April 1991] ShortCommunications 429 emergedinsects than do dippers (Ormerod and Tyler comprehensiveunderstanding of the proximal mech- 1987).Because early dietary experiencecan affectlater anismsunderlying interspecificfeeding, and any eco- choice (e.g. Rabinowitch 1968), the dietary prefer- logical and evolutionary consequences,awaits addi- encesof dipper nestlings fed by wagtails might be tional data. atypical. We are grateful to S. J. Ormerodand S. J. Tyler for We do not understand what benefit, if any, accrued assistanceof many kinds,to S. Cartreelfor help with to the wagtail from feeding the dipper nestlings. the observations,to F. Slater,the Universityof Wales, Dawkins (1976) suggestedthat birds adopting alien and the Llysdinam Charitable Trust for use of the young might benefit in gaining experienceas a par- Llysdinam Field Centre, and to L. Frank and S. Page ent. Although this explanation might accountfor in- for useful commentson an early draft. Funding was experiencedbirds feeding heterospecificnestlings, it provided by National ScienceFoundation Grant INT- seemsunsuitable for the behavior of this Gray Wag- 8807471to Yoerg and by the Central ElectricityGen- tail, given that he simultaneouslyfed his own brood, erating Board of the United Kingdom. at least one of which fledged. Instead, the prolonged investment in the dipper nestlings was more proba- bly a realadaptiveresponse to the proximity of loudly begging chicks.Strong responsivenessto stimuli as- sociatedwith dependent young may have advantages DAWKINS,R. 1976. The selfishgene. New York, Ox- ford Univ. Press. that compensatefor the rare instancesin which that responsivenessresults in maladaptivebehavior. It may OPa•EROD,S. J., & S. J. Tvlam 1987. Dippers (Cinclus be significantthat wagtail nestlingsare much quieter cinclus)and Gray Wagtails (Motacillacinerea) as than dipper nestlings. The dippers' calls may have indicatorsof streamacidity in upland Wales.Pp. acted as a "super-normal" stimulus (Tinbergen 1948) 191-208 in The value of birds (A. W. Diamond to trigger feeding of the alien young as the male and F. L. Filion, Eds.).Cambridge, Int. Council Bird Preserv. passeden route to his own nest with food. One pre- dictionof this hypothesisis that noisynestlings would RAmNowrrcH,V.E. 1968. The role of experience in be more likely to be fed by heterospecificsthan quiet the development of food preferences in gull chicks. Anita. Behav. 16: 425-428. nestlings,especially if the young of the adoptingspe- cies are quiet. SHY,M.M. 1982. Interspecificfeeding amongbirds: A significantaspect of our observationsis that spon- a review. J. Field Ornithol. 53: 370-393. taneousinterspecific feeding, once initiated, may be SONNEMANN,P., & S. SJOLANDER1977. Effectsof cross- self-perpetuating. First, increased feeding by the fosteringon the sexualimprinting of the female heterospecificreduces the parents' contribution and, Zebra Finch (Taeniopygiaguttata). Z. Tierpsychol. 45: 337-348. consequently, their activity near the nest. The inter- specificaggression that might deter the adoptingbird TINBERGEN,N. 1948. Social releasers and the exper- is therefore lesslikely to occur.Second, the sign stim- imental methodrequired for their study.Wilson Bull. 60: 6-51. uli that initially occasionedthe interspecificfeeding (e.g. begging calls, gaping mouths) may become as- TYLER,S.J. 1972. Breedingbiology of the Gray Wag- sociated with the sight of the alien nest and with tail. Bird Study 19: 69-80. approachesto it, thus increasingthe probability that Received29 March 1990,accepted 28 September1990. the adopting bird will return to the alien nest. A

Mate Attractionby Autumnal Songin the NorthernMockingbird (Mimus polyglottos)

CHERYL A. LOGAN AND LAURA E. HYATt DepartmentofPsychology, University ofNorth Carolina, Greensboro, North Carolina 27412 USA The Northern Mockingbird (Mimuspolyglottos) is attraction. Merritt (1985) removed females from the perennially territorial in the southeasternUnited territories of mated males in the spring; the song States. Both mated and unmated males defend autum- productionof maleswhose femaleswere removed nal territories,and both singthroughout the months was greaterthan that of mated malesand approxi- of Septemberand October (Breitwisch et al. 1986,Lo- mated amounts of singing produced by unmated gan 1987).In the spring,unmated males sing more males. Autumnal song is produced from early Sep- than mated males(Breitwisch and Whitesides1987), tember to November, and mockingbirds have been and mockingbirdsong appearsto function in mate observed to form pairs in autumn (Logan unpubl. 430 ShortCommunications [Auk,Vol. 108

gressivecalls including the hew and chatburst(Logan et al. 1983). We measured song production by the

3- number of 15-stime bins per sample in which song occurred. This measure reflects the amount of time spentsinging, but not the quality of the song.Though 2' qualitative aspectsof song may affectmate attraction (Derrickson 1987, 1988), we assumed that males ad-

1 vertisingfor matesshould sing more than matedmales. On this view, the amount of song produced provides one--but not the only--index of song'suse in mate 0 attraction. We also monitored the number of 15-s time Unmated Mated-Removed Mated bins per sample in which the observerlost audio and visual contact with the focal male. If we lost contact Fig. 1. Average ratio (+SE) of postremovalsong with the bird for > 10 min per sample, the sample production to preremoval song production in un- was abortedand begun later; data from abortedsam- mated males (Unmated), mated males whose females pies were not included in the analyses.Unless oth- were removed (Mated-Removed), and mated males erwise indicated, statistical tests used the Wilcoxon whose females were not removed (Mated). The hor- matched-pairs signed-rank test (related samples) or izontal line indicatesa ratio of 1.0: no change in av- the Mann-Whitney U-test (independent samples) erage song production before and after the time at which females were removed from the territories of (Siegel 1956). Values are reported as means (+SE). mated males. * = Kruskal-Wallis, P < 0.025. The amount of songproduced in preremoval sam- piesindicated that matedmales sang significantly less per 30 min than unmated males (mated = 14.0 + 3.0 obs.). We used female removal to determine if autum- 15-s bins with song; unmated = 31.7 + 4.0; U [one- nal song functions in mate attraction outside the tailed] = 9, P < 0.01). Females were removed from 5 breeding season. If this is the case (as it is in the of the 10 mated pairs. One mated male lost his mate breeding season),then unmated males should sing midway through behavioral sampling. After the fe- more than mated males, and female removal should mme left, his average song production per sample increasethe amountof songproduced by maleswhose increasedfrom 20.8 to 52.7 bins with song.Data from matesare removed (e.g. Cuthill and Hindmarsh 1985). this bird are included with the males whose females We studied free-living, color-bandedmockingbirds were removed. We continued "postremoval" sam- that inhabited the residential campus of the Univer- pling on 4 mated males whose females could not be sity of North Carolina at Greensboro.Eight to ten 30- captured and who therefore remained mated min baseline samples per bird were taken from 8 throughout. To examine the change in song produc- unmated and 10 mated males from 13 September tion after female removal relative to each bird's base- through 29 October, 1987, and 7 Septemberthrough line song production, we calculatedthe ratio of mean 19 October, 1988. Becauseautumnal singing begins postremoval song production to mean preremoval at different times for different males, sampling was song production. Ratios were determined for the 6 begun only after the bird producedat least 1.5 min males whose females were removed or left, for 7 un- (mated) or 3 min (unmated) of song per 15 min. mated males sampled at comparabletimes in the sea- Weather permitting, preremoval sampling occurred son, and for 4 mated males. A Kruskal-Wallis one- twice per day for 5 consecutivedays between 0700- way ANOVA revealedsignificant differences among 1100 and 1400-1800.On day 6, or as soon after as the groups (H [one-tailed] = 4.95, P < 0.025). Pair- possible(mean = 3.1 days),the femalesof matedpairs wise comparisonsindicated that the ratio of post- to were captured in baited potter traps and removed. To preremovalsong production was significantly greater equalize disruptionfrom capture,both matedand un- in males whose females had been removed compared mated maleswere alsocaptured and immediately re- with both unmated males (Mated-removed = 3.16 + turned into their territories. Postremovalsampling 0.86; Unmated = 0.81 + 0.10; U [one-tailed] = 7, P < began for mated males the day following female re- 0.03; Fig. 1) and to mated males whose females were moval. The sampling of unmated males resumedthe not removed (Mated: 0.83 + 0.20; U [one-tailed] = 4, day after capture.Postremoval sampling ended on 7 P = 0.057). However, the relative change in song November 1987 and 29 October 1988 either when a produced before versusafter capture in mated males bird added no more than 5% additional singing time whose females could not be captured did not differ to his running averagefor six consecutivesamples or from that of unmated males (U [one tailed] = 17, P on the fifth day after female removal,whichever came > 0.20).The changein songproduction in birds whose first. Following sampling, females were returned to femaleswere removedoccurred quickly. Comparison their home territories. of mean song output in the last two samplesbefore Throughout each sample we noted the occurrence removal with the first two samplesafter removal in- of territorial fights, fights with other species,and ag- dicated a rapid threefold increase in singing (means April 1991] ShortCommunications 431

= 7.75versus 27.33 bins with songper sample).There- Though there is increasinglystrong evidence for fore, variability acrossbirds in the effectof removal autumnalreproductive activity in severalavian spe- is unlikely to be due to variabletimes at which males cies(e.g. Bluhm 1988),we are aware of no prior dem- increasedsong production. Female-removedand un- onstrationthat song functions in the formation of mated males did not differ either before or after re- autumnalpairs in a temperatepassefine. Hegner and movalin averagetime per samplein which audioand Wingfield (1986) suggestedthat the annual cycle of visual contactwith the focal bird was lost (P > 0.20), reproductiveactivities may begin in passetinesin au- and there was no difference in total minutes of ob- tumn. House Sparrows(Passer domesticus) show two servationper bird (485 + 39.8 versus446 _+38.6). phases of reproductive development outside the One of the unmated males (Bird T) attracted a mate breedingseason proper: an autumnalphase from Oc- during precapturesampling. Data obtained on this tober through Januaryin which reproductivephys- bird were excludedfrom the above analyses,though iology changesvery gradually,and a more rapid win- severalaspects of the sequenceare of note.Our preex- ter phasecharacterized by more completegonadal perimental inventory indicated no sign of a second function.During the earlier phasepaired House Spar- bird in the territory, and the male was classedas rows showed defense of nest boxesand gradual in- unmated.During the third of 17behavioral samples, creasesin testissize and androgensecretion. We found Bird T engagedin a courtshipchase with an unbanded that, like House Sparrows,mockingbirds may show bird. Thereafter, an unbanded bird that was not chased reproductivebehavior that includes song and mate out of the territory was seen on 8 of the 17 time attractionduring a period roughly correspondingto samples.Comparison of the mean song production the gradualautumnal phase. per samplein sampleswith the unbanded bird pres- Severalpotential advantages of winter pairing have ent (n = 9) versusthose with no secondbird present been suggestedin waterfowl, including earlier spring (n = 8) indicateda twofold increasein time spentin nesting, enhancedfemale nutrition and mate testing song in the absenceof a secondbird (30.22 versus (Rohwer and Anderson 1988).Hegner and Wingfield 59.33bins with songper sample).Four daysafter the (1986)add to thesethe advantageof a longer breeding first courtshipchase, the unbandedbird fed in the seasonproduced by earlier pairingsin multibrooded territory, and on two of the final samplesthe un- species.In the spring, mockingbirdsappear adapted bandedbird sangin the territory with Bird T present. for temporal efficiencyin an already long breeding Though we cannot be certain that the unbanded bird season(Zaias and Breitwisch1989, Logan et al. 1990). observedon successivesamples was the same bird Becausethey are perennially territorial and highly each time, it would be extremelyunlikely for a res- multibrooded,and may renestwith the samepartner ident to allow a succession of intruders to remain in seasonafter season,mockingbirds are likely to benefit the territory undisturbed. If we assumethat the un- from the increasedbreeding time associatedwith ear- banded bird was a single individual, these observa- ly pairing. If malesuse songto acquiremates in au- tions confirm that mate attractionoccurs naturally in tumn, nestingmay begin the following spring assoon autumn, and they illustrate that when a female is as weather permits, thereby further enhancing the presentthere is a natural decreasein autumnal sing- temporalefficiency of spring and summerreproduc- ing. Moreover, the pattern of changedescribed for tive efforts. this bird parallelsthat seen in a male that acquired We thank Cheryl Ann Carlin, Linda Gregorcyk,and his mate during the breeding seasonin south Florida JackieSpencer for their untiring help in the field. We (Breitwisch and Whitesides 1987). aregrateful to RandyBreitwisch for his valuablecom- Though autumnalsong may function territorially, ments on earlier drafts of the manuscript. The re- it is unlikely that the differencesin song production searchwas supportedby the University of North Car- were due to altered patterns of territoriality. The olina Research Council. groups did not differ in mean number of territorial fights per sample either before (mated-removed= 0.20 + 0.07; unmated = 0.38 + 0.14; U = 19, P > 0.20) or after female removal (mated-removed = 0.20 + 0.06; unmated = 0.43 + 0.16; U = 19, P > 0.20). Sim- BLt•I-Ibl,C. K. 1988. Temporal patternsof pair for- ilarly, there were no differencesin averagenumber mation and reproductionin annual cyclesand of territorial fightsper samplebefore versus after re- associatedendocrinology in waterfowl. Pp. 123- moval in males whose females were removed (T = 3, 185 in Current ornithology,vol. 5 (R. Johnston, P > 0.10). Finally, following the removal of females Ed.). New York, Plenum Press. the groups did not differ in other indices of aggres- Bmm-wISCH,R., M. DIAZ, N. GOTTLmB,R. LEE, & J. sion, including the averageproduction per sampleof ZAIAS. 1986. Defenseof fall territoriesby mated the chatburst call, known to function in territorial and unmated Northern Mockingbirds in south- defense(Logan et al. 1983),or the averagenumber of ern Florida. J. Field Ornithol. 57: 16-21. fights per sample with other species(all P values --, & G. H. WHITEStDES.1987. Directionality of >0.20). singing and non-singingbehaviour of mated and 432 ShortCommunications [Auk, Vol. 108

unmated Northern Mockingbirds, Mimus poly- playback initiates nestbuilding during clutch glottos.Anim. Behav. 35: 331-339. overlapin mockingbirds,Mimus polyglottos. Anim. CUTHILL, I., & A. HIhlDMARSI•. 1985. Increase in star- Behav. 39: 943-953. ling song activity with removal of mate. Anim. MEP,RrrT, P. 1985. Song function and the evolution Behav. 33: 326-328. of songrepertoires in the Northern Mockingbird, DERRICKSOI•I,K. C. 1987. Yearly and situational Mimus polyglottos.Ph.D. dissertation,Coral Ga- changes in the estimate of repertoire size in bles, Florida, Univ. Miami. Northern Mockingbirds(Mimus polyglottos). Auk ROHW•, F., & M. ANVEP,SOU. 1988. Female-based 104: 198-207. philopatry: Monogamy and the timing of pair --. 1988. Variationin repertoirepresentation in formation in migratory waterfowl. Pp. 187-221 Northern Mockingbirds. Condor 90: 592-606. in Current ornithology, vol. 5 (R. Johnston,Ed.). HEGNER,R. E., & J. C. WING•IELD. 1986. Gonadal New Yorkß Plenum Press. developmentduring autumn and winter in House SIEGEL,S. 1956. Non-parametricstatistics.NewYork, Sparrows.Condor 88: 269-278. McGraw Hill. LOGAN, C.A. 1987. Fluctuations in fall and winter Z•IAs, J., & R. B•SCH. 1989. Intra-pair cooper- territory size in the Northern Mockingbird (Mi- ation, fledgling careßand renestingby Northern muspolyglottos). J. Field Ornithol. 58: 297-305. Mockingbirds(Mimus polyglottos).Ethology 80: ß P. D. Bvv•a•, & K. R. Fu•c. 1983. Role of 94-110. chatburstversus song in the defenseof fall ter- ritory in mockingbirds (Mimus polyglottos).J. Received18 January1990, accepted 28 September1990. Comp. Psychol.97: 292-301. ß L. E. HYATt, & L. GREGORCY!C.1990. Song

Measurement Error of External and Skeletal Variables in Birds and Its Effect on Principal Components

$. C. LOUGHEED,T. W. ARNOLD, AND R. C. BAILme Ecologyand Evolution Group, Department of Zoology,University of WesternOntario, London, Ontario N6A 5B7, Canada

Assessmentof measurementerror is important for (Schluter and Smith 1986, Lessellsand Boag 1987, studiesthat use morphometricvariables to make sta- Bailey and Byrnes 1990). In additionßwe examined tistical inferencesabout biologicalphenomena (e.g. the effectsof sucherror on principal componentanal- studies of adaptive radiation, taxonomic relation- ysisusing morphologicalvariables. shipsßinterspecific competition, age and sex deter- The specimensused represent subsets of largercol- mination,body condition,heritability, and growth). lections of birds obtained for other researchobjec- Use of variableswith large measurementerrors can tives. The morphologicalvariables were selectedfor resultin Type II statisticalerrors (i.e. acceptingfalse these researchobjectives and were not chosenspe- null hypothesesßsee Toft and Shea1983). The effect cificallyfor a studyof measurementerror. Before con- of measurementerrorß however, has been ignored in ducting the presentstudy of measurementerror, we most morphometricstudies. Those researchers that had intended that a•l variables be included in our have assessedmeasurement error used techniquesthat respectivestudies. Henceß our dataare typicalof "real identified interobserver or sessionbiases (e.g. Nisbet world" avian morphologicaldata, except that we mea- et al. 1970, Zink 1983ßArendt and Faaborg 1989) or suredindividual specimensmore than once. the absoluteprecision of particularmeasurements (e.g. Twenty-one male Rufous-collaredSparrows were Bortolotti 1984, Francis and Wood 1989). However, mist-netted in Belen, CatamarcaProvince, Argentina measurement error can be assessedproperly only (27ø39'S,67ø02'W). Thirteen skeletal characters were when it is evaluated relative to variation among in- measuredthree times on eachsparrow: skull width, dividualsin a sample(Schluter and Smith 1986,Bailey partial skull length (from the baseof the maxilla to and Byrnes 1990). the foramenmagnum), coracoid lengthß width of the To assess relative measurement error of several ex- proximalend of the scapula,scapula length, sternum ternal and skeletalmeasures in Rufous-collaredSpar- length, keel depth, synsacrumwidth (distancebe- rows (Zonotrichiacapensis), and external measuresin tween acetabulae),width of proximal end of femurß American Coots(Fulica americana), we usedrepeated femur lengthßtibiotarsus length, humeruslength, and measurementsand Model II analysis of variance ulna length'(seeRobins and Schnell 1971for detailed