414 ShortCommunications [Auk, Vol. 107

Molt-Breeding Overlap in Northern

JULIA ZAIAS• AND RANDALL BREITWISCH2 Departmentof Biology,University of Miami, CoralGables, Florida 33124 USA

Molting and breeding are commonlythought to be wisch 1988,Zaias and Breitwisch1989). This pattern mutually exclusiveprocesses, because of the great of multiple nestingswithin a breedingseason and physiologicalexpense of each(Lofts and Murton 1968, instancesof molt-breedingoverlap is not unique to Payne 1972, Skutch 1976, O'Connor 1984; however, this population (observed in North Carolina, C. Lo- seeKing and and Murphy 1985).This generalization gan pers. comm.). The characteristicpattern of molt appearsto be true for most temperate speciesal- for adultNorthern Mockingbirds and their sympatric though exceptionsexist (Newton 1966, Ligon and relatives (i.e. Gray , Dumetellacarolinensis, and White 1974, Nolan 1978, Bancroft and Woolfenden BrownThrasher, rufum) in southernFlorida 1982,Thompson and Slack 1983),especially in trop- is a single,complete postnuptial molt in late summer ical species.Payne (1969) and Foster (1975) both re- to early autumn (Bent 1948). ported low frequenciesof molt-breedingoverlap in Mockingbirdsin the University of Miami popula- large 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 fledgling careand gonadalactivity at the time of collectionof the spec- renesting behavior (Zaias and Breitwisch 1989). We imens.Payne and Fosterindicate, however, that these conducted regular censuses of ca. 35 territories are probablyoverestimates that are due to the diffi- throughout the breeding season.Data presentedare culty of determiningif the observedmolt was sched- the subsetof observationsduring which we moni- uled and completevs. 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 arcticspecies where resourcesare 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 be- 1984).Other specieshave arrestedor protractedmolts tween mid-July and mid-August. presumablyto decreasephysiological stress in un- We recordeda as being in molt if it was ob- predictableenvironments or while they breed(Miller served to be markedly disheveled in appearanceof 1961, Newton 1966, Payne 1972, Ligon and White its torsoand to have lostpaired primaries and 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 were ob- vided quantitative data. Snow and Snow (1964) pre- servedmore than onceover several days. We recorded senteddata on 123tropical ,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). pertain to individuals. We recorded 9 of 14 individuals (64%) with molt- We report molt-breedingoverlap in individually breeding overlap. The onset of molt varied between color-bandedNorthern Mockingbirds (Mimuspoly- 15 July and 14 August.Four of seven(57%) males and glottos)while they cared for fledglings. These data five of six(83%) females molted while they caredfor were part of a 7-yearstudy of a populationof mock- fledglings. Percentagesdid not differ between sexes ingbirds on the main campusof the University of (Fisher exactprobability test, P > 0.05). Parentswere Miami, Dade County, Florida. Approximately 35 ter- first observedin molt a mean of 4.4 + 3.4 (SD) days ritorieswere monitoredeach year, and the population afteryoung fledged (range: 2-11 days,n = 9). For the is continuousinto surroundingsuburbs. The campus five femalesthat fed fledglings,the meanday of first is sparselywooded, suburban lawn (for detailsof the molt was 3.8 + 2.5 dayspostfledging (range: 2-8). For study site, see Breitwischet al. 1984).Mockingbirds the four males,the mean day of first molt was 4.8 _+ in southernFlorida are multibrooded.Typically, they 4.2 days postfledging(range: 2-11). These values for build 3-6 nests in a breeding season,which begins the sexeswere not significantly different (random- in 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: Department of Ecology,Etholo- 1). The mean calendar date of onset of molt for feed- gy, 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 onset for those 2 Present address:Department of Biology, Univer- birds 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 15 costto molt-breedingoverlap, then one might expect •- ß parentsto delay molting until fledglings were inde- • 11 ß pendent (i.e. capableof flight and foraging on 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 _• • 3 was true for males, females, and both sexes combined • < • • 30 (Kendall rank correlation; males: S = -4, n = 7; fe- ,< males: S = -3, n = 6; sexes combined: S = -2, n = 6 • 26 pair means, P > 0.05 for all). LU Feeding rates of fledglingsdid not differ signifi- • •22 ß cantly from nestling feeding rates (Breitwischet al. 1986, Zaias and Breitwisch 1989). This raisesthe ques- •18 ß ß tion of why mockingbirdswait until after nestlings 0 •'•1 I I I I I I I I I I I I I I fledge to begin molt. Defense of nestlings is strong 18 22 26 301 3 7 11 15 (Breitwisch 1988)and is greatestwithin the first week JULY AUGUST of the fledgling stage.Nearly all fledgling mortality DATE MALES INITIATED MOLT occursduring this time before they can fly (Zaiasand Breitwisch1989), and molt might compromiseflight Fig. 1. Synchronyof initiation of molt within pairs maneuverability(Newton 1966,Hunter 1984)during in Northern Mockingbirds. attackson potential predators.Adults may delay or be unable to initiate molt until fledglings are less reproductivesuccess early in the breedingseason may vulnerableto predators.Although mean feeding rates alsoaffect initiation of molt. Theseprior events can per fledgling remain constantthroughout the fledg- influence hormone levels and in turn influence molt. ling stage,the decreasingfledgling vulnerability might Demonstratedsynchrony between mates then allow parentsfinally to initiate molt. and in other species(Newton 1966),despite the wide Our observationsof molt-breedingoverlap are con- range of datesfor molt initiation among pairs, sug- sistentwith Foster's(1974) hypothesisthat an overlap geststhat these factorsmay be important. in molt and breedingeffectively prolongs the poten- Cuesthat initiated molt differ among speciesand tial breedingseason of 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 , such as the tropics, lengthening referencestherein). Molt in other speciesis indepen- the breeding seasonmay be advantageous(Foster dent of breeding schedulesand cued by the environ- 1975).Mockingbirds in Florida have <50% nest suc- ment (Keast1968, Payne 1972and referencestherein). cess,which is typical for a tropical open-cup nester 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 increas- of labor and 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-breedingoverlap in various Molt-breedingoverlap can occur only if energyand stagesof nesting may elucidate factors responsible nutrients are adequate for both activities to occur si- for the onset of molt. multaneously.Hypotheses to accountfor overlapin- We thank B. Broussard,N. Burley,N. Solomon,and clude a dependenceon nutrient reserves,a reduction B. Steinly for helpful insight and revisionsof an ear- or reallocationin energyor nutrient expenditures,or lier version of this manuscript.This work was par- both (Foster1974, 1975;King and Murphy 1985).Al- tially 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 resourcesavail- of Natural History and Tropical Audubon Society. able for overlap to occureven without a reduction in the levels of necessarynutrients. The long nesting LITERATURE CITED period and rapid renesting in this population attest to the continued availability of food. BANCROFT,G. T., & G. E. WOOLFENDEN. 1982. The The range of days of onset of molt indicate that molt of ScrubJays and Blue Jaysin Florida. Or- environmental factors may not be the only factors nithol. Monogr. 29. that influence initiation of molt. For instance,high BENT, A.C. 1948. Life Histories of North American levels of reproductiveeffort early in the seasoncould nuthatches, wrens, , and their allies. U.S. affecttiming of molt. This effort placesthe birds in Natl. Mus. Bull. 195, Washington,D.C., Smith- an energy/nutrient deficit later in the season.Low sonJan Instit. 416 ShortCommunications [Auk,Vol. 107

BREITWISCH, R. 1988. Sex differences in defence of NEWTON,I. 1966. The moultof the BullfinchPyrrhula eggsand nestlingsby Northern Mockingbirds, pyrrhula. Ibis 108: 41-67. Mimuspolyglottos. Anim. Behav.36: 62-72. NOLAN,V., JR. 1978. The ecologyand behavior of , P. G. MERRITT, & G. H. WHITESIDES. 1984. the Prairie Warbler Dendroica discolor. OrnithoL Why do NorthernMockingbirds feed fruit to their Monogr. 26. nestlings?Condor 86: 281-287. O'CONNOR,R. J. 1984. The Growth and Develop- --,& . 1986. Parental investment ment of Birds.New York, Wiley-Interscience. b•the : maleand female PAYNE, R. 1969. Overlap of breeding and molting roles in feeding nestlings.Auk 103: 152-159. schedules in a collection of African birds. Condor BURLEY,N. 1980. Clutch overlap and clutch size: 71: 140-145. alternativeand complementaryreproductive tac- 1972. Mechanismsand controlof molt. Pp. tics. Am. Nat. 115: 223-246. 104-155in Avian biology,vol. 2 (D. S. Farnerand FOSTER,M. S. 1974. A model to explain molt-breed- J. R. King, Eds.).New York, AcademicPress. ing overlapand clutchsize in sometropical birds. PITELKA,F.A. 1958. Timing of molt in the Steller Evolution 28: 182-190. Jaysof the Queen Charlotte Islands, British Co- 1975. The overlap of molting and breeding lumbia. Condor 60: 38-49. in some tropical birds. Condor 77: 304-314. RICKLEES,R. E. 1969. The nestingcycle of HUNTER, $. 1984. Moult of the Giant Petrels Macro- in tropicaland temperateregions. Living Bird 8: necteshalli and M. giganteusat SouthGeorgia. Ibis 165-175. 126: 119-132. SKUTCH,A. F. 1976. Parent birds and their young. JOHNSTON,D. W. 1961. Timing of annual molt in Austin, Univ. Texas Press. the Glaucous Gulls of northern Alaska. Condor SNOW,D. W., & B. K. SNOW.1964. Breedingseasons 63: 474-478. and annual cyclesof Trinidad land birds. Zoo- KEAST,A. 1968. Moult in birds of the Australiadry logica49: 1-39. countryrelative to rainfall and breeding.J. Zool., THOMPSON,B.C., & R.D. SLACK.1983. Molt-breeding London 155: 185-200. overlap and timing of pre-basicmolt in Texas Least Terns. J. Field Ornithol. 54: 187-190. KING, J. R., & M.E. MURPHY. 1985. Periods of nu- WILLtAMS,G.C. 1966. Natural selection, the costs of tritional stressin the annual cycles of endo- therms: fact or fiction? Am. Zool. 25: 955-964. reproduction, and a refinement of Lack's prin- ciple. Am. Nat. 100: 687-690. LIGON,J. D., & J.L. WHITE.1974. Molt andits timing WRIGHT,P. L., & M. H. WRIGHT. 1944. The repro- on the Pinon Jay, Gymnorhinuscyanocephalus. Condor 76: 274-287. ductivecycle of the male Red-wingedBlackbird. Condor 46: 46-59. LOETS,B., & R. K. MURTON.1968. Photoperiodicand ZA•AS,J., & R. BREITWISCH.1989. Intra-pair cooper- physiological adaptations regulating avian ation, fledgling care,and renestingby Northern breedingcycles and their ecologicalsignificance. Mockingbirds,Mimus polyglottos. Ethology 80: 94- J. Zool. London 155: 327-394. 110. MILt,ER, A. H. 1961. Molt cyclesin equatorialAn- dean sparrows.Condor 63: 143-161. Received30 January1989, accepted 31 October1989.

SexualDifferences in Nest Attendanceand Chick-Feeding Rhythms of White Spoonbills

EDUARDO AGUILERA EstacidnBioldgica de Dogana,Apartado 1056, E-41080 Sevilla, Spain

Sexual separationof activity rhythms may be fa- the Yellow-crownedNight-Heron (Nyctanassaviola- vored if eachsex can forageeither at night or during ceus)and the Black-crownedNight-Heron (Nycticorax the day. This shouldbe especiallyadvantageous dur- nycticorax),also forage during the day (Mock 1975; ing reproduction,when breedingactivities (pair for- Fasola 1982, 1984). On the other hand, Wood Storks mation, mateguarding, nest defense, incubation, etc.) (Mycteria americana),Great Blue Herons (Ardea hero- competewith time for foraging, and overall energy dias),Reef Herons (Egrettasacra), and Gray Herons demandsare higher. Although there is extensivelit- (Ardeacinerea), mainly diurnal foragers,are reported eratureon feeding habitsof wading birds,few studies to forage at night (Kahl 1964, Krebs 1974, Black and focus on daily rhythms of foraging (reviewed in Collopy 1982, Draulans and van Vessem1985, van Kushlan1978). Some typical night-herons, including Vessem and Draulans 1986). Knowledge of daily