Mimus Polyglottos)
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J. Field Ornithol., 58(3):297-305 FLUCTUATIONS IN FALL AND WINTER TERRITORY SIZE IN THE NORTHERN MOCKINGBIRD (MIMUS POLYGLOTTOS) CHERYL A. LOGAN Departmentof Psychology Universityof North Carolina,Greensboro Greensboro,North Carolina 27412 USA Abstract.--Many speciesof birdstrack resourcescontained in their feedingterritory. Ob- servationsreported here illustratefluctuations in territory sizefrom fall to winter in peren- niallyterritorial mockingbirds in the piedmontof North Carolina.In coldweather, territory size decreasedfrom fall to winter. The decreasewas neither a competitiveloss of spaceto intrudingconspecifics, nor a directresponse to the competitivethreat posed by otherspecies relying heavily on winter fruit. During the unusuallywarm winter of 1984, territory size decreasedlittle if at all. The decreaseobserved in coldweather may, therefore, be a response to alteredenergetic demands imposed by cold weather in which decreasedterritory size minimizesenergy expenditure. These observationsstress the flexibility of the mockingbird's perennialterritoriality in the face of environmentalvariation. FLUCTUACIONES EN EL TAMAlqlO DEL TERRITORIO DE SINSONTES ~ (MIMUS POLYGLOTTOS) DURANTE EL OTONO Y EL INVIERNO Sinopsis.--Muchasespecies de aveseva16an los recursosque hay en su territorio de ali- mentaci6n.Las observacionesque hice en sinsontesen Carolina del Norte, ilustran fluc- tuacionesen el tamafiodel territorioperemne de estasaves durante el otofioy el invierno. E1 territorio disminuy6en tamafio del otofioal invierno.La disminuci6nno estuvoasociada a perdida de espaciopor competenciacon otros sinsonteso comorespuesta a la amenaza competitivapor otras especiesde avesque dependenen gran medida de frutas durante el invierno.Durante el "tibio" inviernode 1984, losterritorios apenas se redujeron en tama•o. Por endela disminuci6ndel territorioobservada durante inviernos fr•os muy bien pudiera ser una respuestaa la demandaenerg6tica impuesta por las bajastemperaturas en donde la reducci6ndel tamafio del territorio reduce tambi6n el gastoenerg6tico. Estas observaciones recalcan la flexibilidad del territorio peremne de los sinsontescon relaci6n a variables climato16gicas. The adaptivebenefit of territoriality dependsupon a net advantage that accruesto the residentthrough exclusiveaccess to resourcescontained in the defended area (Brown 1964, Davies and Houston 1984, Schoener 1983). Changes in the costsof defense,the value of resources,or the energeticdemands of maintenancemay alter patternsof territorialdefense over both the long and short term (e.g., Carpenter and MacMillen 1976, Hixon et al. 1983). Changesin patternsof territorial defenseshould be especiallypronounced in speciesexhibiting year-round territoriality in a variableenvironment. Differences in resourcequality or distribution,in weather, and in the nature of competitorsmay producealtered patterns of territoriality.The observationsreported here chart the courseof changes in the fall and winter territorial behaviorof the Northern Mockingbird (Mimus polyglottos). In the SouthernUnited States,mockingbirds exhibit year-rounddefense of an all-purposeterritory. Throughout the year most of the residents' activitiesare focusedon the defendedarea (Laskey 1936, Michener and 297 298] C.A. Logan j. FieldOrnithol. Summer 1987 Michener 1935). However, patternsof resourceuse may changeconsid- erably. For example,demands imposed by breedingare relaxed in the fall and winter, when benefits of territorial defenseaccrue primarily through food availability (Hailman 1960, Laskey 1936, Michener and Michener 1935). Competitivepressures change as well. In the fall, the number of conspecificintruders increases with the large influx of strange conspecificsattempting to establishterritories (Breitwisch et al. 1986, Laskey 1936, Logan et al. 1983, Michener and Michener 1935). In addition, competitionwith other speciesmay increasein the winter, as mockingbirdsvie for increasinglyvaluable fruit (Moore 1978). Such changesmay substantiallyalter patternsof resourcedefense. METHODS I observedresident wild mockingbirdsin Guilford County, North Car- olina, in the fall and winter of 1981 and 1984-1985. All territories were locatedin residentialor urban areason or around the University of North Carolina at Greensboro.This area providesabundant resourcesfor res- ident mockingbirdsthat remain in the area year-round. With minor shifts in territorial boundaries,many residentsappear to remain in the same area for life. I engagedin systematicfocal animal sampling(Altmann 1974) of 12 males in three sampling periods in 1981 and 1984-1985. Six were ob- served in 1981 and 6 in 1984-1985. In 1981 observations were collected from 5-27 Sept., from 17 Oct.-7 Nov., and from 28 Nov.-19 Dec. All 6 were color-bandedand with one exception,each bird sharedhis territory with a female. In 1984-1985, observations were conducted from 2-20 Oct., from 30 Nov.-22 Dec., and from 14-31 Jan. Of the 6 birds mon- itored, 3 were mated and 3 were unmated. One of the mated males and 3 of the unmatedmales were color-banded.Conspecific aggression peaks during the early fall (Laskey 1936). Therefore, to distinguishconspecific aggressionfrom that directedtoward other speciesdifferent observation periods were used in 1984-1985. Each sampleconsisted of 30 min of continuousmonitoring of the bird's behavior.During this period, the locationsof all activitieswere recorded on a map drawn to scalerepresenting each bird's territory. I alsorecorded the temporalsequencing of all behaviorobserved. The behavioralpatterns monitored included conspecificfights, chasesand the ritualized dance (Hailman 1960), aggressiveinteractions with other species,all vocaliza- tions, and all major perch changesand foragingpositions. Because fights and chaseswith conspecificintruders occurredthroughout the occupied space,I defined the occupiedspace as a true territory rather than as a home range. Total numbers of samplesobtained in 1981 for each bird in each sampling period were 32, 32, and 18, respectively,totalling 41 h/bird. In 1984-1985, samplestotaled 12, 10, and 12/bird, respectively for the three samplingperiods. Total sampletime was 17 h/bird. As mockingbirdsare accustomedto the presenceof humans, no blinds were used. However, observersmaintained a distance of at least 5 m from the vol.58, •o. • Fall Territorialityin Mockingbirds [299 focalbird. Observationswere evenly distributed throughout each sampling period,and nearly equal numbersof sampleswere obtainedduring the morning(0800-1200) and afternoon(1300-1700) hours.Territory sizes were calculatedby determiningasymptotic observation-area curves as definedby Odum and Kuenzler (1955). This methodadds the observed area acrosssuccessive samples to determinethe point at which additional samplesadd no more than 1% to the total area observed.The 1% criterion was usedin 1981; in 1984, thoughfewer sampleswere used,the number of sampleswas sufficientto ensure that no more than a 5% increasein area was addedwith additionalsamples. Exceptions are notedbelow. An electronicplanimeter was usedto calculateterritory areas. RESULTS Changesin territory size.--Observation area curvescalculated from September 1981 forward acrosssuccessive samples through December 1981 and from December1981 backwardacross successive samples in- dicatedstable changes in the sizeof the 1981territories. Figure 1 illustrates thesechanges in 2 of the 6 males. In bird C, two different asymptotic sizesare apparent, one in the fall (calculatedfrom Septemberthrough December)and onein the winter (calculatedfrom Decemberto Septem- ber). Bird X exhibitedtwo fall asymptotes,both considerablylarger than the asymptoticsize of the Decemberterritory. A winter decreasein ter- ritory size distinguishingthe larger fall space from the smaller winter spacewas observedin 5 of the 6 birds sampledin 1981. In these5 birds the December spacewas, on the average, over 50% smaller than the Septemberspace (Table 1, P < 0.06, two-tailed, Walsh Test [Seigel 1956]). The locationof the 6th bird's spaceshifted continuously through- out the fall and winter of 1981, and no asymptoticvalue could be obtained. The pair disappearedfrom the area in the springof 1982; no data from this pair are includedin the analysesbelow. The early fall representsthe period of greatestconspecific territorial intrusionin mockingbirdsinhabiting the SouthernUnited States(Laskey 1936, Logan et al. 1983, Michener and Michener 1935). It is, therefore, possiblethat the winter lossin territory sizereflects a competitiveloss of spaceto conspecifics.Two observationssuggest that this is not the case. First, the territoriesof three adjacentneighbors each shrank away from the other, and no bird occupiedthe interveningspaces. In the following spring,each bird's territory expandedto includethe vacatedwinter space. Secondly,in 1981, the fall influx of intruding conspecificspeaked before measurabledecreases in territory sizeswere observed.The number of conspecificintrusions was reducedby 50% during the Octobersampling period,though sizeable decreases in territory size were not evidentuntil mid-November. Becausefewer sampleswere conductedin 1984, direct comparisons cannotbe made betweenterritory sizesin the two years.However, sizes calculatedin 1984 were within the range of variation observedin 1981, and in all but two cases(see Table 1) in 1984 an averageof lessthan 300] C.A. Logan J.Field SummerOrnithol. 1987 bird C .60 1 .30- .30 bird X .60-- .60 2nd Ist J .30 September •) • December to criterion to criterion SUCCESSIVE 30