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414
ShortCommunications
[Auk,Vol. 107
Molt-Breeding Overlap in Northern Mockingbirds
JULIA ZAIAS• AND RANDALL BREITWISCH
2
Department
of Biology,Universityof Miami, CoralGables,
Florida33124USA
Molting and breedingare commonlythought to be
mutually exclusiveprocesses,becauseof the great
physiologicalexpenseof each(Loftsand Murton 1968,
Payne 1972, Skutch 1976, O'Connor 1984; however,
seeKing and and Murphy 1985).This generalization
appearsto be true for most temperate speciesalthough exceptionsexist (Newton 1966, Ligon and
wisch 1988,Zaiasand Breitwisch1989).This pattern
of multiple nestingswithin a breedingseasonand
instancesof molt-breedingoverlapis not unique to
White
1982,Thompsonand Slack 1983),especiallyin trop-
BrownThrasher,Toxostoma
rufum)in southernFlorida
is a single,completepostnuptialmolt in late summer
ical species.Payne (1969) and Foster (1975) both re-
to early autumn (Bent 1948).
1974, Nolan
1978, Bancroft and Woolfenden
this population (observed in North Carolina, C. Logan pers. comm.). The characteristicpattern of molt
for adultNorthernMockingbirdsandtheir sympatric
relatives (i.e. Gray Catbird, Dumetellacarolinensis,
and
ported low frequenciesof molt-breedingoverlap in
Mockingbirdsin the University of Miami populalarge collectionsof African (3.8%)and Neotropical tion were observedthroughout the entire 1986breed(10%) species.Both authors characterized molt and
ing seasonas part of a study on fledglingcareand
gonadalactivityat the time of collectionof the spec- renesting behavior (Zaias and Breitwisch 1989). We
imens.Payneand Fosterindicate,however, that these conducted regular censusesof ca. 35 territories
are probablyoverestimatesthat are due to the diffi- throughout the breeding season.Data presentedare
culty of determiningif the observedmolt wassched- the subsetof observationsduring which we moniuledandcompletevs.replacement,
interrupted(Payne tored molt. Mockingbirds on seven territories were
1969), or adventitious (Foster 1975).
observedto molt while they caredfor fledglings.In
Molt-breeding overlap is observed in arctic/sub- all cases,the overlap occurredduring the last broods
arcticspecieswhere resources
are abundantfor only of the season(i.e. broods not followed by another
a brief period (Pitelka 1958,Johnston1961,Hunter nesting attempt). Most last broods were initiated be1984).Other specieshave arrestedor protractedmolts tween mid-July and mid-August.
presumablyto decreasephysiologicalstressin unWe recordeda bird as being in molt if it was obpredictableenvironmentsor while they breed(Miller
served to be markedly disheveled in appearanceof
1961, Newton 1966, Payne 1972, Ligon and White
its torsoand to havelostpairedprimariesand rectrices.
1974). Although many specieshave been noted to Complete recordsof the extent and duration of molt
overlapmolting and breeding,few studieshave pro- were not obtained, but all molting birds were obvided quantitative data. Snow and Snow (1964) pre- servedmorethan onceoverseveraldays.We recorded
senteddataon 123tropicalpasserines,
where 41 (33%) whether the parent was molting and whether it fed
specieswere observedto molt and breed in the same fledglings.Occasionally,only one parent caresfor
month. These were, however, general molt and fledglings of the last brood of the season(Zaias and
breeding recordsfor speciesand did not necessarily Breitwisch 1989).
We recorded 9 of 14 individuals (64%) with moltpertain to individuals.
We report molt-breedingoverlap in individually breeding overlap. The onsetof molt varied between
color-bandedNorthern Mockingbirds(Mimuspoly- 15 Julyand 14 August.Four of seven(57%)malesand
glottos)while they cared for fledglings. These data five of six(83%)femalesmoltedwhile they caredfor
were part of a 7-yearstudyof a populationof mock- fledglings.Percentagesdid not differ between sexes
ingbirds on the main campusof the University of (Fisherexactprobability test,P > 0.05). Parentswere
Miami, Dade County, Florida. Approximately 35 terfirst observedin molt a mean of 4.4 + 3.4 (SD) days
ritorieswere monitoredeachyear,andthe population afteryoungfledged(range:2-11 days,n = 9). Forthe
is continuousinto surroundingsuburbs.The campus five femalesthat fed fledglings,the meanday of first
is sparselywooded,suburbanlawn (for detailsof the molt was3.8 + 2.5 dayspostfledging(range:2-8). For
study site, seeBreitwischet al. 1984).Mockingbirds the four males,the mean day of first molt was 4.8 _+
in southernFloridaare multibrooded.Typically, they 4.2 days postfledging(range: 2-11). Thesevalues for
build 3-6 nestsin a breeding season,which begins the sexeswere not significantly different (randomin mid-March and extends through August (Breit- ization test P > 0.05).
Molt onsetwas synchronouswithin pairs(Kendall
rank correlation,S = 14, n = 6 pairs,P < 0.01; Fig.
• Presentaddress:Departmentof Ecology,Etholo- 1). The mean calendar date of onset of molt for feedgy, and Evolution, University of Illinois, 606 East ing maleswas 23 July, for feeding femaleswas 28
Healey, Champaign, Illinois 61820 USA.
July. The mean calendardate of molt onsetfor those
2Present address:Department of Biology, Univerbirds not feeding was 4 August(n = 4).
sity of Dayton, Dayton, Ohio 45469 USA.
In theory,the costof rearinga broodincreases
with
April 1990]
ShortCommunications
415
brood size (Williams 1966). If there is a significant
costto molt-breedingoverlap,then one might expect
parentsto delay molting until fledglingswere independent (i.e. capableof flight and foragingon their
own). We found no correlationbetween the first day
the parent was observedin molt after the young
fledged and the number of fledglings present. This
was true for males, females, and both sexes combined
(Kendall rank correlation; males: S = -4, n = 7; females: S = -3, n = 6; sexes combined: S = -2, n = 6
pair means,P > 0.05 for all).
Feeding ratesof fledglingsdid not differ significantly from nestling feeding rates(Breitwischet al.
1986, Zaias and Breitwisch 1989). This raisesthe ques-
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tion of why mockingbirdswait until after nestlings
fledge to begin molt. Defense of nestlingsis strong
18
22
26
301
3
7
11
15
JULY
AUGUST
(Breitwisch 1988)and is greatestwithin the first week
of the fledgling stage.Nearly all fledgling mortality
DATE MALES INITIATED
MOLT
occursduring this time before they can fly (Zaiasand
Fig. 1. Synchronyof initiation of molt within pairs
Breitwisch1989),and molt might compromiseflight
maneuverability(Newton 1966,Hunter 1984)during in Northern Mockingbirds.
attackson potential predators.Adults may delay or
be unable to initiate molt until fledglingsare less reproductivesuccess
earlyin the breedingseasonmay
vulnerableto predators.Although meanfeeding rates alsoaffectinitiation of molt. Theseprior eventscan
per fledgling remain constantthroughout the fledg- influence hormone levels and in turn influence molt.
ling stage,thedecreasing
fledglingvulnerabilitymight Demonstratedsynchronybetweenmockingbirdmates
then allow parentsfinally to initiate molt.
and in other species(Newton 1966),despitethe wide
Our observationsof molt-breedingoverlapare con- range of datesfor molt initiation among pairs,sugsistentwith Foster's(1974)hypothesisthat an overlap geststhat thesefactorsmay be important.
in molt and breedingeffectivelyprolongsthe potenCuesthat initiated molt differ among speciesand
tial breedingseasonof an individual, which increases acrosshabitats.Molt is induced hormonally in some
the probabilityof producingoffspring.In areasof species(Wright and Wright 1944, Payne 1972 and
high nest predation, suchas the tropics,lengthening
referencestherein).Molt in other speciesis indepenthe breeding seasonmay be advantageous(Foster dent of breedingschedulesand cuedby the environ1975).Mockingbirdsin Florida have <50% nest suc- ment (Keast1968,Payne 1972and referencestherein).
cess,which is typical for a tropical open-cupnester It is possible,however, that molt schedulesof species
(Ricklefs 1969). The birds use time-saving mecha- in stable environments may be influenced by their
nismsin renestingin the form of a temporaldivision relative reproductive successor effort. It is increasof labor and clutch overlap of eggs and fledglings ingly apparent that no one factor functions as the
(Zaias and Breitwisch 1989) as predicted by Burley same cue for initiation of molt in all species.Addi(1980).
tional incidencesof molt-breedingoverlapin various
Molt-breedingoverlapcanoccuronly if energyand stagesof nesting may elucidate factors responsible
for the onset of molt.
nutrients are adequatefor both activities to occur simultaneously.Hypothesesto accountfor overlapinWe thank B. Broussard,N. Burley,N. Solomon,and
cludea dependenceon nutrient reserves,a reduction B. Steinly for helpful insight and revisionsof an earor reallocationin energyor nutrient expenditures,or lier version of this manuscript.This work was parboth (Foster1974, 1975;King and Murphy 1985).Altially funded by grants to Breitwischfrom Frank M.
though there is no evidence of reduction in mock- Chapman Memorial Fund of the American Museum
ingbirds,there may be sufficientfood resourcesavailof Natural History and Tropical Audubon Society.
able for overlap to occureven without a reductionin
the levels of necessarynutrients.The long nesting
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to the continuedavailability of food.
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Received
30 January1989,accepted
31 October1989.
SexualDifferencesin Nest Attendanceand Chick-Feeding
Rhythms of White Spoonbills
EDUARDO AGUILERA
Estacidn
Bioldgica
deDogana,Apartado1056,E-41080Sevilla,Spain
Sexual separationof activity rhythms may be favoredif eachsexcanforageeither at night or during
the day. This shouldbe especiallyadvantageous
during reproduction,when breedingactivities(pair formation,mateguarding,nestdefense,incubation,etc.)
competewith time for foraging,and overall energy
demandsare higher. Although there is extensiveliteratureon feedinghabitsof wading birds,few studies
focus on daily rhythms of foraging (reviewed in
Kushlan1978).Sometypicalnight-herons,including
the Yellow-crownedNight-Heron (Nyctanassa
violaceus)andthe Black-crowned
Night-Heron(Nycticorax
nycticorax),
also forage during the day (Mock 1975;
Fasola 1982, 1984). On the other hand, Wood Storks
(Mycteria americana),Great Blue Herons (Ardea hero-
dias),Reef Herons (Egrettasacra),and Gray Herons
(Ardeacinerea),
mainlydiurnal foragers,arereported
to forage at night (Kahl 1964,Krebs 1974, Black and
Collopy 1982, Draulans and van Vessem1985, van
Vessem and Draulans 1986). Knowledge of daily