Territory, Breeding Density, and Fall Departure in Cassin's Finch

TERRITORY, BREEDING DENSITY, AND FALL DEPARTURE IN CASSIN'S FINCH

FRED B. S^•rSON

WHETHERterritory during the breeding seasonacts as a mechanism to regulate breeding density or total numbers within a population of birds has aroused much interest (Hinde 1956, Wynne-Edwards 1962, Lack 1966, Brown 1969, Klomp 1972, Von Haartman 1972, Watson 1973), but controversyremains. Kluyver (1970), after studying Great Tit (Parus major) populationson the Dutch Island of Vlieland in the North Sea, indicatedthat tit numbersare regulatedin early autumn after breeding. In the presentinvestigation, different populationsof Cassin'sFinch (Carpodacuscassinii) were studiedduring the breedingseason and early autumn of 3 years, to determinefactors influencing breeding density and population numbers. I selectedCassin's Finch to study becauselittle is known of its breeding biology or postbreedingbehavior. Published descriptionsof reproductive activities are few, most often by residentnaturalists or those engagedin faunistic surveys (Ridgway 1877, Henshaw 1879, Grinnell 1908, Taylor 1912, Dawson 1923, Bailey 1928, Grinnell and Miller 1944• Jones and Baylot 1969), presumablybecause accessibility to montane breeding populationsis limited. Fall studies are restricted to noting numbersseen or distributionallists (Orr 1968). The breedingrange for Cassin'sFinch extendsfrom southernBritish Columbia,Alberta, and Manitoba into northernArizona and Baja Cali- fornia (Bailey and NeJdtach 1965). These authors suggestedthat Cas- sin's Finch, like the crossbill(Loxia spp.), breed whereveran abundance of food exists within their range, with specificlocations changing from year to year. For breeding, Cassin'sFinch prefers a mixed forest habitat of the alpine meadow zone generally above 1500 m (Salt 1952). Most nests are located on ends of upper branchesin tall pines (Orr 1968), and pairs may nest in coloniesor singly (Lack 1968). After breeding, Cassin'sFinch movesto lower elevations,with congregationsas large as 5000 reported in December (Chapin 1958). Two plumagesare evident in Cassin'sFinch. All femalesand males until approximately14 monthsold have a streakedgray-brown plumage, but they are separableby winglength. Older males are mainly reddish pink and are easily distinguished.

STUDY AREAS AND METIKODS I investigated Cassin'sFinch's breeding biology principally near the Beaver Moun- tain ski area in the Cache National Forest, Cache County, Utah. This study area of

477 The Auk 93: 477-497. July 1976 478 FRED B. S^3•soxT [Auk, Vol. 93 approximately 250 ha involves a northeast-facingslope located in the Bear River Mountains 48 km northeast of Logan, Utah, at an elevation of 2225 m. Lodgepole pine (Pinus contorta), quaking aspen (Populus tremuloides), subalpine fir (Abies las•ocarpa),Engelmann spruce (Picea Engelmanii), limber pine (Pinus flexilis), and Douglas fir (Pseudosugamenziesii) form a discontinuousforest canopy. Meadows interspersedamong timbered areas are coveredwith sagebrush(Artemlsia trldentata), grasses(Bromus spp., Festuca spp., Agropyron sp.), sedges (Carex spp.), and other annual and perennial herbs. I also used a second tract of approximately 20 ha, 4.6 km southeastof Beaver Mountain, referred to as site 143. This tract is similar in altitude and vegetation to the Beaver Mountain site but is separated on all sides from surrounding forested areas by open shrubland. Snow commonly remains on the ground at both sites until late May with open bare patches of ground evident in mid- to late April. Snowstormsoccur into June (17 June 1973 and 6 June 1974). Finches were collectedin the spring months of 1972 and 1973 at three other locations with similar vegetation and elevation: (1) Amazon Hollow, 2.5 km southeast of Beaver Mountain; (2) Limber Pine, 8.6 km east; and (3) Summer Camp, 8.8 km southwest. The studies of finch breeding and fall departure were conducted from June to October 1971, February to October in 1972 and 1973• and April through June 1974. I visited the principal study area, Beaver Mountain, at least 4 days a week from July through August 1971, February through July in 1972 and 1973, and weekly in 1974. Other study areas were visited less frequently and only for specific purposes, such as collecting males. To investigatetotal populationsize and sex and age ratios, fincheswere captured systematicallyand banded with USFWS numbered aluminum bands. Nearly 50% of these I marked with unique combinationsof plastic colored leg bands permitting individual recognitionwithout recapture. I used mist nets at Beaver Mountain at a site within the study area, setting the nets once a week for 3 h, usually 0700-1000, during July 1971, from early May through July 1972, and from late May through early August 1973. Banding started in the spring when finches began to appear regularly on the banding site and stopped in late summer when no finches were being taken. During the winter months, the banding site served as a parking lot for a ski area, and salt was used to remove snow. The species'salt-eating habit (Henshaw 1879, Kelley 1921, Bailey 1928) attracted birds to the site after late May. Winglengths of birds netted at Beaver Mountain and site 143 were measured to the nearest 0.1 mm with a vernier caliper. Plumage color was describedfor all birds captured, and presenceor absenceof an incubation patch was noted. I collected44 finches in 1972 and 20 more in 1973 by shooting throughout April, May, and June to compare testicular development in adult and yearling males and to determine if females were part of spring flocks. In 1972 all collecting was at the Summer Camp site, and finches were shot as encountered. Finches in 1973 were shot as seen at the Limber Pine and Amazon Hollow sites. I used no systematic approach either year, but rather tried to make the collecting as random as possible. Events of the breeding cycle of Cassin'sFinch (i.e. song, courtship activities, territory, and nesting) were recorded by studying and describing individual or flock behavior, movements, and activities with observations of color~marked birds empha- sized. Field notes of location of events were transcribed later to maps of the study area for analysis; further details on methods will be presented in later sections. Common and scientific nomenclature for birds follows the A.O.U. check-list (1957) as emendedby the Thirty-second supplement (A.O.U. 1973); for mammals, Blair July 1976] Cassin'sFinch 479

TABLE 1

NU/•BERS OF CASSIN'SFINCH BANDEDAND RECAPTUREDAT BEAVERMOUNTAIN ANDSITE 143, U•AH•

May June July August Number Number Number Number Number recap- Number recap- Number recap- Number recap- banded tured banded tured banded tured banded tured

1971 38 1 33 1972 151 4 194 7 106 3 1973 19 25 7 56 4 12 All data from BeaverMountain exceptfor 57 birds bandedand 3 recapturedat site 143 in i971. et al. (1957); and for plants• Holmgren (1965). Data were analyzed by a Burrows 6700 computer utilizing statistical programs of Hurst (1972). Tolerance limits and Chi-square tests were calculated following Dixon and Massey (1969).

BREEDINGPOPULATIONS, TERRITORY, AND BREEDING •DENSITY This paper considersfour major aspectsof Cassin'sFinch breeding: (1) What were total numbersand sex and age ratios of finchespresent each summer?(2) Do all adult and yearling males breed or do differencesin their testicular developmentor dominance-subordinationre- lationshipsexist that limit their breeding? (3) Is delayedor restricted participationin breedingby femalesevident? (4) What effect does territory have upon breedingdensity? 1. Populationsize and sex and age ratios.--Population levels of finches varied from summer to summer. In 1971 at site 143 and Beaver Moun- tain, 71 fincheswere bandedduring 54 net h (6 days, 3 h/day, 3 nets) for a mean of 1.31 birds per net h. At Beaver Mountain in 1972, 451 fincheswere bandedduring 117 net h (13 days, 3 h/day, 3 nets) for a mean of 3.85 birds per net h; and in 1973, 112 finchesin 90 net h (10 day, 3 h/day, 3 nets) for a mean of 1.24 birds per net h. Table 1 givesnumbers banded and numbersrecaptured. Once caught, fincheswere wary of mist nets, and recaptureswere few in any summeralthough banded finches continued to use the bandingsite. To estimatethe percentof populationsbanded, proportions of banded to unbanded birds using the site over a 1-h period were recorded once weekly during the bandingseason in each of the three summers. Counts were made with 9 x 35 binocularsor a 20-powerspotting scope usually at distanceswithin 35 m. From these observations,mean monthly proportionsof bandedto unbandedbirds werecalculated (Table 2). Estimatesof total finch populationsduring each summerof study at BeaverMountain were obtained with a modifiedLincoln index (Lincoln 480 FRED B. SAMSON [Auk, Vol. 93

TABLE 2 MEANMONTI-ILY PROPORTIONS • ON TI•IEBANDING SITE AT BEAVERMOUNTAIN, UTAI•I

May June July

1971 15.1% 1972 19.1% 22.3% 19.4% 1973 12.1% 24.9% 23.4% Expressedas percent of banded to unbanded birds.

19'30), by dividing the mean proportion of marked birds visiting the bandingsite during a particular month into total banded prior to and during that month. If numbers of finches visiting the site during July 1971 and June 1972 and 1973 are estimated (95% confidencelimits), the populationin 1972, estimatedat 1548 (NL = 27, Nu = 285), exceededthat of 1973, estimatedat 177 (NL = 3, Nu = 33), and both are larger than the populationin 1971, estimatedat 93 (NL = 2, N•. 17). Confidencelimits on N were estimatedfollowing Chapman (1948). These estimatesand the numberof birds caughtper net hour suggestsubstantial changesin finch numbersfrom summerto summer. Farner (1952) also reported marked fluctuations of summer Cassin's Finch populations during eight summersfrom 1941 to 1952 at Crater Lake, Oregon. Two assumptionsof the Lincoln index are that the population sampled is well definedand that the averageprobability of observinga marked individual is equal to that of an unmarkedindividual. No exchangeof bandedbirds was evident during the three springsand summersof study between Beaver Mountain and site 143 and their surroundingterrain. This suggeststhat finch populations,including breeders and nonbreeders, may limit themselvesduring breedingand postbreedingperiods to certain topographicalregions and, therefore,are defined populations;but con- clusiveevidence is lackingas interchangeof finch flocksbetween Beaver Mountain and adjacent mountain terrain other than site 143, though possible,was not observed.The only changein behaviorof bandedbirds noted was in the segmentof the banding site used, always away from nets; and no change was evident until July when site use decreased. Thus prior to July the averageprobability of observinga markedbird is judgedequal to that of an unmarkedone. During the breedingseason female fincheshave an incubationpatch. The winglengthsof capturedgray-brown birds with an incubationpatch can be separatedfrom those of other capturedgray-brown birds (data pooled 1971, 1972, 1973; P < 0.001). Thus, on the breedingground, femalesdiffer from yearling malesby both winglength(see below) and possessionof an incubation patch. July 1976] Cassin'sFinch 481

TABLE 3 SEx RATIO8:•OR CASSIN'S FINCH AT BEAVERMOUNTAIN AND SITE 143, UTAH

Male to Adult Yearling female males males Females ratio •

1971 May June July 25 7 6 August 24 7 2 Total 49 14 8 63:8

1972 -• May 76 51 24 June 63 85 46 July 43 36 26 August Total 182 172 96 354:96 1973 :• May 12 4 3 June 19 6 July 38 14 August 6 4 2 Total 75 28 5 103: 5

TOTALS 306 214 109 520: 109

All proportionsvary significantlyfrom expected1:1 (P • 0.001). One brown bird escapedbefore its wing was measured and is excluded from the data. Four brown birds escaped before their wings were measured and are excluded from the data.

During June and July in the San BernadinoMountains in California Grinnell (1908: 89) reportedan imbalancein Cassin'sFinch sex ratios stating that "there seemsto be a large excessof males." In all three summersof this study,the ratio of near 5:1 (Table 3) in favor of yearling and older males to females varies significantly (P < 0.001) from the expected1: 1 ratio. This may not reflect the actual ratio, as most banding occurredduring the breedingseason when the malesfeed the incubating femalesor femaleswith young nestlingsat or near their nests. Females are inactive in comparisonto the foraging males and may be under- representedin May and June bandingsamples. Of 57 pairs seen building nests in 3 years of study, 54 (94.6%) involved older males. Data indicate that all older males (see section 2) and femalesof all ages (see section3) attempt to breed and, therefore, are presumedequally representedin breedingpopulations. By constructing a population wherein number of females of all ages equals number of older males captured (n = 306) and yearling males are representedby number caught (n = 214), femaleswould then representan estimated 37%, older males 37%, and yearling males 26%. These estimates are 482 FazD B. S•so•v [Auk, Vol. 93 significantlydifferent (P < 0.01) from an expectedsex ratio of 1:1. If sex ratios are adjustedusing banding data for each summerto reflect the as- sumptionthat femalenumbers equal thoseof older males,a significantdis- parity in sexratio wouldhave existedin 1972 (P < 0.001) and 1973 (P < 0.05) but not in 1971. In 1971 banding was not started until 6 July, which may have influenced banding samples; and, more importantly, no data were obtained to determine if all adult males obtained mates. My studiesshow that malesattain adult plumagewhen approximately 14 monthsold. Over the 3 years, yearling males represented41.1% of all malesbanded. In 1971 yearlingscomprised 22.2% of the males; in 1972, 48.6%; and, in 1973, 27.2%. 2. Breedingof males.--The study of male Cassin'sFinch breedingin- volvesthe relationship,both physiologicaland behavioral,between adult and first-yearmales and the extent of breedingor nonbreedingby these two groups. To determineif yearling males were handicappedphysiologically for breeding, first-year and adult male finches were collectedduring early spring 1972 and 1973 to comparetesticular development. In the spring of 1972,21 adult and 21 yearlingmales were collected at SummerCamp; and in the springof 1973, 6 adult and 6 yearling maleswere collected at Limber Pine and 6 adult and 2 yearling males at Amazon Hollow. Testes were immediatelyremoved, measured for length and width with a vernier caliper to the nearest0.1 mm, and placed in individual vials of Bouin'ssolution. Left testisvolumes were calculatedusing the formula for an ellipse,V ----4/3 a.b2. The rate (slope) of increasefor testiculardevelopment in first-year maleswas less than that of adult malesboth years (Fig. 1). In both years, testis volume of adult males exceededthat of yearling males. These differencesin testis volume suggestsome advantage for adult malesthrough earlier productionby the testesof steroidhormones that influencereproductive behavior, particularly for pair formation; but testicular developmentof yearling males, although slightly delayed particularlyin 1973,was adequate to allow them to breed. To determineif adult maleswere competitivelylimiting yearlingmales from breeding,three adult males,each a memberof a nestingpair, were shotin June 1973 (2 on 13 Juneand 1 on 17 June) at AmazonHollow. All three nestswere fully constructed,but it couldnot be determinedif clutcheswere complete. Finches were present in AmazonHollow during each springof studybut no estimatesof populationcomposition or size werepossible. Two of the threeadult malesremoved were replacedwithin a 3-dayperiod by yearlingmales and both nests were successful in raising young. The third nest was abandoned for reasonsunknown. July 1976] Cassin'sFinch 483

400 o •=18.85+5.24X o •=-87.21+5.57X r2 = 81.8 /• r2 = 82.1 ß '•'=2.578+5.26X ,,•/' ß •'=-61.72+3.87X r2 = 84.0 oo /•/ r2 = 46.4 300

200

100 • I ales ••earling males

April May June May June 1972 1973

Fig. 1. Testis volumes of adult and yearling male Cassin'sFinches.

In addition,two adult males with known histories,both paired and with nests completed,disappeared at Beaver Mountain in 1973. One adult, marked with a distinct leg-bandcombination, disappeared 1 June and was replacedby an unmarkedfirst-year male within 3 days. The secondadult male, unmarkedbut paired to a distinctivelymarked female, was not observedon 9 June and was replacedby an unmarkedbrown male, also within 3 days. No surplusadult maleswere seenon or near this study area after their disappearance,suggesting that both died. Suchreplacement patterns of adult maleslost or removedfrom breeding populationssuggest they do limit the participationof yearling males in competitivebreeding. If not excluded,first-year males are both behaviorally and physiologicallycapable of breeding. The mechanismsof replacementare not clear. In 18 of 20 pairs watched building nests at Beaver Mountain in 1972 the male was adult. No adult males appeared unpaired beyond mid-May. In 1973 at Beaver Mountain, all nine pairs that built nests initially involved an adult male, and no single adult males were seen by late May. At site 143 in 1973, 13 of 14 pairs starting nestswere composedof an adult male and a female, and all adult males appeared paired by early June. At all 14 nestsinitiated at both sitesin 1974 the male of each pair was an adult. Several other observationssubstantiate that all adult males paired. At the onsetof nest construction,mated malesstop singingand do not use songto defendtheir territory. Weekly playbackof adult male song 484 FRED B. SAMSON [Auk, Vol. 93 in the springsof 1972 and 1973 failed to elicit any responsefrom territorial males. Male song is important in many avian speciesto attract unpairedfemales (Nottebohm 1972) and probably servesthis function in Cassin'sFinch populations. In 1972 the last nest was started 1 June, and no adult males were heard singing at Beaver Mountain after that date. Adult males at Beaver Mountain in 1973 stoppedsinging 16 June, and constructionof the last nest began 18 June. At site 143 in 1973, no adult malessang after 12 June, and the last nestlocated was started 10 June. If any adult male remainedunpaired, it is reasonableto as- sume that song would have continued. This was not observed; but numeroussingle, first-year males continuedto sing near the colony at Beaver Mountain until 9 June 1972 and 11 August 1973, and at site 143 until 17 August 1973, suggestingthat at both sites in these years yearling malesremained unpaired. These nonbreedingyearling males when not singing formed flocks of 7 to 50 birds and were seen regularly on or near the study area. Flock size varied with males leaving the flock to sing or forage at a particularlocation. No instancesof any defenseof a segmentof habitat or of another brown bird were evident. These flocks traversed the breedinghabitat without any interferenceunless they violated a male's territory. Estimation of total numbers forming these flocks was not possibleeven though they contained color-markedbirds both in 1972 and 1973. As data suggestthat all femalesand all adult malesbreed, these flocks were apparently composedentirely of yearling males. Estimates from bandingdata discussedabove indicate that yearling males repre- sent approximately 26% of the total finch population. 3. Breeding of females.•An important considerationis whether all femalesbreed. Data from three sources,(1) the possessionof incubation patchesby a group of gray-brownbirds exhibiting a discreterange of winglengths,(2) similarwinglengths of ageclasses of malesand females, and (3) compositionof nonbreedingfinch flocks, support the contention that all females breed. Winglengthsof all brownbirds with incubationpatches are significantly shorter (P < 0.001) than of those lacking incubation patches. This separationof all brown birds capturedinto two distinct groupsindicates nearly all femalesbreed. Second,at 95% confidence,95% of the winglengthsof first-year males are within a 3.8 mm span (91.9 -+ 1.9 ram). For adult males, the 95% tolerance limits with confidenceat 95% equals93.4 -+ 1.6 ram, for a toleranceinterval of 3.2 min. Thus for the two age classes,yearling males and adult males,winglength intervals extended from 90.0 to 95.0 mm or 5.0 min. The 95% tolerance intervals at 95% confidencefor femalesare 88.3 -+ 2.3 mm or 4.6 ram. This similarity July 1976] Cassin'sFinch 485 in distribution of winglengths,4.6 and 5.0 mm, between two age classes of males and all females suggeststhat two age classes,yearling females and adult females,were captured. As they possessedincubation patches, both yearling and adult femalesare presumedto have bred; and neither delayed maturation nor lack of participation in breeding by either age class of females was evident. Third, if somefemales were not breeding,they shouldhave been part of nonbreedingflocks. On 2 July 1973 I collectedfour birds at random from a nonbreedingflock of nine gray-brown finchesand four more from a similar flock on 6 July. All birds were collectedat site 143 and all were yearling males. This would be expectedif femalesof all ages were on nests or were with their mate caring for young. The probability of collectingeight consecutivemales is less than 1 in i0 • when considering only gray-brownbirds, of which yearling males are estimatedto repre- sent 27.2%. These data supportthe premisethat all femalesbreed. 4. Territory.--Conder (1949) described a "mated-female distance" for severalavian specieswithin which the male defendsthe female. The location,at least prior to nesting,is dependentupon the location of the female,which doeschange. Definitionsof territory that includea moving datum rather than a localizedtopographical area are, as pointed out by Hinde (1956: 342), usually excludedin the commonusage of "territory." Territory accordingto Tinbergen (1957) is the result of two distinct tendencies,site attachmentand hostility. He assertedthat, "Either can occur without the other." For Cassin'sFinch, territory is a mate-defendedarea with the male excluding other finch males from the vicinity of his mate by means of chasesor threat postures,the defendedarea being defined by female location. The female does centralize her activity at the nest site, but this "attachment" does not appear to become an integral part of the male territorial behavior. A male will readily leave the nest site at any time during the nesting seasonif the female does. In addition, if the female disappearsor is experimentally removed as two were on 18 June 1974, the male will not continue to defend the nest site actively nor attempt to attract a new female to the site. Rather, he abandonsthe nest site and presumablyseeks an unmated female elsewhere. Male territorial behavior involving defense of mate was not seen prior to nest site selectionin any spring even though finch arrival pre- cedednesting by nearly 3 monthsin 1972 and 3-4 weeksin 1973 and 1974. Presumedpairs involving an adult male and a brown bird were noted in these early spring flocks each year. On 9 April 1972, two of these pairs were collected at Summer Camp. In both instances, the brown birds were females, providing evidencethat at least for adult 486 FREDB. S;•so• [Auk, Vol. 93 males pair formation may occur either prior to or immediately after arrival on the breeding ground and while finches are still in flocks. As yearling males also were collectedin early spring of 1972 and 1973, flocksarriving on the breedingground were composedof females,yearling males,and older males. Males began defenseof mate when the female first soughta nest site. To measurethe defendedarea, female location and the point of male defensewere projectedfrom the forestcanopy to groundlevel, and intra- pair intervals were measured. In each breeding season,males extended their defended area from a mean radius of 4.1 m (n = 31, SD = ñ 0.91 m) during nest site selectionto a mean of 15.6 m (n = 35, SD = ñ 1.66 m) during late stagesof nest construction,which in many species of birds coincideswith the peak in copulatoryactivity. Differencesin length of defended radii were not evident between years, but the in- tensity of defensedid vary. After nest constructionthe defendedarea around the female decreased,and defenseceased when incubationbegan. Territorial behavior was particularly evident in the spring of 1972 at Beaver Mountain when the population level was high. That spring paired males invariably accompaniedand defendedtheir femalesagainst other males while the femalesforaged, selected the nest site, gathered nestingmaterials, and built the nests. In 1973 when the populationof fincheswas substantiallylower than in 1972, males always accompanied and defended females while foraging and during nest construction. In several instanceswhile the female gathered nest material, males left the nest site with the female, perchedin a tree near her and far closer to her than to the nest site, but did not join her on the groundas they did in 1972. This changein behavior pattern from 1972 to 1973 apparently was due to the presenceof fewer bachelormales on or near the colony,which reduced the pressureon mate defense. Whether springterritorial behaviorlimits densityof breedingpasserine birds has been testedexperimentally by the removalof territory holders by Orians (1961) and Krebs (1971). In both studies,most replacement birds were first-year individuals. In this study two of three adult males collected at Amazon Hollow were replaced by yearling males. Neither replacementmale exhibitedany defenseof mate. The lack of defense of mate by replacementmales would be expectedif clutcheswere complete, but this was not known. Yearling males also replacedtwo adult males that disappearedfrom the colony in 1973, each memberof a pair in which at least one member was marked. The replacementyearling male of nest 3 did defend the female while she completedher clutch, and I saw this ptiir copulating. I noted no defenseof mate by the other replacementmale. Both replacementmales did feed their females dur- July 1976] Cassin's Finch 487

o 1972 ß 1973 I 1974 20 m

Fig. 2. Cassin's Finch nest locations at Beaver Mountain, Utah. Stippled area includes pine-aspen canopy. ing incubationand the subsequentnestlings. This lossof two adult males and their replacementby two yearling males did not appear to influence the territorial behavior of other males. As one of the widowed females was marked, replacementwas not by a pair. Along with the existence of bachelorflocks, these data suggestthat somemales capable of breeding were preventedfrom doing so. As Brown (1969) pointed out, if total reproductionis to be limited by territorial behavior, some femalesmust be prevented from breeding. Data already presentedindicate that all females did attempt to breed and they were not replacedwhen lost or experimentallyremoved at nestsas maleswere, suggestingthat the mated-femaleform of behavior did not limit total reproductiveeffort in these finch populations.In 1972, 1973, and 1974, finches at Beaver Mountain nested in colonies usingrestricted areas of quaking aspen,lodgepole pine, and subalpine fir (Fig. 2). In eachyear, vacanthabitat similarto that usedfor nesting extendedon three sidesof the colonyfor distancesgreater than 200 m. At site 143, finch colonieswere in the samegeneral 8-ha area of mixed aspenand conifer during 1972, 1973, and 1974 breedingseasons with extensivevacant but similar habitat adjacent to or close to breeding sites. Attempts to separatenesting habitat from vacant similar sur- roundinghabitat by tree speciespresent, foliage volume, or height were not successful.Habitat did not limit colony size or restrict colony location. The availability of suitable habitat is evident in that the site used 488 FR•n B. SA•rSO• [Auk, Vol. 93

TABLE 4

IIq'IERNESI AND NESI-Io-CoLoNY CENTROID DISTANCES1

Beaver Mountain Site 143

Nest-to-colony Nest-to-colony Internest centroid Internest centroid Year distances distances distances distances

1972 n • 20 22.2 q- 11.1 rn 53.0 q- 33.9 rn 1973 n : 9 n = 14 36.1 q- 32.5 47.3 q- 31.2 34.4 4- 29.1 rn 49.9 4- 44.4 rn 1974 n • 6 n ---- 8 37.0 q- 20.7 29.6 4- 22.8 39.6 q- 22.8 40.3 4- 31.0

1 (• __• I SD). for colonial nestingat Beaver Mountain in 1973 and 1974 was available but not used in 1972, and the site used in 1972 was available to 1973 and 1974 nesters but not used. Mean internest distancesand mean distancesto colony centroid for all nestsinitiated were comparedfor the five coloniesstudied intensively (Table 4). No significantdifferences are evident amongthe mean internest distancesor nest distancesto colony centroid, suggestinga rather uniform nest dispersion pattern within finch colonies. Internest dis- tanceswere larger in 1973 and 1974 when the colony size was smaller. Nests were difficult to watch becausethey were usually on either the terminal end of upper branchesof tall lodgepolepines or subalpinefir or against the tree trunks within 1 m of the top. Close observationof the few accessiblenests was inadvisablebecause young finchesleft the nest before they were fully featheredif disturbed,causing needless mortality. Nest successwas measuredby whether or not pairs remained actively feeding young at the nest until the day the young left. At Beaver Mountain in 1972, 10 of 20 (50%) nestswere successful;in 1973, 7 of 9 (77.8%); and at site 143 in 1973, 9 of 14 (64.3%). No particularpattern of nest losswas evidentother than early nestswere lesssuccessful (Table 5). Height of nestswas not an influence. Two of three nestsinitiated by pairs involvinga yearling male were successful.These pairs all nested near the colony centroid. The male of the secondpair to start nest construction at Beaver Mountain and site 143 and of the eighthpair found building a nest at Beaver Mountain in 1972 were yearlings. Most nest constructionat Beaver Mountain and site 143 was spaced sufficiently,either temporallyor spatially,to precludeintense aggressive behavior. For those pairs attempting to build nests simultaneously within a limited segmentof habitat, territorialbehavior did temporally July 1976] Cassin'sFinch 489

TABLE 5

SUMiVI.ARY OF NESTING•

Beaver Mountain Site 143

1972 1973 1973

Number of nests 20 9 14 Height above ground ( 15 m 5 (2) 2 2 (2) 1 (1) > 15 m 12 (6) 7 (5) 11 (7) > 25 m 3 (2) 2 (1) Number successful 10 7 9 Number early nestslosff 5 2 4 Number peripheralnests lost 4 5 1 3 • 1974 nesting data excluded as five nests were experimentally removed. 2 Number successful. 3 Nests initiated before median date of nest construction. Beaver Mountain, 1972 - 26 May; 1973 = 9 June; site 143, 1973 - 8 June. 4 Outside mean distance to colony centroid.

spacenesters within a segmentof habitat.After completionof the clutch when defenseof female ceased,males assumeda "sentinel"position (Mundinger1971) at somedistance from the nest. Anotherpair could then nestwell within the originaldefended area of the first male. The male of the secondpair toleratedthe male of the first pair, traversing his defendedarea only when the first male was flying to his own nest to feed his mate and if he exhibited no overt aggressivebehavior. The secondmale never interfered with the female of the first pair. An ex- ampleoccurred on 28 May 1972when two females,both mated to adult males,had selectednesting sites 1 m apart. Vigorouschases and combat occurredover a 4-h period beforeone pair withdrew; 10 days later a pair alsowith an adult malenested 10.2 m awayfrom the victorof the 28 May contestand wasneither challenged nor interferedwith. Anotherexample occurred at site 143 in 1972 after the mate of an adultmale began building a nestnear the top of a lodgepolepine on 25 May. On the sameday the femaleof a yearlingpair selecteda nest site in the same tree but on a lower branch less than 2 m away. The adult male vigorouslychased one brown bird assumedto be the male for over an hour before both brown birds left the site. By 16 June two morenests had beenbuilt by pairswith adult males,each well within the originaldefended area of the first male. The temporalspacing of nests allowedfor increaseddensity of breedingpairs within a segmentof breedinghabitat, but did not limit the total breedingpairs in a colony.

FALL DEPARTURE After the breedingseason in all three summers,one or more family groupsof adultswith youngformed loose flocks of from 7 to 12 mem- 490 FRED B. SA•so>• [Auk, Vol. 93 bers. Theseflocks dispersed from colonysites to forage. They were hard to follow and watch exceptwhen they returnedto the bandingsite for salt. Observationsof color-markedbirds on the site and a few recaptures indicate that these flocks remainedin the general vicinity of Beaver Mountain or site 143 throughoutthe summerand early fall. What was evident in all three summerswas an earlier departure from the study areaby flocksof adult males,their mates,and youngthan by yearling male flocks. In 1971 adult males were last seen on 17 Sep- temberwhereas the flocksof yearlingmales remained until 2 October. In 1972, adult maleswere last notedon 16 Septemberwhile yearlingmale flocks remaineduntil 29 September. To verify if all membersof non- adult flocks were males, 17 birds were collectedat random at site 143 on 16 September. All were males molting into adult reddish plumage. In 1973 adult males,their mates,and youngleft the study area 22 Sep- tember and yearling malesdeparted 30 September.On 11 September I collected 6 males at random at site 143 from the remaining flocks. Utilizing the estimatedsex ratios, the probability of collecting17 con- secutive males in 1972 would be less than 1 in l0 s and less than 1 out of 1000 for the 6 males collectedin 1973, again supportingthe inter- pretationof essentiallyall male compositionfor theseflocks.

DISCUSSION AND CONCLUSIONS Findingsof this study that warrant discussionare (1) the disparity in sex ratio observedin Cassin'sFinch populationsand its effect on (a) reproductiveeffort, (b) mated-femaledistance, and (c) timing of fall departure;and (2) the fluctuationof finch numbersfrom one breeding season to the next. 1. D{spar{tyin sex ratios.--In the three summersof study, both the sex ratios observedfrom banding data and those estimated favored males. For 15,183 finchesbanded from 1955 to 1973 (data from Bird Banding Laboratory, USFWS), 39% were reported as males, nearly exclusivelyadult males,31.3% as females,and 29.7% as unknown. Some banders (Sheppard pets. comm.) failed to recognizemale dimorphism and reported ai] gray-brownbirds as females. These data must then be viewed with considerablecaution, but they do suggestsimilar per- centagesfor adult males and females in other populationsof Cassin's Finch to thoseestimated in this study. Disparities in tertiary sex ratio favoring males in many speciesof birds are not uncommon(Lack 196(5),but few data have been published for primary or secondarysex ratios in passetines.Morton eta]. (1972) found more males than femalesbeing hatchedin nestsof White-crowned Sparrows (gonotrich•aleucophrys oriantha) during 3 years of study, July 1976] Cassln'sFinch 491

this imbalancelasting through adult life. Disparityin sexratios favoring malesin limited samplesof juvenal Black Rosy Finches (Leucosticte atrata) shotby French(1959) suggestsmore males may havebeen pro- ducedin the nest,and their recruitmentmay have influencedthe im- balancefavoring males noted in adult sexratios. Data on secondarysex ratios obtainedin this study are limited. Two nests at Beaver Mountain and 3 at site 143 were removed in spring 1974. No data were obtained for Beaver Mountain because nest con- tents were lost during removal. Three nests were accessibleat site 143. Of the 13 nestlingsand 1 egg removed,8 were male, 5 were female, and 1 was unidentified.This sex ratio is not significantlydifferent from the expected1:1 ratio but doessuggest that further investigationinto sec- ondary sex ratio in Cassin'sFinch is needed. Estimates of annual mortality and recruitment that may have in- fluencedobserved tertiary sex ratios are difficult to obtain for Cassin's Finch as populationsappear nomadic to breedinggrounds. Of 14 finches banded in 1971 at Beaver Mountain, 6 (42.9%) were recaptured or seenthere in 1972. All 6 weremales and 5 of the 6 were in adult plumage when banded. Of 57 finchesbanded at site 143 in 1971, 7 (12.2%) were recordedthere again in 1972, and all were adult males when banded. In 1973, 6 adult males of 451 birds (1.3%) banded in 1972 were recaptured at Beaver Mountain. No birds banded in 1971 were noted in 1973. These data are insufficient to calculate annual mortality but suggestthat year to year site attachmentis strongestin adult males. la. Reproductiveeffort.--In somestudies, Ribaut (1964) on European Blackbirds (Turdus merula), Delius (1965) on Skylarks (Alauda ar- yensis),and Carrick (1963) on AustralianMagpies (Gymnorhina tibicen), femaleswere preventedfrom breedingby territorial behavior;but this was not found in Cassin's Finch. The mated-female distance did not re- ducereproductive effort by forcingpairs into marginalhabitat, as indi- cated for many species(see reviewsby Brown 1969, Klomp 1972, and Watson1973). The coloniesstudied were very similarto the generalized colonialpattern providedby Newton (1972) for the subfamilyCar- duelinaeand for membersof the subfamilyincluding Carduelis cannab•na by Tast (1970) and Carduelisflammea by Peiponen(1962) and Hilden (1969). Tast reportedasynchrony in nestingwithin coloniesand fluctuations in numbersof summerpopulations, both findingssimilar to what this study reportsfor Cassin'sFinch. Both bandingdata from all 3 yearsand the sexesof nonbreedingbirds shot at randomduring the breedingand postbreedingseasons indicate that surplusflocks were composedentirely of males. All femalesat- 492 FREt) B. SAMSOW [Auk, Vol. 93 tempted to breed, as shownby the discretedistribution of winglengths of individuals that had incubationpatches that included first-year and adult females. Unpairedyearling males,physiologically and behaviorally capableof breeding,expressed their readinessto breed beyondthe nesting season,but their participationwas limited by a shortageof females. Femalesalso were not replacedat nestsas maleswere. Polygyny was not observedand is not a characteristicof the subfamily Carduelinae (Lack 1968, Newton 1972). Nu•l•er of females is consideredlimiting for reproductiveeffort in this study. lb. Mated-femaledistance.--With femalesand not habitat the limiting factor, it wouldbe advantageousfor a male to defendits mate and not a fixed segmentof nestinghabitat. A mobiledefense of femalesimilar to that seenin this studywas described by Twining (1938) and French (1959) for the Black Rosy Finch, Johnson(1960) for the Gray-crowned Rosy Finch (Leucostictetephrocotis), Thompson (1960) for the House Finch (Carpodacusmexicanus), Marler and Mundiger (pers. comm.) for the Twite (Carduelisfiavirostris), and is generallycharacteristic of Carduelinefinches (Newton 1972). Somenest defensewhen it contained youngwas suggestedin the studiesof Johnsonand Thomlosonbut was not notedin this study. Males were estimatedto outnumberfemales by 5:1 or 6:1 by Frenchand 4:1 by Johnson,ratios that exceedthose estimated in this study. Both authorsconsidered number of femaleslimiting for breedingpoloulation size similarto the resultsof this study. Lackingsite attachment, certain functions of territory(i.e. knowledge of terrainto avoidioredation, to ensurean adequatesupply of foodfor young,or to facilitateioair formation) appear unimportant. Without an intimatefamiliarity to reducepredation within a territoryvia site attachment, Cassin'sFinch has reducedboth visual and auditory cues for ioredatorsthrough cryptic coloration of femalesand nests,lack of con- spicuousdisplays near the nest•and cessationof songat onsetof nesting. Femaleexposure to predationmay alsobe reducedwhen females are fed at the nestduring incubation and whenwith very youngnestlings. These ioatternsdo suggestthat predationis importantin femalemortality. Possiblepreclators at BeaverMountain includethe Goshawk(Accipiter gentilis) , Cooper'sHawk ( A. cooperil), Sharp-shinned Hawk ( A. striatus) , AmericanKestrel (Falco sparverius),Pygmy Owl (Glaucidiumgnoma), and red squirrel(Tamiasciurus hudsonicus), but no predationon free- flying or nestingfinches was actually seen. A Sharlo-shinnedHawk did selectan adult male from several adult males and brown birds caught in a mist net 8 June 1972, but this was the only instanceof predation on a Cassin'sFinch notedduring the breedingseasons. July 19761 Cassin's Finch 493

During nest constructionand when the female was completingher clutch, finch pairs invariably fed away from the nest and colony site. Distancesof feedingsites from the colonyvaried from 30 to 300 m, but were generallyat somedistance from the colony. When feedingfemales or young, males always foraged at some distance,usually more than 100 m, from the colonysite. Femalesfeeding young were seenforaging near the nest on several occasions,but generally joined their mates and fed away from the colony. The radius of defended area did not vary from year to year even thoughboth total numbersand colony size varied. Basedon the preceding,and becauseof the limited interval of preincubationexpression, the mated-femaledistance does not function to ensurean adequatefood supply for young. In many passerinespecies, males arrive beforefemales and establish territories. This did not occur in Cassin's Finch nor was any defense of site by a singlemale noted. Certain reproductiveactivities of Cassin's Finch, includingcourtship behavior and copulation,are limited only by the female's location and male and female interest. Courtship feeding and copulationwere both seenat considerabledistances, often exceeding 100 m, from nestsand colony site. Pair formation is not a function of or facilitated by the mated-femaledistance. Three functionsof the mated-femaledistance appear important: (1) The male gainsan advantagein ensuringthe passageof his genesto future generationsby defendinghis mate whereverher locationmay be whenshe is mostreceptive to fertilization. (2) The protectionprovided the female allows her to feed without interference from other males when energymust be mobilizedto build a nest and, more importantly,to complete productionof eggs. (3) The female is provided continuouspro- tection from predationby both the watchfulnessof the accompanying male and, if pairedwith an adult male, a potentiallymore visibletarget for predatorsto select. The finch breedingpopulation would benefit with each of thesefunctions. Adult maleshaving survivedat least 20 months providea geneticcontribution reflecting a capabilityfor resourcelocation and exploitation,predator avoidance,and possibleprior breedingex- periencethat yearlingmales may lack. For a breedingpopulation with number of femaleslimiting the reproductiveeffort, female survival is important, and the mated-femaledistance provides for this. lc. Timing of fall departure.--The importanceof numbersof females may also be reflected in the timing of departure frmn the study area. Each fall adult males, females,and young left the study areas prior to yearling males. Magee (1924) reported a similar differential fall departure for anotherNorth Americanmember of the genusCarpodacus, 494 F•x• B. S•rso• [Auk, Vol. 93 the Purple Finch (C. purpureus),when after a particular date in early fall• no adult maleswere capturedbut yearling finchescontinued to be caught. Peiponen (1974) reports that Scarlet Rosefinches(C. e. erythrinus) older than 1 year leave southernFinland beforejuveniles. Birds gain two advantagesby leaving earlier. If food suppliesafter breedingand molt are diminished,earlier departureallows exploitation of new food resourcesat placesaway from the breedingground in early fall. Secondthese flocks gain an advantagein locatingwinter food sup- plies and in familiarity with their location. If great distancesto winter- ing groundsexist as in the ScarletRosefinch, the earlier departurewould be beneficial (Peiponen 1974). Becauseof the differential departure• theseadvantages benefit survivalof females,adult males•and young in the early nonbreedingseasons. As no hostility was observedbetween Cassin'sFinch flocks of breeding birds with young and yearling male flocks,the reasonfor differentialdeparture is not entirely evident. Yearling male molt (duration of all molts estimated by regression analysis) was complete3 days prior to departureof adults and young in 1971 and 10 days prior in 1973 (Samson1974). Yearling male molt in 1972 extended4 days beyondadult and young departureand thereby may have delayedtheir departure. This is questionableas molt for adults is estimatedto have extendedbeyond their fall departure date in 1973. The differential departure may representa geneticallyinfluenced behavior selectedparticularly to favor female survival. This favorability may also be reflected in the female dominanceand associatedlow mortality found in Cassin'sFinch winter flocksduring 1972-1973 and 1973- 1974 (Samson1974). 2. Annual Jluctuationin breedingnumbers.--Cassin's Finch numbers fluctuated substantiallyfrom one breedingseason to the next. Other carduelinefinch populationsalso fluctuate from year to year, apparently in relation to food supply (Newton 1969, 1972). Contentsof digestive tracts from 58 of 61 finches collectedprior to and during the early nestingseason in 1972 and 1973 were over 94% by volume staminate buds of quaking aspen. No animal material was evident in any digestive tract. It is known (Graham et al. 1963, Bogdanov 1968) that aspen pollen productionvaries from year to year with an unusuallylarge pro- duction every third or fourth year. Conceivablyfinch utilization of region for breedingand lack of total annual fidelity to a breedingsite may be influencedby availability and changein aspen staminatebud pro- duction,but total Cassin'sFinch numbersin a breedingseason may be influencedby other factorsincluding winter food supply similar to the Bullfinch (Pyrrhula pyrrhula) (Newton 1964). July 1976] Cassin'sFinch 495

ACKNOWLEDGMENTS Appreciationis extendedto Keith L. Dixon for the helpful suggestionsand advice throughoutthe study. The use of equipmentand facilitiesof the Department of Biology,Utah State University, and that of Allen W. Stokesgreatly facilitated the research.Keith L. Dixon, Ian Newton, and Peter Marler improvedan earlier draft with constructivecomments. Special thanks are extended to Sue Samson for her assistancethroughout the study and in preparationof figures. The researchwas partially financedby grantsfrom SigmaXi and the ChapmanFund, AmericanMuseum of Natural History, New York, New York.

SUMMARY Breedingpopulations, the mated-femaledistance form of territorial behaviorand its effecton nestingdensity and fall departureof Cassin's Finch are described.Finch numbersvaried from year to year with an estimated 15-fold increase from 1971 to 1972 and a decreasein 1973 to near the 1971 level. Males significantlyoutnumbered females in 2 of 3 years,and the femaleis consideredthe limiting resourcefor reproductive effort. Male territorialbehavior, expressed as a mated-femaledistance, centers around the female and her location. It does not limit total breedingnumbers but may spacenesting pairs temporally. Finches nestedin colonies,and habitat did not appear limiting for colony size or location. Adults with young left the study area prior to yearling males in each of three summers.

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Departmentof Zoology,Ecoloey Center, Utah State University,Logan, Utah 84321. Presentaddress: Schoolof Forest Resources,The Pennsyl- vania State University,University Park, Pennsylvania16802. Accepted 31 January 1975.