Female-Biased Delayed Dispersal and Helping in American Crows

Home , Brown jay, Helpers at the nest, Northwestern crow

The Auk 109(3):609-619, 1992

FEMALE-BIASED DELAYED DISPERSAL AND HELPING IN AMERICAN CROWS

CAROLEE CAFFREY Departmentof Biology,University of Californiaat LosAngeles, LosAngeles, California 90024, USA

AnsTRAcr.--Observationof a resident population of WesternAmerican Crows (Corvusbra- chyrhynchoshesperis) over five yearsrevealed a socialstructure that deviatesfrom that of a majority of cooperativebreeders, including other corvids.Breeding density was unusually high (0.8 pairs/ha). Core areasoccupied by pairs and familieswere small,overlapped exten- sively with thoseof neighbors,and were not defendedagainst conspecifics. A nonbreeding flock was resident on the study site. Juvenile dispersalpatterns were highly variable; individuals dispersed at various ages after two months postfledging. They either joined the nonbreedingflock or left the studyarea. Some individuals delayed dispersal for one or more years,while somereturned home after extendedabsences. As yearlings,more femalesthan males were resident in their natal core area during the breeding season.Most, but not all of thoseat home servedas helpers.More femalesthan malesassisted their parentsin breeding. The female bias in dispersaland helping is unusual.It suggeststhat the costsand benefits associatedwith these behaviors differ between the sexes,and may be different from those postulatedfor many other cooperativebreeders. Received 28 January1991, accepted 13 January 1992.

ALTHOUGHMUCH diversity exists in life-his- common to many cooperativelybreeding spe- tory traits among avian cooperativebreeders, a cies, including other corvids.I use patternsof majority of speciesshare a number of ecological, spaceuse, dispersal,settlement, and breeding demographic and social characteristics.Most to characterize the unusual social organization speciesare primarily monogamouswith helpers in this population. at the nest (Brown 1987, Smith 1990). Breeding groupstend to be permanentresidents on all- STUDY AREA AND METHODS purpose territories (e.g. Lewis 1981, Brown et al. 1983,Rabenold 1985, Curry and Grant 1990); I studied a resident population on the BalboaGolf colonially breeding species are uncommon Course in Encino, California, from March 1985 to (Brown 1974, 1978,Heinsohn 1987,Emlen 1990). March 1990. The habitat consists of tracts of grass Delayed dispersalby offspringrarely occursif separatedby rows of trees,with additionalclumps of dispersaland floating are an option and, there- trees scatteredthroughout the site. The tree flora is fore, within-population nonbreeding flocksare dominated by conifers and eucalypts, but also in- cludessycamores, oaks, and severalexotics. The course uncommon (Brown 1978, Koenig and Pitelka is watered and mowed regularly, and is surrounded 1981, Koenig et al. 1992). Dispersalin most co- by parkland, cornfields, and residential areas. The operativebreeders is female-biased(e.g. Gaston climate is southern Californian Mediterranean, with 1978a, Mumme and de Queiroz 1985, Marzluff hot, dry summers and occasionalwinter rains be- and Balda 1988, Reyer 1990), as in most birds tween Decemberand February. (Greenwood 1980). The majority of helpers I captured173 free-flying individuals (63 males,54 acrossspecies, therefore, are sonsfrom previous females,56 unsexed)using large walk-in traps (n = broods(e.g. Rowley 1981,Austad and Rabenold 52) and a cannon net (n = 121). Trapped crows were 1986, Lennartz et al. 1987, Curry 1988, Rowley weighed,measured and marked.I took five morpho- and Russell 1990). logicalmeasurements: weight; total head length (from back of head to tip of bill = THL); culmen length; Here I describethe socialorganization of a bill depth at anterior end of nares;and tarsuslength cooperativelybreeding population of Western (TL), measured as distancebetween intertarsal joints American Crows (Corvusbrachyrhynchos hesper- bent at right angles. Each individual received two is). I found that breeding pairs regularly were identical, 3.5 x 6.5 cm patagial tags bearing two let- assistedby nonbreeding auxiliaries,as in other ters, a U.S. Fish and Wildlife leg band, and a unique cooperative breeders. However, their social combinationof colored leg bands.Nestlings (n = 97) structure deviates in several ways from that also were weighed, measuredand marked between 609 610 C^ROLEEC^FFRE¾ [Auk,Vol. 109

core areas based on their location: interior core areas

• 96 o o E borderedon all sidesby golf course;peripheral areas E in which nestswere placedin treesalong the edge, 92 Oo o %0oOoø øø%%o Oo so that one side of the nest tree faced off the course 0 0000 0000 00 0 0 (e.g. a streetor the driving range). Individuals were classified as breeders, auxiliaries, o o or membersof the nonbreedingflock. Breedersen- T 84 : ß gaged in courtshipactivities, remained in closecon- •- •0 tact during the egg-layingstage, and were never assistedin nest building. For 31 of 59 breeding pairs 44 ' 4•6' 4•8' 5'0 observedover five years, at least one member was Tersus Length (mm) marked.Auxiliaries were nonbreedersregularly as- Fig. 1. Relationshipbetween tarsus and total head sociatedwith a breedingpair and their core area.Of length for individuals older than two months post- 45 individual auxiliaries, 28 were marked. Members fledging. Femalesand malesrepresented by solid and of the nonbreedingflock often were with eachother, open circles,respectively. and were not tied to a particular core area. Fourteen nonbreedingflock memberswere marked.Of marked nestlings,12 malesand 9 femaleseither were caught 32 and 38 days posthatching(mean nestling period aftertwo monthspostfledging or eventuallywere ob- = 41.0 + SE of 0.9 days, n = 17; unpubl. data). served breeding. These individuals are included in I distinguishedyearling crowsfrom adultsby the the above sample sizes. following characteristics:yearlings have a dull brown I defined helpers as auxiliariesthat fed nestlings. castto their plumage in contrastto the glossyblack Determination that nonbreeders associated with of adults;yearling retricesand remigesare pointed breeding pairs were not acting as helpers was based or rounded in shape,in contrastto the more truncated on at least three 4-h periods of observationat nests feathersof adults;yearling tail shapeis squaredcom- after hatching.Individuals were classifiedas having pared to the rounded appearanceof adult tails (Emlen dispersedonce they no longer concentratedtheir ac- 1936). Marked individuals were sexedbehaviorally tivity in their natal core area. or by using a discriminant function based on mor- Survivorshipof nestlingsand juveniles was mea- phological measurements.Thirty individuals were suredat four stages:fledging, two weeks postfledg- sexed from observation of breeding behavior--fe- ing, two months postfledging,and one year. Two males performed all incubation and brooding and weeks is the approximatetime before fledglings first were fed on and off the nest by males,as in other come down to the ground successfully;before this crows (Good 1952, Goodwin 1976). A discriminant time they cannot generateenough lift to return to function was generated basedon the measurements trees.Two months was the earliest observedage of of these individuals plus those of 42 found or ob- independence.For the groupclassified as having sur- tained dead from local rehabilitation facilities and vived to one year, I included only those individuals sexedby dissection.The best discriminantfunction, alive at two months whose subsequentfates were obtainedby a stepwiseprocedure, correctly classified known. the sex of 96% of these 72 individuals based on total Survivorship,dispersal and settlement patterns, and head and tarsuslength (Fig. 1; F = 102.74,df = 2 and sex-relatedbehavior were analyzedonly for marked 76, P < 0.0001). This discriminant function (Z = individuals.I usedANOVAs and t-teststo analyze 1.782THL - 0.503TL) was then used to sex 17 indi- relationships between dispersalbehavior and individuals caught at least two months postfledging. vidual characteristics.Sex biasesin dispersaland Overlap in body sizewas too greatto distinguishthe helping were analyzedusing only yearlingsso that sexof nestlingsand young juveniles. eachindividual was included only once.All statistical I conductedapproximately 5,300 h of observation, testsused are two-tailed. In all casesthe most pow- concentrated during the breeding season (March erful test, appropriatefor samplesize, was used. For through June).I made observationsusing 10x bin- variableswhere no significantbetween-year differ- ocularsand a spottingscope, and my car as a blind. enceswere found, datawere pooledacross years. De- The roadsthroughout the courseprovided accessto scriptive statisticsare presented as means and stan- all areas.I censusedthe study site and noted the lo- dard errors. cationof individuals four to sixtimes per week during the breedingseason and one to three timesper week RESULTS at other timesof the year.Nest locationswere plotted on a map of the study site each year. I calculated SPATIAL DISTRIBUTION internestdistances using a GTCO Digitizer. Residents ranged widely both on and off the course;however, During the breeding season, crows were during the breeding seasonpairs concentratedtheir memberseither of breeding groupsor a resi- activity in a specificcore area. I defined two types of dent nonbreeding flock. July1992] CrowDispersal and Helping Behavior 611

Breedinggroups.--Breeding groups occupied core areas distributed throughout the course (Fig. 2). Breeding density on the course as a whole remainedrelatively constantover the five years at approximately 0.8 pairs/ha. All core areas contained large trees and parts of fair- ways, and overlapped widely with those of neighbors. Interior core areas appeared larger and less overlapping than peripheral ones. Nearest-neighbordistances for interior nests were significantly greater than those for pe- ripherals, and nests in interior core areas had fewer other active crow nests within 110 m (the furthest nearest-neighbordistance measured; Table 1). Rows of trees along the edges of the coursetherefore supporteda higher density of nests than in the interior. The core areas of Fig. 2. Core areasand nest locations(o) of pairs observedin 1989.Core areas subjectively drawn. North many peripheral pairs were almost completely (top) and westborders of studyplot (indicatedby fine overlappedby thoseof others.Some peripheral lines) are boundariesof golf course.Lined area near pairs even nested in the same tree; in each of southeastcorner is driving range. The golf course three years, one tree was shared by two differ- extends to both the east and south. Broken lines in- ent pairs. dicateapproximate core areas of pairsnot part of study. Occupancy of interior core areas was more Scalebar on bottom left represents200 m. stable than that of peripheral ones. All interior pairs bred in the same location for as long as they were monitored (2-5 years). Four pairs curred during the breeding season; his mate breeding for their first time also established went on to fledgeone of the four nestlingsalive themselvesin the interior. Peripheral core areas at the time of his death. The other occurred in containedboth long-term residentsand several November. In both cases, the core areas of these pairs each year that "squeezed in" to these ar- malesremained unoccupiedfor the following eas.Some of the "squeezer" pairs were resident two breeding seasons.One was still vacantfour in the area but nestedoff the study site in some years later. Neighbors did not expand into the years (e.g. in a park acrossthe street). Some of availablespace and prevent new pairs from set- the long-term peripheral residentshad nearby tling; adjacent core areas remained approxi- off-the-courseforaging areaswhere they could mately the same, and there were conspicuous be found regularly. Three pairs breeding for vacancieswhere the former pairs had resided. their first time established themselves on the In three other years,two females(and presum- periphery over the five years. ably their mates)and one family of three "dis- Core areas did not appear to be defended appeared" for the breeding seasonand then against conspecifics.Neighbors often foraged returned to the courseand their coreareas early together, members of breeding groups were in the summer of the same year. In all three regularly observed in others' core areas, and cases, these interior core areas also remained breedersrarely prevented othersfrom entering unoccupied for the duration of the breeding their nest tree or landing on or near their nest. season. Vacancyand establishment of coreareas.--In two I observed the establishment of seven core different years,an interior male died after pre- areasby pairs breeding for the first time (four viously having bred successfully.One death oc- malesand two femalestagged as nestlings, one

TABLE1. Nest spacingin interior versusperipheral core areas.Values are • + SE (n). P-valuesbased on t-tests.

Interior Peripheral P Nearest neighbor (m) 63.6 + 3.6 (47) 47.4 + 4.5 (38) <0.01 Active nests within 110 m 2.64 + 0.21 (33) 4.33 + 0.38 (33) <0.001 612 C•ROLEEC•a•FR• [Auk, Vol. 109

T^BLE2. Percentageof breeding groupscontaining auxiliary birds (helpersand nonhelpers)and with only helpers.

1985 1986 1987 1988 1989 All years Breeding groupsobserved 7 16 26 40 26 115 Percentwith auxiliarypresent 57 56 39 28 31 37 Percentwith helper 29 47 28 25 26 30

male caught when two yearsold and a member slight shifting of neighbors. Overt aggression of the nonbreeding flock). Whether in the in- was never observed.All six individuals tagged terior or on the periphery, settlement by all as nestlings (regardlessof sex, dispersalbehav- pairs appearedto involve simply squeezinginto ior, or helper experience) settled within two already occupied areas, accompanied by the core areas of their natal one. Nonbreedingfiock.--Each year this flock con- sisted of 20 to 30 birds. Members of the non- A • 100 breeding flock often were seenin one particular area of approximately 4 ha; no pairs bred at the center of this area, but the periphery merged x (•o) = 75 into the core areas of several breeders. Non- breederswere not restrictedto this area. They were also regularly seen in the core areas of 50 breeding pairs. The flock included individuals from four months to severalyears old. One male who bred from at least1986 through 1988joined the nonbreeding flock for the breeding season (8) E 20 of 1989 when at least seven yearsold. His mate had been unmarked and her fate unknown. Of • 5 • (2) 14 birds known to have spentat leastone breed- 1 2 ing seasonin the nonbreeding flock, 11 were male and 3 were female. The nonbreedingflock Number may have served as a focus for pair formation. At least seven individuals of both sexespaired, settled, and attempted to breed the year after 100 B their tenure in the flock.

(39) COMPOSITION OF BREEDING GROUPS • 78 In each year a substantial proportion of breeding groups contained nonbreeding, aux- <• 50 iliary birds. The proportion of groups with auxiliaries ranged from 28 to 57% annually, aver- aging 37% over all years (Table 2). The majority of groups contained only one auxiliary, al- (7) though up to three were sometimespresent (Fig. •- 18 3). Of known-ageauxiliaries (n = 50), most(78%) (2) (1) (1) were yearlings (Fig. 3B). Acrossall years of the study, 61% of auxiliaries of known sex were 1 >,5 female. Significantly more female than male Age yearlings were at home during the breeding Fig. 3. (A) Distribution of auxiliary birds among season.Of 15 females, all were present in their breeding groupswith auxiliaries, 1985-1989. (B) Age natal core areas; of 14 males, 9 were at home (P distribution of known-age auxiliaries. = 0.0169, Fisher's exact test). This sample in- July1992] CrowDispersal and Helping Behavior 613

TABLœ3. Sexdifferences in helping by yearlingsres- TABLE4. Annual mortality of breedersfrom March ident on golf course.(A) All individuals of known to March. Sample sizes larger for males because sex.(B) Individuals taggedas nestlingsor members they were trappedmore frequently than breeding of whole broods caught postfledging. Chi-square females. testfor part A significant(P < 0.001),as was Fisher's exact test for part B (P = 0.0198). Males Females

Percent Percent Acted as helper disap- disap- Yes No No. pearing No. pearing All individuals 1985-1986 6 0 1 0 Females 10 1 1986-1987 7 14 2 0 Males 3 10 1987-1988 9 0 5 20 1988-1989 17 12 9 0 Taggedindividuals 1989-1990 23 4 12 0 Females 6 1 Total 62 7 29 3 Males 3 9

taggedas nestlings or membersof whole broods cludes all individuals tagged as nestlings and caught and marked between fledging and the those caught postfledging where all members following breeding season.Again, significantly of the brood were known to be marked. more femalesthan malesacted as helpers (Table Auxiliaries were prior offspringof the breed- 3B). This was not simply a function of delayed ing pair. All 27 auxiliaries marked as nestlings dispersalby females;I alsoanalyzed the differ- or juveniles associatedwith their parents (n = ence between the sexes in their likelihood to 17) or, if both parents were unmarked, they help if at home for the breeding season.Ten of were in their natal core areas(n = 10). The other 11 females,but only 3 of 8 malesassisted their marked auxiliary was a widowed female who parents. This difference is significant (Fisher's in 1989joined another pair and actedas a help- exact test, P = 0.0408). I excluded individuals er. She was at least five years old and had bred who were home for the breeding seasonbut on the study site in 1988. Her genetic relation- whose helper status could not be determined ship with the breeding pair she helped was becausenest failure occurredbefore feeding be- unknown. gan. Three marked females and two marked males

HELPING remained home with their parents for an additional one to three years after their first. In Most auxiliaries(82%) acted as helpers.In ad- every case, these individuals helped in some dition to feeding nestlings,helpers also fed the years but not in others. breedingfemale during incubationand brooding, fed fledglings,guarded nestlings and fledg- SURVIVORSHIP lings, and attended to nest sanitation. Non- helping auxiliariescontinued to associatewith Table 4 summarizesthe annual disappearance breeding pairs and to be based in their core rates of breedersby sex from one breeding sea- areasthroughout the breedingseason, but were son to the next (March to March). Survivorship never observedat active nests.All helpers and over the five yearswas 93% for malesand 97% most nonhelping auxiliariesbegged from and for females.This is an exactfigure for females; were fed by one or both breeders,particularly the single elimination was hit by a golf ball. Of the male. the four males, three were found dead. The Acrossall years of the study, 72% of helpers fourth, at least eight years old and an interior of known sex were female. Among yearlings resident for the first four years of the study, resident on the golf course,helpers were pri- disappearedand was presumeddead. marily female (Table 3A). To control for the Table 5 summarizessurvival of nestlingsand possibilitythat femaleswere more likely to be juveniles. In total, 71% of fledglings survived caughtafter fledging, I also analyzed the rela- two months.Approximately 68% of all fledg- tionship using only those resident yearlings lings survivedto age 1. Known causesof death 614 CAROLEECAFFRE¾ [Auk,Vol. 109

TAI3LE5. Nestling and juvenile survivorship be- suggestthere was no effect of dispersalbehavior tween four life stages.Survival from two months on survivorshipor the outcomeof first breeding postfledgingto oneyear excludesnine individuals of unknown fate. attempts. For individuals alive as yearlings whose subsequentfates were known, 1 of 11 Survived males and 2 of 11 females died. The first breed-

Total No. Percent ing attemptsof eight malesand six femalesall were unsuccessful. Fledging 97 68 70 Two weeks postfledging 68 54 79 Two monthspostfledging 54 48 89 DISCUSSION One year 39 37 95 In social organization, Western American Crows depart from the few generalizationsthat of fledglings and juveniles included falling to apply to a majority of cooperativelybreeding the ground before they could fly, predationby species.Within this dense population, histori- Great Horned Owls (Bubovirginianus) and Red- cally successfulbreeding areaswere available shoulderedHawks (Buteolineatus), disease (lung and unoccupied,and a nonbreeding flock was tumors found upon dissection), and injuries present.Nonbreeding individuals of both sexes sustainedfrom golf balls. No carcassesshowed dispersedfrom home for the breeding season, any indication of starvation. delayeddispersal and actedas a helper, or delayed dispersalbut did not help. Femalesde- JUVENILEDISPERSAL layed dispersaland assistedtheir parentsmore frequently than males. These characteristics Young crowsexhibited variable dispersalpat- combinewith other aspectsof spaceutilization terns after two months old. Figure 4 summariz- to distinguish these birds as unusual among esthe first-yearhistories of 43 individualsalive both cooperativebreeders and corvids. two monthspostfledging (early August). Of in- Spaceutilization.--The use of spaceby crows dividuals known to be alive the following Jan- in this population differs from the majority of uary, 18% had dispersedfrom their natal core other Corvusspecies that have been studied in areas in their first fall and either joined the detail, and corvids in general. Core areas ap- nonbreeding flock or left the golf course.One- pearedto be smallerthan reportedterritory sizes half of theseindividuals returned home asyear- for other crows and corvids (e.g. Shank 1986, lings the following breeding season.I found Chamberlain-Auger et al. 1990, Richner 1990), that 82% of the juveniles remained home until and breeding density was higher (Loman 1980, at least January.In February,prior to the be- Vines 1981, Parker 1985, Davis and Davis 1986, ginning of the breeding season,12% of those at Fitzpatrickand Woolfenden1986, Richner 1989). home left to join the nonbreedingflock or dis- The lack of any obviousarea defensealso con- perse off the course. I found that 55% of all trastsstrikingly with the describedbreeding- juveniles remained at home throughout their seasonspace-utilization patterns of other cor- first year and at leastinto their secondsummer. vids, mostof which (includingcolonial species) A few remained for one or more additional years, maintain exclusive access to certain areas as indicated above. (Goodwin 1955, 1976, Antikainen 1980). Con- I examinedthe relationshipbetween first-year geners such as Common Ravens (Corvuscorax; dispersalbehavior and severalcharacteristics of Newton et al. 1982, Davis and Davis 1986), individuals, but found no significant patterns. Hooded Crows (C. corone cornix; Loman 1984, Dispersalpatterns were statisticallyunrelated 1985), Carrion Crows (C. c. corone;Richner 1990), to: (1) incubation date; (2) prefledging size and Black Crows (C. capensis;Skead 1952) are (weight, THL and TL); (3) brood size at fledging, describedas classicallyterritorial. two weeks, and two months; and (4) whether Among crows in North America, space-use or not individuals were members of helped patterns are diverse. Northwestern Crows (C. broods. Some of the above sample sizes were caurinus)defend small nesting territories only small. However, pooling data into home/not during the breeding season(Butler et al. 1984), and golf course/not categoriesfor Januaryand whereas larger, all-purposeterritories are de- March did not alter the results. fendedfor at leastthe breedingseason by Flor- For two- and three-year-olds,preliminary data ida Crows (C. brachyrhynchospascuus; Kilham July1992] CrowDispersal and Helping Behavior 615

Home

Off 45 Nonbreeding Course Flock

August - November 27 (6t

2 (1 Februory

18 (6 7 o"o")

Fig. 4. Dispersal patterns of juveniles marked before or just after fledging. Numbers and sex (where known) of individualspursuing different strategiesindicated. A number of individualsof unknown fate (not shown) disappearedfrom study area: 10 between August and November; and 5 (including 1 male) during February.

1984) and year around by Eastern American ican Crows in Bent (1946) and Goodwin (1976) Crows (C. b.brachyrhynchos; Kilham 1985,Cham- suggestthat this maybe a regularphenomenon. berlain-Auger 1990,K. McGowan,pets. comm.). Delayed breeding.--Breeding-habitatsatura- Despitethe documentedvariation in the timing tion and a shortageof mates are the limiting of defense and the size or type of territory, factors most often invoked as the basis for de- members of these populations all were ob- layed breeding among cooperativebreeders served to actively exclude conspecificsfrom (Koenig and Pitelka 1981, Emlen 1982, Emlen breedingareas. However, referencesto the lack and Vehrencamp 1983, Brown 1987). Each year of overt territoriality in other populations of on the golf coursethere were identifiable un- American Crows (Black 1941, Good 1952) and occupiedinterior areas.The densityof nestsand Fish Crows (C. ossifragus;McNair 1984) exist. high degree of core-areaoverlap on the pe- The tendency for Western American Crows to riphery indicated successfulbreeding did not nest in colonies (Emlen 1942) adds to the di- requirea greatdeal of space.Settlement by new versity. Breedingdensity in my study popula- pairs did not involve obviousaggressive inter- tion was similar to that in the colony studied actions.This suggeststhat breeding habitat was by Emlen (0.8 vs. 0.56 pairs/ha). Anecdotal ref- not limited. Additionally, nonbreedersinclud- erencesto breeding coloniesof Western Amer- ed both malesand females,suggesting no short- 616 CAROLEECAFFREY [Auk,Vol. 109

age of potential mates. The lack of any area- Given delayed breeding, nonbreeders in a defense and high degree of sociality resulted majority of avian speciesdisperse and float until in easy accessto other obvious breeding re- a breeding vacancy becomesavailable (Stacey quirements (e.g. food and water). Thus, it is and Ligon 1987, Koenig et al. 1992). Among difficult to arguelimiting resourcesas the cause cooperative breeders, delayed dispersal by of delayed breeding. young birds is commonlyinterpreted as an ad- The majorityof nonbreedingauxiliaries (75%) aptationproviding: (1) group-membershipben- were yearlings. Yearling American Crows re- efits (Alexander 1974, Gaston 1978b, Bednarz tain juvenileplumage and lack full gonadalde- and Ligon 1988);(2) current accessto resources velopment (Black 1941). Delayed maturation (Alexander 1974,Stacey and Ligon 1987,Ligon appearsto be a characteristicof the genus,as et al. 1988);or (3) future accessto breeding sta- suggestedby the lack of breeding by yearlings tus (Brown 1974, Woolfenden and Fitzpatrick in American Crows (Good 1952, K. McGowan, 1978, Koenig 1981, Ligon 1981). None of these pers. comm.), Hooded Crows (Loman 1985), interpretationseasily explains delayed dispers- Rooks (Holyoak 1971, Marshall and Coombs al in my study population, or its sex bias. The 1957 [in Roskaft et al. 1983]), Common Ravens presence of a nonbreeding flock into which (Coombs 1978, Davis and Davis 1986), and Aus- young birds commonlydispersed, and the ex- tralian Ravens(C. coronoides;Rowley 1969[in treme tolerance displayed by individuals re- Davis and Davis 1986]).Thus, a large fraction garding spatial distribution makes it unlikely of delayed breeding may be an ancestrallife- that disperserssacrificed any benefits of group history trait in the genus Corvusrather than a membership. That breeders did not prevent response to immediate demographic condi- othersfrom foraging in their coreareas suggests tions. However, delayed maturation does not that accessto food also was unlikely to be af- explainall delayedbreeding in this population. fected by dispersal.These conclusionsare un- Althoughseveral individuals attempted to breed derscoredby preliminary data,which show no when two yearsold, 25%of nonbreedingaux- difference in mortality rates between individ- iliaries were at least that age. All of the five uals that did delay dispersalversus those that marked individuals that delayed breeding for did not. Finally, given the extensiveranging of two or more yearseventually bred on the golf individuals throughout the area, it seemsun- course. likely that birds in the nonbreedingflock sac- Delayed dispersal.--Yearlingsof both sexes rificed information on local breeding oppor- employseveral dispersal options. Juveniles may tunities. dispersefrom their natal coreareas at any time I considered two further hypotheses that after two months postfledging.They may join might explain both delayed dispersaland its the resident nonbreeding flock or dispersedif- sex bias. First, delayed dispersalmight provide ferent distancesoff the course.Some dispersers benefits via extended parental care. For exam- never were seen in the area again, some main- ple, auxiliaries continued to beg from and be tained various amounts of contact with their fed by the breeding pair (see results).Second, families, and some returned after extended pe- competitionwithin the nonbreedingflock might riods.Alternatively, individuals remained in the impose costswhich could be avoided by re- immediate area for one or more years.Individ- maining home (Eden 1987).Although I did not uals that delayed dispersalvaried in their be- study the social structureof the nonbreeding havior: some maintained residencyand family flock, suchflocks often are the setting in which relationships but were observed regularly in dominance relationships are established (Gar- other locations;some were virtually inseparable nett 1981,Eden 1987).Both hypothesespredict from their parents. This situation is very dif- that smallerand lighter birds should delay dis- ferent from that in Hooded Crows, where es- persal. Females more commonly delayed dis- sentiallyall offspringhad left homeby Septem- persal and are smaller and lighter than males, ber and were not seenwith their parentsagain but I found no differencesbetween dispersers after November (Loman 1985). The observa- and nondispersers in prefledging size and tionsof Verbeekand Butler(1981) suggest that weight. However, the sample sizes for these yearling Northwestern Crows also may pursue tests were small. variable dispersaloptions. Helping.--For individuals of other speciesthat July1992] CrowDispersal and Helping Behavior 617 delay dispersal,the main benefitsof remaining Interpretation of these unusual patternsawaits home are often acquired through helping. carefulstudy of the costsand benefitsassociated Helping has been suggestedto provide both with delayedbreeding, dispersal behavior, and indirect benefits through increased inclusive helping for each sex. fitness (e.g. Brown and Brown 1981, Hunter 1985,Emlen 1990),and direct benefitsas a step- ACKIqO•MEIqTS ping stoneto breedingstatus (e.g. Koenig 1981, Kinnaird and Grant 1982, Woolfenden and Fitz- At least45 people have assistedin trapping crows patrick 1984) or as an opportunity for socially over the years;I thank them all. Fritz Hertel, Edmund learned breeding skills (e.g. Woolfenden 1975, Alkaslassy,Virginia Nichol, GerdaUllrich, Tony Dow and Franco Bonamassadeserve special thanks in this Lawton and Guindon 1981, Fairbanks 1990). In regard.This work would not havebeen possible with- this population,helpers are siblingsof the nest- out the support and cooperationof the patrons and lings they help and so would gain an inclusive- especiallythe GroundsCrew of the Balboaand Encino fitness benefit if their behavior increased pa- Golf Courses in Encino, California, to whom I am rental breedingsuccess. Over six years,helpers extremely grateful. Walt Koening, Marc Hauser and were not found to have such an effect (Caffrey Joan Silk provided valuable commentson an earlier 1991). It is unlikely that helping is a stepping draft of this manuscript.I thank ThomasHowell and stoneto breeding statusin ways that have been Ed Caffrey for their unwavering support, Kevin demonstrated in other cooperative breeders. McGowanfor sharedthoughts and accessto unpub- lisheddata, and Charlie Petersonfor his specialtouch. Breedinghabitat doesnot appearlimited; there- Simply acknowledgingmy advisor,Robert Gibson, fore, helping does not function as a means of does not do him iustice;his help and editorial com- obtaining breeding space(Gaston 1978a, Wool- mentshave been invaluable.And Bryan, you know fenden and Fitzpatrick 1984, Rowley and Rus- what you mean to me. I could not have done this sell 1990).Nor is helping a meansof inheriting without you. This work wasfunded in part by awards a mate (Reyer 1990) or producingfuture allies from SigmaXi, the LosAngeles and E1Dorado Chap- or helpers (Ligon and Ligon 1978, Rabenold tersof the Audubon Society,the Frank M. Chapman 1985). Finally, it is possiblethat helping pro- Memorial Fund, the AmericanOrnithologists' Union videsopportunities for the learningof breeding JosselynVan Tyne Fund, and Lida Scott Brown and skills. I have observed helpers literally follow- OrganismicBiology Funds of the UCLA Department of Biology. ing in their parents'footsteps and manipulating discardednesting material, sitting next to and Lrrm•vo• Crr•o watching their parents during nest building, being thwarted by breedersas they attempted ALrOC•Nr•R, R.D. 1974. The evolution of social be- to feed nestlingsinappropriate items, and jos- havior. Annu. Rev. Ecol. Syst.5:325-383. tling with the breeding female for the oppor- A•,rr•Nraq, E. 1980. The breeding successof the tunity to brood eggsand nestlings. Jackdaw Corvusmonedula in nesting cells. Ornis Fenn. 58:72-77. Extant theories regarding potential benefits AUSTAD,S. N., ANDK. N. RABENOLD.1986. Demog- do not offer an easy explanation for the ob- raphy and the evolution of cooperativebreeding servedsex bias in helping. Possibly,given their in the BicoloredWren, Campylorhynchusgriseus. greater role in nest building and early brood Behaviour 97:308-324. care (pers. observ.), females may benefit more Bm•NAUZ,J. C., ANr•J. D. LIC,ON. 1988. A study of from social learning. However, there is little the ecologicalbases of cooperativebreeding in evidence to support this idea. the Harris' Hawk. Ecology99:1176-1187. In conclusion,this study has demonstrated Braw'r,A. C. 1946. Life histories of North American that Western American Crows exhibit many of jays, crows and titmice, part II. U.S. Natl. Mus. the phenomena commonly observed in coop- Bull. 191. eratively breedingbirds: delayed breeding, de- BI•CK, C.T. 1941. Ecologicaland economicrelations of the Crow, with special reference to Illinois. layed dispersal,and helping at the nest by rel- Ph.D. thesis, Univ. Illinois, Urbana. atives. However, Western American Crows differ BROWN,J. L. 1974. Alternate routes to sociality in from other cooperativebreeders by exhibiting jays--With a theoryfor the evolutionof altruism obligatedelayed breeding, by showingvarying and communalbreeding. Am. Zool. 14:63-80. degreesof delayed dispersal,and by the strong BROWN,J. L. 1978. Avian communalbreeding sys- female bias in delayed dispersaland helping. tems. Annu. Rev. Ecol. Syst. 9:123-155. 618 CAROLEECAFFREY [Auk,Vol. 109

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