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Cooperative Breeding in Lanius Shrikes

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Cooperative Breeding in Lanius Shrikes COOPERATIVE BREEDING IN LANIUS SHRIKES. I. HABITAT AND DEMOGRAPHY OF TWO SYMPATRIC SPECIES STEVEZACK • AND J. DAVID LIGON Departmentof Biology,University of New Mexico,Albuquerque, New Mexico87131 USA ABSTRACT.--Twosyrupattic speciesof Laniusshrikes were studied near Lake Naivasha, Kenya, in an effort to understandthe ecologicalfactors favoring the evolution and mainte- nanceof cooperativebreeding in one of them. The Gray-backedFiscal Shrike (L. excubitorius), a cooperativebreeder, occupied territories of significantlyhigher tree and shrub coverthan did the Common Fiscal Shrike (L. collaris),which is not a cooperativebreeder. Areas with greatervegetational cover held significantlymore insectsin the dry monthsof the year than did relatively open sites. Possiblyassociated with the shrikes' differencesin habitats and resourceswas their significantdifference in disappearancerates during the study. Turnover of individualsamong gray-backs was abouthalf that of commonfiscals. Similarly, territorial stability (asmeasured by the percentof territoriescontinually occupiedduring the 18-month study) was nearly twice as high in the gray-backs.Within the restrictedacacia woodland gray-backswere dominant to commonfiscals, and in four instanceswe observedgray-backs expel common fiscalsfrom the latter's territories. We suggest that cooperative breeding in gray-backsis related to occupancyof a temporally stable,but spatially restricted,habitat of high quality. This in turn may lead to relatively higher survivorshipin gray-backsand, as a result,their habitatbecomes relatively "saturated."In suchan ecologicaland demographic setting, options for juvenile dispersalare restricted,and one evolutionary solution is group living. Received5 October1984, accepted 11 April 1985. COOPERATIVELYbreeding birds have received Stacey(1979) related differences in groupsize, considerableattention in recent years (see re- site tenacity, and reproductiverates between views by Brown 1978; Emlen 1978, 1984). Se- New Mexico and California populationsof the lander (1964) first suggestedhabitat saturation Acorn Woodpecker(Melanerpes formicivorus) to asa critical factorfavoring the evolution of such differences in the degree of habitat saturation socialsystems. Habitat saturation occurswhen and in demographic patterns in the two areas. all usable habitat in an area is filled with breed- Koenig and Pitelka (1981) refined the habitat ing pairs. An important consequenceof habitat saturationhypothesis (emphasizing "resource saturationis the inability of nonbreedingbirds localization" and "habitat localization") and to establisha territory in the local habitat. Re- postulatedthat a relative scarcityof marginal tention of young in the natal territory until a habitat is instrumental in the evolution of co- breeding vacancy occurs is one response to operative breeding. Atwood (1980) used this habitat saturation.Cooperative breeding often line of reasoningto explain why Santa Cruz is thought to be a result of such habitat restric- Island ScrubJays (Aphelocoma coerulescens insu- tions and retention of offspring in the natal laris), which occupyareas with extensive mar- territory. Only recently, however, has the in- ginal habitat, are not group-breeders,whereas fluence of habitat on the development or main- Florida ScrubJays (A. c. coerulescens),which are tenance of cooperativebreeding systemsbeen more habitat-restricted, do exhibit cooperative investigated in a quantitative manner (e.g. breeding (Woolfenden 1975, Woolfenden and Brown and Balda 1977, Gaston 1978a, Craig Fitzpatrick 1978). 1979, Dow 1980a, Trail 1980, Koenig 1981a, In this paper we comparetwo sympatriccon- Brown et al. 1983). generic shrikes, one species a cooperative breeder and the other a more typical (for the •Present address:Department of Biological Sci- genus) noncooperativebreeder. We then con- ences, Purdue University, West Lafayette, Indiana sider the possibleroles of habitat and resources 47907 USA. in the evolution and maintenance of these two 754 The Auk 102: 754-765. October 1985 October1985] ShrikeCooperative Breeding 755 socialsystems. The Gray-backedFiscal Shrike num band on one leg and a unique color combina- (Laniusexcubitorius) is a cooperativelybreeding tion of two color bands on the other. Nestlings of species,with socialunits ranging in size from both specieswere similarlybanded at 10 daysof age. 2 to 9 or more birds. Gray-backsappear to be Groups(gray-backs) and pairs(common fiscals) were monogamous,with only one breeding pair per censusedat irregularintervals, but generallyat least once a month. Both speciesare sexuallydimorphic group (Zach MS). The supernumerarybirds in in plumage,as femaleshave auburnflanks (Grimes a flock act as "helpers." The breeding biology 1979)that, althoughdifficult to observein the field, of gray-backsis very similarto that described are readily observedin the hand. for anothercooperatively breeding laniid, the Both shrike speciesnested with the onset of the Yellow-billed Shrike (Corvinella corvina; Grimes majorrains. Most nests were inaccessible due to their 1980). In contrast,the sympatricCommon Fis- heightand frailty of the supportinglimb. The num- cal Shrike (L. collaris)occurs only as breeding ber of fledgedyoung was recorded from all success- pairs (Zack MS). First-year Common Fiscal ful nestsfor both species. Shrikes disperseupon maturity, usually coin- Habitat sampling.--Twelvegray-back and 6 com- mon fiscal territories were measured using a modi- cident with molt into adultlike plumage. Gray- backs are restricted to Acacia woodlands in east fied point-quartermethod (Cottamand Curtis 1956) asfollows: Beginning 50 m eastof the southwestcor- and central Africa, whereas common fiscals oc- ner of a given territory, a north-southtransect was cur throughoutmost of the Afrotropicalregion established. Each transect consisted of five 40-m in- in a variety of habitats--most commonly in tervals,with eachinterval having one randomly de- openand disturbed(especially cultivated) areas terminedpoint. Fromeach point, the distance(in cm) (Hall and Moreau 1970, Britton 1980, Mack- to the nearesttree (>1.5 tn in height) to the west, worth-Praed and Grant 1960). and then againfrom the randompoint to the nearest individual tree to the east, was measured. Occasion- STUDY AREA AND METHODS ally, this led to repeatedsampling of the sametree in relativelyopen areas. These measures formed the Studyarea.--The study site is locatedon Morendat basisof the point-treedata. From eachof thesetrees, Farm near Lake Naivasha, Kenya, in the Rift Valley the distanceto its nearestneighboring tree was mea- of eastAfrica (0•40'S, 36ø23'E). Ligon and Ligon (1978) sured for use in dispersionestimates. Dispersion es- have describedthis area.The vegetationis character- timatesare from the equation• p2/(• p2+ • n2),where ized by a virtually monotypicstand of the yellow- p is the point-to-plantdistance and n is the plant-to- barked acacia (Acaciaxanthophloea) with an under- nearest-neighbordistance (Hopkins and Skellam 1954, story of Naivashastar grass(Cynodon plectostachyus) Batcheller and Bell 1971). The diameter at breast and a few speciesof perennial shrubs (most com- height (DBH) was measuredfor each tree encoun- monly Achyranthesaspera, Hypoestes verticillaris, and tered. DBH measureswere taken only once and were Solanumincanurn). Thickets of Euphorbia,Aloe, or not usedagain if the treewas encountered more than Opuntiaoccur uncommonly in woodland openings. onetime. A l-m: grid alsowas established at the orig- To the eastand south of the study site are more open inal random point, and the percent coverageof pe- areaswith only scatteredacacias. To the west toward rennial shrub cover was determined visually. This Lake Naivasha is a denser woodland composed of shrub measurementwas repeatedin l-m: quadrats5 several tree species.To the north is a large plowed and 10 m west of the point and 5 and 10 m east of field under constantcultivation (see Fig. 1). The farm the point, making five such measurementsper ran- is grazed by domesticcattle, domesticsheep, six dom point. At each quadrat,tree cover was recorded speciesof native bovids,and hippopotamus(Hippo- (presentor absent),permitting a relativemeasure of potamusamphibius ). percent shade cover. This processwas repeated 15 Rainfall measurements were taken from records times per territory (3 parallel transectscontaining 5 kept by the farm managersof Morendatand adjacent random point lines spaced75 m apart), giving 30 Marula farms. Rainfall sometimes occurs in a bimod- point-to-treemeasures, 30 tree-to-treemeasures, and al patternbut is highly unpredictablein both timing 75 perennial shrub cover and tree cover measures. and quantity.The generalpattern is that of substan- Five territories (2 gray-backand 3 common'fiscal) tial rains in April-May with lighter rains during oth- were too small to accommodate three transect lines, er months. and for these only two transects(for a total of 20 Shrikes were studied from June 1979 to December random point measurements)were sampled. All 1980 and from August 1981 to early January 1982. measurements were made in the woodland; no mea- Birdswere trapped in mist nets or caught in spring- sureswere taken in the plowed field. Three common loaded traps baited with large insectsor small frogs, fiscalterritories (osf, gpf, and tf in Fig. 1) contained and then banded with a uniquely numbered alumi- no acaciasand were not measured.The plowed field 756 ZACKAND LIGON [Auk, Vol. 102 TABLE1. Resultsof the vegetationalsampling per
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