. from early August to late September (Fig. 2).
None of these birds was recaptured, there- Mean date for onset of molt was 10 August
1 ).
fore we lack information on rate of growth. (±2.1 days; females 15 August ± 3.4 days, n
Loss of PI marked the beginning of molt = 10; males 7 August ± 2.6 days, n = 17).
in remiges. From there, molt proceeded se- Molt of rectrices and remiges lasted 38.4 ±
—
quentially, with P9 the last feather replaced. 0.7 days (n 17) with no significant differ-
Similar size in adjacent growing remiges in ences (t-test, t = —0.90, P > 0.10) between
five individuals suggested that replacement of males (38.9 ± 0.8 d; n — 11) and females
these feathers may have occurred simulta- (37.5 ± 1.6 d; n = 6) Complete molt lasted
.
neously or nearly so. In four individuals, molt 52.0 ± 1.1 days ( = 21), and was longer (t-
of primaries in both wings showed some de- test: t = —1.80, P < 0.10) in males (53.2 ±
gree of asymmetry. Molt of secondaries S7— 1.1 days, n = 15) than females (49.2 ± 2.3
S9 (tertials) and S1-S6 was independent. Re- days; n = 6). Our data suggest that Wood
placement of tertials apparently began at the Thrushes that started molting earlier in the
same time as that of the inner primaries. The season extended the molt over a longer period
order of replacement of tertials varied, and (Pearson correlation; r = —0.83, P < 0.001;
=
was asymmetrical between wings for two n 21; Fig. 3).
FIG. Molt stage of adult Wood Thrushes captured Aug.-Oct. 1995-1996 in Quantico, Virginia. Dark bars
I .
represent old feathers; light-grey bars represent new full-grown and partially growing feathers; narrow-dark bars
represent pin feathers. Birds with one wing had symmetrical molt. All birds had heavy feather growth on body.
Vega Rivera el al. • WOOD THRUSH MOLT 387
—
3
388 THE WILSON BULLETIN • Vol. 110, No. 3, September 1998
65
F568(94) - J31L (40)
F696(95) - (36)
M552(94) ' (38) (50) 60
M933(95) - (47)
M930(93) - (47)
F991(95) J J40L (45) £ 55
c/) M951(93) J J40) ;roo,
CD F929(93) ^ J41) o
cno FF565063((9945)) -- (39) -((4593)) joc! 50
F509(95) . (37) -(54) o
TO M907(93) - (52) 15
O M997(93) - -(56)
M555(94) - (37) -(50)
03 F516(94) (14) 40
=3 M928(94) - (37) -(49)
~o
> F544(94) _ (39) (54)
73 M583(95) (40)
C | 35
M527(94) - -•(55)
F904(93) - (53)
July August September
M975(95) - -(56) (69)
M514(94) J36L -(59) Dateforstartofmolt
F900(94) - 1(32) FIG. 3. Correlation between duration of molt (20
M546(94) - (44) (60) July = day 1) and date of onset of molt in adult Wood
M599(95) - -(56) Thrushes (Pearson Correlation Coefficient; r = -0.83,
M919(94) - (35) 1(41) P < 0.001).
M902(94) m p-mJ41L —(56) (70)
j-n-|-r|'rrn'|Tn-|'p'n
20 30 9 19 29 8 18 28 8
Jul Aug Sep Oct we compared the mean distance between con-
Date
secutive radiolocations in nesting versus molt-
FIG. 2. Molt chronology in adult Wood Thrushes ing sites for 23 birds (114 ± 9 m and 67 ± 8
breeding in Quantico, Virginia. Numbers above and at m, respectively; Wilcoxon test: Z = 3.13, P
the end of the lines indicate the duration (days) of —
flight-feather and complete molt, respectively. A dot- 0.002) and size of the area used (95% con-
ted line represents a bird staying in the molting area vex polygon, 2.6 ± 0.5 ha and 1.4 ± 0.3 ha,
after the end of the molt. A circle indicates that the respectively; Z = 1.88, P < 0.1).
transmitter fell off the bird. A square indicates that the Overlap between reproduction and molt
.
bird was killed. For molting birds for which we obtained
breeding data, molt was initiated after repro-
— duction was completed. For these birds, molt
Adult behavior during molt. According to began 1-4 days after their young reached in-
our observations of radio-tagged adults, heavy dependence or after nest failure. We observed
molt seriously impaired maneuverability and only one pair caring for fledglings while molt-
ability to sustain flight. We observed five in- ing. This pair was captured when we radio-
dividuals that for at least 10 days could barely tagged their three nestlings, on 22 August.
fly. For instance, while we tracked a molting The male had general, light body molt. On 6
m
female, she was observed to hop 1 up a September, while still caring for one fledgling,
branch from which she attempted to fly but the male had lost his rectrices and showed
could not. She fell to the ground, and disap- heavy general body molt. It was not until sev-
peared, hopping and flapping her wings. Molt- en days later that the fledglings became in-
ing adult Wood Thrushes, at least during the dependent. The female attended two of the
heaviest molt stage, were very difficult to ob- three fledglings. At the time of capture she
serve, and almost impossible to capture in was not molting. On 14 September, while still
mist nets. They hid on the ground in the dens- caring for one fledgling, she was in heavy
est patches of vegetation where, on occasions, general body molt. Next day the fledgling had
m
we had to approach within 3 to make them dispersed, and the female was without rectri-
move, and ensure that they were alive. These ces.
behavioral changes were also evident when Overlap between molt and migration de-
Vega Rivera el al. • WOOD THRUSH MOLT 389
—
parture. Twenty-three of 30 birds that bred individuals for which we had information on
and molted at the study area, and for which breeding activities suggest that emancipation
we had data, left between 15 September and from parental care by one parent corresponded
14 October. All of these birds had completed to initiation in molt in that parent. This ob-
flight feather molt by the time they left. Five servation agrees with published information
individuals were known to still have head from studies of other passerine species where
molt in progress on the day be—fore they left. known breeding pairs have been identified
Prebasic molt in juveniles. We captured (e.g., Morton and Welton 1973, Sealy 1979,
23 juvenile Wood Thrushes in moderate to Tiainen 1981).
—
heavy general body molt. The earliest date on Behavior during molt. Flight efficiency in
which a juvenile in molt was captured was 27 adult Wood Thrushes was reduced during ex-
July. This was an individual with incomplete- tensive molt. Twenty-one birds on which we
ly grown rectrices and light molt on the sides. had observations lost all rectrices at once and
Nine more juveniles were captured from 9-20 five lost several primaries over a short time.
August and 12 from 2-5 September. The last Simultaneous replacement of flight feathers in
date on which a molting juvenile was captured waterfowl, resulting in temporary flightless-
was 18 September. Juveniles left the study ness, is well known (Jehl 1990). In passerines,
area in September and early October, with there are scattered reports that extensive molt
47% leaving in September. may impair or even prevent flight (e.g., Hau-
kioja 1971, Verbeek 1973, Rimmer 1988).
DISCUSSION
— Haukioja (1971) suggested that this condition
Molt in adults. The replacement of remi- in passerines is probably more common than
ges in Wood Thrushes, including the tertiaries has been estimated from captures or visual ob-
which were molted ahead of the other second- servations, but furtive behavior associated
aries, was typically passerine (Dwight 1900). with this condition made its detection difficult.
Simultaneous molt of rectrices, which oc- Lack of observations or captures of molting
curred routinely in our population, is not a individuals on their nesting sites have been
common
pattern in passerines (Pyle et al. used as justification for the conclusion that
1987). In Wood Thrushes, its occurrence was birds actually have left the nesting area (for
previously reported by Weaver (1949). Roth instance Sealy 1979). We caution against this
and coworkers (1996) reported 3 adults that interpretation. In this study, if molting birds
dropped all rectrices at once. The only other had not been radio-tagged, they would likely
member of the Turdiniae for which data are have passed unnoticed because they were fur-
available is Swainson’s Thrushes (Catharus tive, silent, and flew very little. Capturing ra-
ustulatus; Cherry 1985). Based on calculation dio-tagged molting Wood Thrushes using pas-
from recaptured individuals, this species re- sive mist netting was difficult. On several oc-
quired 32 days to complete flight-feather molt, casions, we surrounded molting birds with
which is close to the measure reported here mist-nets and yet failed to capture them. Only
for the Wood Thrush (38 —days). active chasing of the birds toward the nets
Sex differences in molt. Our data suggest proved to be effective.
that adult male Wood Thrushes commenced Reduced ability to fly may result in in-
molt prior to adult females. Average date of creased predation, but during this study only
onset of molt in males was eight days earlier two radio-tagged adults were predated while
than for females. These results agree with nu- molting, whereas six were predated when they
merous reports for other passerine species were nesting. Survival during extensive molt
(e.g., Verbeek 1973, Rimmer 1988, Norman can be enhanced by behavioral inconspicu-
1990). In our population, the difference in on- ousness and/or selection of safe areas (Jehl
set of molt between the sexes could have been 1990). Both strategies apparently were adopt-
influenced by female dispersal. In this study, ed by molting Wood Thrushes; they were se-
15 females, but only 3 males that bred at the cretive and they molted in sites with vegeta-
study area, left before molting. Thus, we did tion attributes, such as a dense understory, that
not get information on when those females may have enhanced protection (Vega Rivera
started molting. Our observations of molting 1997).
—
390 THE WILSON BULLETIN • Vol. 110, No. 3, September 1998
Relation between reproduction and molt. parents probably are low. Also, at this time
Separation of breeding and molt is considered fledglings are capable of full flight and thus
a general strategy that allows these processes they are less vulnerable to predators which
to occur with minimal nutritional stress makes parental role in this aspect less critical
(Payne 1972, Morton and Welton 1973, Ginn (Vega Rivera 1997). Second, at the study area
and Melville 1983; but see King and Murphy the onset of adult molt coincided with a time
1985, Murphy 1996). The initiation of molt when production of fruits and berries was
while still caring for fledglings has been re- high. Based on our observations as well as
ported for several species (e.g., Rimmer 1988, information in the literature (Martin et al.
Norman 1990, Zaias and Breitwisch 1990, 1951, Conway et al. 1994, Roth et al. 1996),
Evans Ogden and Stutchbury 1996). In our these resources are commonly eaten by Wood
study, all known breeding Wood Thrushes ap- Thrushes in late summer and fall. —
parently began molt after their young reached Relation between molt and migration. Si-
independence or after nest failure, but two in- multaneous occurrence of flight feather or ex-
dividuals were found molting while caring for tensive body molt and migration is considered
fledglings. incompatible because of the negative effects
It is possible that more Wood Thrushes on flight efficiency and nutritional condition
started molting while still taking care of fledg- (Kjellen 1994). Indeed, for most species it is
lings, but we lack documentation because of assumed that molt is scheduled so that migrant
our inability to detect incipient body molt birds have fully functional flight feathers at
without actually capturing the bird. When we the time of migration (Ginn and Melville
recorded a bird as molting based on obser- 1983). Based on our observations, we have no
vations only, it was the “notable molt” of No- evidence that Wood Thrushes molting at the
lan (1978) that we recorded, i.e., a bird with study area left before the replacement of rem-
heavy molt on the head or with missing/grow- iges and rectrices was complete. Departure
ing flight feathers. None of the birds in this with light body molt was not uncommon.
condition, with exception of the two birds There are, however, reports that individuals
mentioned, were in the company of fledglings. of some species migrate while molting (e.g.,
In our radio-tagged population, reproduc- Hyytia and Vikberg 1973, Sealy 1979). We
tive effort was apparently extended to a point suggest that many records of birds showing
where it did not compromise the timing of flight feather or heavy body molt found out-
molt and migration. Reproduction, from build- side the breeding range may represent relo-
ing of the nest to fledgling independence, took cation to special molting sites rather than mi-
about 5 1 days. Molt of flight feathers took an gration. Rappole and Ballard (1987) proposed
average 38 days. If we consider that 91% of that birds of many species may move to spe-
the adults left the study area between 22 Sep- cial molting areas safer from predators and/or
tember-10 October, then the latest any pair with better food resources than nesting habi-
should have initiated a clutch was 25 June-13 tats. Cherry (1985) captured molting Swain-
July. These dates agree with our data. Except son’s Thrushes 30, 150, and 250 km from the
for one pair, which began nest building on 22 closest known breeding areas for the species.
July, all breeding pairs initiated their last He concluded that Swainson’s Thrushes are
clutch before 13 July. unlikely to initiate actual southward migratory
An overlap between care of fledglings and movement during the middle of molt. In our
onset of molt would effectively prolong the study, 18 adults left the study area after nest
potential reproductive period for an individual failure or fledgling independence. These birds
(Foster 1974, Evans Ogden and Stutchbury were probably not migrating, but were moving
1996). We suggest that Wood Thrushes at- elsewhere to renest or initiate molt. We base
tending the last brood of the season may ini- this conclusion on the fact that intensive study
tiate molt a few days before fledgling inde- of this species documents that transients do
pendence with minimal nutritional stress for not arrive in south Texas until late to mid-
two reasons. First, close to independence. September (Rappole and Blacklock 1995) or
Wood Thrush fledglings feed mostly by them- in Veracruz, Mexico, until early October
selves; therefore their demands for food from (Winker et al. 1990).
Vega Rivera et al. • WOOD THRUSH MOLT 391
We
propose that birds that left the study nutritional stress in the annual cycles of endo-
area relocated some distance from their nest- therms: fact or fiction? Am. Zool. 25:955-964.
ing sites to specific molting areas. They re- Kjellen, —N. 1994. Molt in relation to migration in
birds a review. Ornis Svec. 4:1-24.
mained for several weeks in these areas while
Jehl, R. H. 1990. Aspects of the molt migration. Pp.
completing molt and preparing for migration, 102-113 in Bird migration: the physiology and
following a time sequence very similar to the ecophysiology (E. Gwwinner, Ed.). Spring-Verlag,
birds that remained on the study area. Testing Berlin.
of this hypothesis will require additional stud- Martin, A. C., H. S. Zim, and A. L. Nelson. 1951.
American wildlife and plants: a guide to wildlife
ies.
food habits. Dover Publications, Inc., New York.
ACKNOWLEDGMENTS Morton, M. L. and D. E. Welton. 1973. Postnuptial
molt and its relation to reproductive cycle and
We thank Dari Fletcher and David Condoulis for body weight in Mountain White-crowned Spar-
excellent help with field work. We are grateful to the rows (Zonotrichia leucophrys oriantha) in central
staff of the Natural Resources Branch at the Quantico Siena Nevada. Condor 75:184-189.
Corps Marine Base, especially Tim Stamp, for their Murphy, M. E. 1996. Energetics and nutrition of
unconditional support and help during our work at the molt. Pp. 158-198 in Avian energetics and nutri-
Quantico Marine Corps Base. J. B. Cruz, R. R. Root, tional ecology (C. Carey, Ed.). Chapman and Hall,
and three anonymous reviewers provided comments on New York.
the manuscript. This research was supported by the Niles, D. M. 1972. Molt cycles of Purple Martins
U.S. Department of Defense, Legacy Program, as a (Progne subis). Condor 74:61-71.
grant to the Smithsonian Institution. Nolan, V., Jr. 1978. The ecology and behavior ofthe
Prairie Warbler Dendroica discolor. Ornithol.
LITERATURE CITED
Monogr. 26:1-595.
Cherry, J. D. 1985. Early autumn movements and Norman, S. C. 1990. Factors influencing the onset of
prebasic molt of Swainson’s Thrushes. Wilson post-nuptial moult in Willow Warblers Phyllo-
Bull. 97:368-370. scopus trochilus. Ringing & Migr. 11:90-100.
Conway, C., W. R. Eddleman, and K. L. Simpson. Payne, R. B. 1972. Mechanisms and control of molt.
1994. Seasonal changes in fatty acid composition Avian Biol. 2:103-155.
of Wood Thrushes. Condor 96:791-794. Pyle, R, S. N. G. Howell, R. P. Yunick, and D. F.
Dwight, J. 1900. The sequence of plumages and DeSante. 1987. Identification guide to North
moults of the passerine birds of New York. Ann. American passerines. Slate Creek Press, Bolinas,
N.Y. Acad. Sci. 13:73-360. California.
Evans Ogden, L. J. and B. J. Stutchbury. 1996. Rappole, J. H. and K. Ballard. 1987. Postbreeding
Constraints on double brooding in a Neotropical movements of selected birds in Athens, Georgia.
migrant, the Hooded Warbler. Condor 98:736- Wilson Bull. 99:475-480.
744. Rappole, J. H. and G. H. Blacklock. 1985. Birds of
Foster, M. S. 1974. A model to explain molt-breed- the Texas Coastal Bend: abundance and distribu-
A&M
ing overlap and clutch size in some tropical birds. tion. Texas Univ. Press, College Station.
Evolution 28:182-190. Rappole, J. H. and A. R. Tipton. 1991. New harness
Ginn, H. B. and D. S. Melville. 1983. Moult in design for attachment of radio transmitters to
birds. British Trust Ornithological Guide 19, Hert- small passerines. J. Field Ornithol. 62:335-337.
fordshire, U.K. Rimmer, C. C. 1988. Timing ofthe definitive prebasic
Haukioja, E. 1971. Flightlessness in some moulting molt in Yellow Warblers at James Bay, Ontario.
passerines in Northern Europe. Ornis Fenn. 48: Condor 90: 141-156.
101-116. Roth, R. R.. M. S. Johnson, and T. J. Underwood.
Hunt, C. B. 1967. Physiography of the United States. 1996. Wood Thrush (Hylocichla mustelina). In
W. H. Freeman and Co., San Francisco, Califor- The birds of North America, no. 246 (A. Poole
nia. and E Gill, Eds.). Academy of Natural Sciences,
Hunter, W. C. 1992. Species and habitats of special Philadelphia; The American Ornithologist's
concern within the southeast region. Pp. 159-169 Union. Washington, D.C.
in Status and management of Neotropical migra- Sealy, S. G. 1979. Prebasic molt of the Northern Ori-
tory birds (D. M. Finch and P. W. Stangel, Eds.). ole. Can. J. Zool. 57:1473-1478.
U.S. For. Serv. Gen. Tech. Rep. RM-229. Tiainen, J. 1981. Timing of the onset of postnuptial
Hyytia, K. and P. Vikberg. 1973. Autumn migration molt in the Willow Warbler Phvlloscopus trochi-
and moult of the Spotted Flycatcher Muscicapa lus in relation to breeding in southern Finland.
striata and the Pied Flycatcher Ficedula hypoleu- Ornis Fenn. 58:56-63.
ca at the Signjlskar Bird Station. Ornis Fenn. 50: Vega Rivera, J. H. 1997. Premigratory movements
134-143. of a long-distance migratory species: the Wood
King, J. R., and M. E. Murphy. 1985. Periods of Thrush (Hylocichla mustelina). Ph.D. diss. Virgin-
392 THE WILSON BULLETIN • Vol. 110 No. 3, September 1998
,
ia Polytechnic Institute and State University, Winker, K., J. H. Rappole, and M. A. Ramos. 1990.
Blacksburg. Population dynamics of the Wood Thrush in
Verbeek, N. A. M.1973. Pterylosis and timing of molt southern Veracruz, Mexico. Condor 92:444-460.
of the Water Pipit. Condor 75:287- 292. Zaias, J. and R. Breitwisch. 1990. Molt-breeding
Weaver, F. G. 1949. Wood Thrush. U.S. Nat. Mus. overlap in Northern Mockingbirds. Auk 107:414-
Bull. 196:101-123. 416.