Dickcissel. Photo by G. Ronald Austing from the Audubon Collection/PR.

Is the Dickcissel a threatened species?

Does what the males and females eat on wintering grounds affect the breeding successof the species?

by Stephen Fretwell

Introduction hayfields are the typical . In Iowa, Nebraska, Kansas, Oklahoma, and Texas, HE DICKCISSEL(Spiza americana) is one of the most ubiquitousand abundant Dickcissels occur in weedy roadside edges summeringbird speciesof the tall and mid- where those edges border wheat, corn, or grass prairie states. In Illinois, it would ap- soybean fields. They also occur in substan- pear to breed in every clover field in the tial coloniesin alfalfa and clover hayfields, central part of the state. In Missouri, timothy in patches of ragweed near river bottoms,

Volume 31, Number 5 923 and in ungrazedpatches of prairie. On the the species'niche. This is presumedto result edges of its range, the species is rather in fewer males survivingthan females, leav- erratic and unpredictablein abundance.In ing more females than males. th•s article I want to showthat in spiteof its Applying this aspect of the hypothesisto apparent abundance, the Dickcissel could the Dickcissel reveals a discrepancy. In well be a threatenedspecies. Some species almost every observed situation, there ap- that have becomeextinct in the past typi- pears to be more male than female Dick- cally have been common, but erratic in winter cisselspresent. Richardffrench found this to occurrence, as is the Dickcissel. be true in Trinidad (1%7), and I verified •t there (Figure 1); I also found from 2 to 2.5 Introductionto the species--Breeding malesper female in Panamaand in Acariqua, Biology Venezuela. John DeGrazio of the Denver F•sh and Wildlife Center found similar ratios in Mexico (pers. comm.), and in various HEAugust.DICKCISSEL Its nestBREEDS sitefrom usuallyMay involves through places near Acariqua, Venezuela. In Cala- some dead or woody vegetation from the bozo, Venezuela, our data suggestedthat previous year, unless a clover or hayfield males and females might be about equally xs the nesting habitat. The nest is sited in a abundant, but these data were likely to be crotch or clump of this old material, con- biased (Fretwell ms a.). cealed by this year's green foliage, but not tied to the vegetationin any way. The eggs are pale blue. Both the eggs and young are the sole 'NMATEDvery commonMALE inDICKCISSELS the breedingare seasonalso responsibility of the female. The male is (Fretwell ms a.; Fretwell and Calver, 1970) polygynous, and is mated to up to eight Only in Texas in May are there more females in a season. The mated females females than males, but this is not the case xn the territory of any one male are always later in the season. In sum, it seems that dxssynchronousby about 4-5 days in their overall there is about one female for every nestingschedule (Zimmerman, pers. comm.); two males in the total population;contrary exadentlythe male takes his many mates one to the prediction of Selander'ssexual selec- at a time, attending each one until she is tion hypothesis. well into her nestingcycle, and then seeking Why are there so many male Dickcissels9 another. There may be a simple answer to this Male Dickcissels are both more brightly question.Dickcissels wintering in the tropics colored and larger than the female: 30 grams are similar to the polygynous blackbirds •n for the males, 25 for the females. North America in other respects besides sexual dimorphism and polygyny. Like the blackbirds, Dickcissels form giant wintenng Sexual selection and sex ratio roosts of several million . Great flights leave these roosts in the morning for nearby HioDICKCISSELfall into a atgroupfirst ofglance species appears with feeding grounds. Often, thesefeeding grounds polygynous mating systems (Zimmerman are rice and grain sorghumfields. 1966), discussed by Selander (1965) and Dickcissels on their wintering grounds Onans (1969). These speciesbreed in single indicate a preponderanceof males in crop layer environments (Verner and Willson (Fretwell and Shane, 1975). Sex 1966, 1%9), the males provide little or no ratios in crops averaged3 to 5 males per food to the young, and the males are sub- female, while natural grasslandshad either stantially bigger than the females. Selander an equal number of males and females, or (op cit.) argued that this difference in size more females than males. Crop are was due to the polygynous mating system. generally bigger than weed seeds (they are Males evolve a larger size because of size bred for size), and it is known that larger advantagesin the aggressiveinteractions be- sparrowseat larger seeds(Newton 1967). My tween males for mates, and in spite of the hypothesisis that recentincreases in plant- ecologic disadvantage of being too big for ings of these crops have provided an •n-

924 American B•rds, September,1977 Table 1. North-south trends in Dickcisselnesting success.

Rates of cowbird parasitism (per cent Rates of of nests affected) nest survival Location Author

47% 0 Nebraska Von Steen(1965) 52% -- Nebraska Hergenrader(1962) 60% 25% Kansas Zimmerman(1971) 31% 30% North Oklahoma Overmire(1962) 20% 45.5% SouthOklahoma Wiens (1963) 5% 50% North Texas Fretwell, Francis, and Shane (1974)

creased food supply for the larger male more females, the nesting successis higher Dlckcissels. If females are too small to eat Yet, how could the sex ratio affect nesting these seeds, these crop increases will not success? expand the females' resourcesand may ac- tually reduce them. Thus, the males, instead The Allee effect and the sex ratio of beingfood stressedbecause of their larger size, now find more to eat than the smaller RETWELLAND LUCAS (1970) considered females.The postulatedresulting high sur- he case of a species with an Allee reval of males leads to a sex ratio skewed effect in its breeding success.They demon- heavily in favor of males. This may well strated that at certain lower densities, in- have seriousimplications for the breeding creases in density resulted in an increase m successof the species. nestingsuccess. We found that such species should have an erratic breeding distribution Thus, certain habitats or geographicregions are predicted to be unoccupied in some Breeding successand sex ratio years, but densely occupiedin others, owing to very slight changes in population size A nestingNUMBER successOFSTUDIES in the ofcentralDickcissel areas Tabor (1947) and Emlen and Wiens (1965) of the species'range have producedalarming have documentedthe "erratic breedingrange data. Von Steen (1965), working in Nebraska, fluctuations" of the Dickcissel, suggesting found zero nesting success. Zimmerman that this species might well be under the found similar results in eastern Kansas in influence of an Allee effect. This is con- 1965 and 1966, but found 1967 to be a more sistent with the observation that females are successfulyear. Even in good years, Dick- more successfulin nesting where they are clssels succeededin fledging only about 20 more densely distributed. It remains to relate to 30% of eggslaid, sometimeslosing whole this to the sex ratio. nests, but often losing part of a clutch, Zimmerman(1971) has brilliantly shown owing to cowbird parasitism. that male Dickcisseldensities are set by the Farther south, Dickcisselsare moderately territorial behavior of the males. As the male successful(Table 1). densitiesare set at levels that do not vary In an attempt to account for the failures much, variationsin the overall population in the northern part of the range, I noted sex ratio (females/male)owing to differen- that in Kansas in 1967, when successwas not tial winter survival of the sexes leads to zero, the sex ratio was considerablyhigher. extensive variations in the densities of breed- Males seemednot to be especiallyabundant, ing females in a particular field. If there are but females were quite common, and most 3 females/male,and 1 male per hectare,there males were mated. In fact, most were will be 3 females per hectare, but when the polygynous.Also, in Texas, where success sex ratio drops to 1 female/3 males, still is consistentlyhigher, there are, as noted, with one male per hectare, there will be more females than males. So, where there are only 1 female in 3 hectares,or 0.33 females/

Volume 31, Number 5 925 40 SEX

RATIO 87:

MALE SIZE 3O 82

81 WL ß

2O 80

YEAR 19__

Figure 1. Sex ratio and male size. The data on this figure were largely gatheredby R. ffrench in Trinidad, I measuredthe last point. They show sex ratios averaging30 per cent females and decliningwith years. Also, male wing lengths increase over the same period. hectare. This is a nine-fold drop in female all in 1971. The resultsare plotted in Figure density. Thus, the lower the sex ratio 2. They fit in well with the hypothesized (females/male),the lower the female density. Allee effect, and seem to further confirm But the evidence suggeststhat the lower that at low densities female Dickcissels are the sex ratio, the lower the nesting success. not as successfulat breeding. This seems to imply that lower densities of females have lower rates of success, Mechanisms for breeding failure which is the critical aspect of an Allee effect. Thus, the Allee effect predicted from the distribution theory and the erratic HEaboutDISTRESSING the DickcisselASPECT rangeOf myin 1971 tour distribution appears to be confirmed, and is was the almost uniform nesting failure consistentwith the sex-ratio, nesting success throughout 40% of the area covered. In all correlation.There also appearsto be an Allee the northern states, females were much effect in male Dickcissels (Zimmerman, scarcer than males (about one male m 1971, p. 606, Fig. 9). three was mated) and, apparently due to the I wanted to further confirm the Allee Allee effects, females were only rarely found effect in females. As an index to nesting feeding young. What has affected the female success,I recorded the per cent of females Dickcissels in the north? Could it spread seen that were carrying food in nesting to Oklahoma and Texas where success was colonies. As an index to density, I took still high? the number of females seen per acre. I Zimmerman conducted his detailed studies obtained samplesfrom south Texas to South on the evolution of the breeding activity Dakota, and from western Kansasto Indiana, in Dickcissels (1966, 1971; also an unpub-

926 American Birds, September, 1977 lished A.O.U. presentation, 1969). He found 8O that the major sources of nesting failure in Dickcissels were cowbird parasitism and PREDATION nest predation. Zimmerman found that cow- RATE parasitismwas a major source of loss 6O early in the breeding seasonand that preda- tion was a major source of loss later. His evidence suggested that mid-June, when

Dickcissels start more nests than any other 40 time, was a relatively safe period, falling between the times of cowbird parasitism LOST and nest predation, I have found that nest predation in the Field Sparrow and in other old-field species is density dependent, so that when densities DENSITY FEMALES PER I00 e rise, nest predation rises also (Fretwell Figure 3. Density dependenceand nest mortahty 1972b). A summary plot of data from the The pointson this graphare from publishedstudies literature on Dickcissels(Figure 3) confirms of Dickcissels, where both densities and predatmn the density dependenteffect. Therefore, nest rates were measured. Four of the six points are from John Zimmerman's studies (1966, 1971) w•th predation cannot produce an Allee effect, oneby Fretwell, and oneby JanetHarmeson (Auk and female Dickcissels should not group 91: 348-359, 1974). togetherto escapepredation. anecdotal evidence to this effect in the Red- OWBIRDPARASITISM, onthe other hand, wingedBlackbird literature), so that the cow- is a likely candidatefor Allee effects. birds are unable to lay. Or the density of It is well known about the Red-winged Black- female cowbirdsmay be constrainedby other bird that nests in colonies are much less than the availability of nests, so that where parasitizedby cowbirdsthan isolatednests there are many nests, the cowbirds present (Friedman 1963: 128). If this effect is owing are simplytoo few to parasitizethem all to density, and not to habitat differences, That cowbird parasitism causes extensive then it clearly implies an Allee effect. A nest failure in Dickcissels is shown in F•gure colony of birds may be able to harass fe- 4, where losses owing to other factors male cowbirds (and Friedman reviews some (mostly unknown) averagemuch higher •n studies where cowbird related losses were high. When cowbirds are such a serious parasite that some female Dickcissels are driven to desert their nests, then other 35

30 problems are more severe as well. Thus it appears that cowbird parasitism couldproduce an Allee effect in Dickc•ssels At low densities,excessive numbers of cow- bird eggsare depositedin Dickcisselnests (Zimmerman1966, noted five cowbirdeggs •n one day in one nest, and eight eventually), Figure2. Visits to breedingareas were madethree leadingdirectly to desertion(about 19% of the weeks after the females had arrived and had started time) or to other sorts of failures (about nesting.Each female seenwas scoredfor the stage 25% of the time). As 50% of the nests of the nestingcycle that she had reached. Females were scoredeither as feeding nestlingsor fledglings, fall to predators, a population with heavy or beingin someother stageof nesting.The density cowbird parasitism should lose 93% of all was estimated from the number of females per acre. nests, as opposedto 50% without parasitism The per cent females that were sufficiently success- This is similar to the values in Figure 2 ful to be feeding young was computed, and com- In more detailed studies (Fretwell ms b) I pared to the density. Females at low densities appearedto be least successfulin rearing young. have confirmed these effects in both Red- (From Fretwell, 1972b). wingedBlackbirds and Dickcissels, and •n the Volume 31, Number 5 927 Other Losses birds and they suffer high rates of nest predation.

20. Prognosisfor the future--factors determining %nests the distributionof nestingsuccess

10. ET US ACCEPTthat the sex ratio of Dickcisselshas been distorted by plant- ing of sorghum, which feeds males but not females, and that this distortion in sex ratio Losses due to Cowbirds has led to the observed nesting failures %nesls througha low-densityAllee effect. The breed- Figure4. Effect of excessivecowbird parasitism on ing female density is lowered by the poor accidental nest losses. The data are from Zimmer- winter survival of females, which leads to man (1971, 1966)and plot lossesof unknown cause high cowbird parasitismand other associated against per cent nests deserted due to excessive nesting failures. cowbird parasitism. I have noted that these failures and the associatedsex ratio problems are largely processhave discovereda competitiveeffect confined to the northern•tier of states in between these two species, where Red- which the Dickcisselbreeds; Texas and Okla- wingeds attract both cowbirds and predators homa populationsappear to have more suc- to Dickcissel nests. These studies confirmed cessfulnestings (Table 1). I am concerned that density is a major factor in cowbird about whether this pattern of failure could parasitism, quite capable of producing an spread to the southern part of the range. Allee effect. They also introduce another This concern is fostered by the recent na- factor, however, the presenceof Red-winged ture of the present situation. Sorghum and Blackbirds. When Dickcissels nest with Red- rice plantings have increased substantially wlngeds,they are heavily parasitizedby cow- in the tropics in the last decade. In Figure

Table 2A. Geographicvariation in wing lengthsin wintering populations,after food movementsin mid-January. (See Figure 5)

Western edge Center Eastern edge of range range of range

Acariqua, Calabozo, Trinidad Trinidad Panama Venezuela Venezuela (ffrench)* 1972

Males (n,X)** 10 86.00 79 83.03 28 81.19 141 82.8 132 84.35 (SE) .63 .22 .38 .17 .17 Females (n,X) 5 77.9 37 76.10 74.05 74.7 32 76.34 (SE) .89 .33 .4 .23 .35

Table 2B. Geographicvariation in wing lengths (ram) in breeding males.

Western Kansas 84.0 (10) South Dakota 85.1 (8) Missouri 82.67 (6) Kansas Upland 83.3 (10) Kansas Lowland 82.6 (9) Oklahoma 81.5 (5) North Texas 81.2 (7)

* ffrench and I checked for bias in measurements by both measuring the same bird; we found no d•fference. ** n = sample size; • = mean wing length in mm; SE = standarderror of mean.

928 American Birds, September, 1977 1, I plot the sex ratio estimates in Trinidad A: TRINIDAD since 1959, showing the trend for more and more males in recent years. In order to know whether the distorted ,•, • / B=CALABOZO sex ratio in the north might appear in the • •. '•,: C:ACARIGUA south, we need to know what causes such a difference in the distribution in the first place. Why are the northern populationsand WINTERRANGE )••) not the southern ones affected? DICKCISSFL ) • • • Winter distribution I \ ?

rEgrounds.FIND AN DickcisselsANSWER on leavethe thewintering United States by the first of October (except for vagrant lingerers), and appear in Central America through October. By November the birds seem to have disappeared;very few are reported in that month. Those records that do exist which are representative of the Figure 5. Winter range of the Dickcissel. The large numbers typical of the wintering speciesis localized until mid-December somewhere grounds are from southeastern Venezuela, in eastern Venezuela. From this region, it spreads out as winter progresseswith reports from as far but this area has not been studied ade- north as Mexico. Most of the information is quately for Dickcissels. By mid-December anecdotal, and refers to major population move- the Dickcissels are back on the move, and ments. Isolated Dickcissels are reported from all grain depredationsin western Venezuela are regions north of Venezuela into the United States all winter long (A.O.U. 1957). reported then. Also, Richard ffrench de- scribes some December arrivals in Trinidad, to the east. These movements do not appear to be migrations, but are probably wonderings Trinidad, or Acariqua) are more like those searchingfor food (Fretwell ms. c). of males that breed in the northern United I saw no birds in Panama until early States (Table 2). Also, the sex ratio in January, and no big flocks until mid-January. Calabozo (femalesper male), was higherthan Local birdwatchers noted that this was the elsewhere (about 1:1). This may have been time that the birds normally arrived in this owing to a biased sample, since no birds region in late winter food flights. Pacora, were taken at a roost where the sexes are Panamais a thousandmiles west of Acariqua, randomly mixed. As noted earlier, females Venezuela, in western Venezuela (see map, occur more frequently in weeds, but males Figure 5). occur more frequently in crops. Shane, who But where are the Texas males in winter? collected the Calabozo data, had difficulty I believe that many are in central Venezuela, finding mature crop habitats, and saw most near a town called Calabozo (map, Figure birds in "green" fields. Only samplesfrom 5), where there is a large reservoir with watering areas or roosts are unbiased, and irrigatedrice plantings.Local peoplesay that we lack such data from Calabozo, so that Dickcissels abound there all winter. Our we are less sure of the sex ratio there. Calabozo samplewas in late March, and was It seems possible, however, that Texas- probably of the birds that apparently never Oklahoma Dickcissels winter in central Vene- made much of a food flight. Shane measured zuela, are smaller, and do not make exten- some birds there (Fretwell and Shane, 1975), sive food flights. and we found that the males had much shorter wings than males caught elsewhere. PPARENTLYIN FALL most Dickcissels In fact, Calabozo male wing lengthswere the wintering in the tropics migrate to same as those of Texas males while wing Calabozo, or points to the southeast in lengths of food movement birds (Panama, Venezuela. By December, some, but not all,

Volume 31, Number 5 929 of the birds are somehow motivated to fly advantage of it were the northern breeding out of this region. Probably food is the moti- birds. First, the northern breedersare prob- vating factor in thesemovements. In any case, ably bigger birds, obeying Bergmann's rule it is mostly long-wingedmales and females They do have larger wings and bills, sug- (see Table 2) that leave. Those birds that gesting a larger size. Zimmerman's physio- leave apparently wander east or west from logical studieson the Dickcissel(1963) imply Calabozo. No one knows how far these birds a cold sensitivity that might well be alle- eventually move; some seemto wander all the viated by large size. South Dakota has many way to Mexico. In early April, some, per- near freezingmornings in May when the males haps all, the Dickcisselsfatten, and in mid- arrive, yet Dickcisselsnormally can barely April migrate to the breeding grounds. It tolerate such temperatures. seems that the birds that never left Calabozo Larger -eatingbirds typically eat larger breed in Texas, while the males that had seeds. My analysis of Tom Quay's data in wandered off on foraging travels migrate on North Carolina (Fretwell 1972b) provides an to Nebraska, Iowa, South Dakota, and other intraspecificproof of this point in Savannah northern mid-western states. The Calabozo Sparrows. The idea is also well established population seems not so crop dependent, between species(see Newton 1967). Thus the and so the sex ratio is not so skewed. larger and, coincidentally, more .northern Hence, when they breed in Texas, they Dickcisselswere the ones to take advantage are successful.Only the wandering ecotypes of the crops. And only the males of this that feed on crops have too many males population were large enough to be success- and dispersedfemales. ful in the exploitation of crops. Fretwell We do not have very many measurements and Shane (1975) provide further evidence of females from these northern states because for this relationship. females are so scarce. So we cannot con- e DONOT I•NOW whether the northern finn the trends that we see in males. Texas females followed them on the food females are similar in size to Calabozo fe- flights for crops or whether they stayed males somewhat smaller than either Trinidad behind--probably some of each; but, in or Panama or western Venezuela females. either case, it does not matter. The females The females in the outlying regions of the did not survive as well, and thus the sex wintering grounds do not show increasesin ratio was distorted and the breedingfailures size nearly as much as the males (Table 2). followed. Smaller males, normally the more southern breeders, avoided these food wan- Stability of the system derings, and they probably survived even PPARENTLY,THESEX RATIO distortion less well than the females. As a result, is rather recent (Fig. 1), and is likely surviving small males found many mates to be gettingworse as more and more crops available in Texas, enabling this• population are planted. A mitigating factor is the clear- to breed successfully. ing of forest land in the tropics. Such The stability of this system in time there- cleared land, as it grows up in weeds and fore, depends upon how genetically fixed grass, apparently is beneficial to female, but are the migration patterns of different seg- not to male, Dickcissels. However, much ments of the populations.What would happen land is cleared for grazing, and I have not if large, northern males stopped in Texas to foundDickcissels wintering in grazedhabitats breed? If they were able to do so, they (Fretwell, 1972a). would increase the male/female ratios with How stable is the present breedingsitua- unfortunate results. Clearly any large male tion9 The winter-eruptive population goes that stops in Texas (and many do--the on north to breed but fails at breeding,while range of measured male wing lengths in the winter-stable(Calabozo) population stops Texas is 77 to 85 mm, the average of eight in Texas and Oklahoma, where it breeds males in South Dakota was 85.1 mm) successfully.Can this imbalancepersist? should be a successfulbreeder. Being bigger, We may reasonably suppose that when he should get a better territory in these sorghum and rice first became widely avail- southern regions, shouldhave more females able the Dickcissels that were able to take because there are fewer males to compete 930 American Birds, September, 1977 Dickcissels, male above, from a painting by J McAleavey with, and, until the dilution effect predomi- Table 3. Changes in the distribution of Dickcissel nates, his mates should have more young. males, 1967 and 1968. Enoughgenerations of this sortof "accident" Northern Tier Southern and large Dickcisselswill become common States States in Texas. Then the Texas population will llli- Ne- Okla- be taken over by the crop-type population nois Iowa braska homa Texas and will suffer breeding loss accordingly. Perhaps (Table 3) these trends are already 64* 99 54 1967 33 6 underway, and revealed in data published 44 86 37 1968 41 21 by Robbinsand Van Velzen (1969). * Birds per survey route. Summary Friedman, H. 1963. Host relations of the parasitic cowbirds. Bull. U.S. Natl. Museum 233:1-276 Harmeson, Janet. 1974. Breeding ecology of the I gardedSUBMITas THAT a threatenedTHE DICKCISSELspecies (Blue be List).re- Dickcissels. Auk 91:248-359. Although my interpretation of the population Hergenrader, G. L. 1962. The incidence of nest regulationof this speciesis still largely ten- parasitismby the Brown-headedCowbird (Molo- thrus ater) on roadsidenesting birds in Nebraska tat•ve and theoretical there does seem to be a Auk 79:85-88. real and tangible possibility that Dickcissels Johnston, R. F. and R. K. Selander. 1964. House have a low-density limit. When the females sparrows: rapid evolution of races in North of the species fall much below this limit America. Science 144:348-550. over most of the range, then we may expect Lack, D. 1966. Population Studies of Birds Clarendon Press, Oxford. 341 pp. the speciesto fade into . Already, Orians, G. H. 1969. On the evolution of mating the females frequently fall below this limit, systems in birds and mammals. Amer. Nat and apparently the factors that have caused 103:589-603. th•s decreaseare recent and increasing. Overmire, T. G. 1962. Nesting of the Dickcissel in Oklahoma. Auk 79:115-116. References Newton, J. 1967. The adaptive radiation and feed- ing ecology of some British . Ibis Emlen, J. T. and J. A. Wiens. 1965ßThe Dick- 109:33-90. c•ssel invasion of 1964 in Southern Wisconsinß Robbins, C. S. and W. T. Van Velzen. 1969 PassengerPigeon 27:51-58. The Breeding Bird Survey, 1967 and 1968 ffrench, R. P. 1968. The Dickcissel on its winter- Washington,D.C. Bureau of Sport Fisheries •ng grounds in Trinidadß The Living Bird 6: 123-140. and Wildlife, SpecialScientific Report, 124. Fretwell, S. D. Ms a. Sexual Selection and Sex Selander, R. K. 1965. On mating systems and sexual selection. Amer. Nat. 99:129-141. Ratios in the Dickcissel: a test of Selander's Tabor, R. D. 1947. The Dickcissel in Wis- hypothesis. consin. PassengerPigeon 10:39-46. -- Ms b. Competitive effects on the breeding grounds between Red-winged Blackbirds and Verner, J. and M. F. Willson. 1966. The influence D•ckcissels. of habitats on mating systems of North Amen- can passerinebirds. Ecology 47:143-147. -- Ms c. Winter ecology of the Dickcissel. Verner, J. and M. F. Willson. 1969. Mating 1972a. The regulation of bird populations on Konza Prairie: the effects of events off of systems,sexual dimorphism and the role of male the Prairie. Third Midwest Prairie Conference North American passerinebirds in the nesting cycle. Omithol. Monogr. No. 9. Proceedings,L. C. Hulburt, ed. Von Steen, D. A. 1965. A study of nesting -- 1972b. Populations in a seasonal environ- Dickcissels in Nebraska. Neb. Bird Rev. 33 ment, Princeton, N.J. Princeton Univß Press 22-24. 217 + xxiii. Wiens, J. A. 1963. Aspects of cowbird parasitism Fretwell, S. D. and J. S. Calver. 1970ß On terri- in southern Oklahoma. Wilson Bull. 75:130- tonal behavior and other factors affecting habitat 139. d•stnbution in birds. III. Sex ratio variation in the Dickcissel. Acta Biotheoretica 19:37-44ß Zimmerman, J. L. 1963. The bioenergeticsof the Dickcissel,Spiza americana. Ph.D. Dissertation, Fretwell, S. D. and H. L. Lucas. 1970. On Univ. of Illinois. territorial behavior and other factors affecting habitat distribution in birds. I. Theoretical --. 1966. Polygyny in the Dickcissel. Auk 83:534-546. developmentßActa Biotheoretica 19:16-36. ß 1971. The territory and its density de- Fretwell, S. D. and T. G. Shane. 1975. Ecotypic pendent effect in Spiza americana. Auk 88 variation in wintering Dickcissels.EBBA News. 591-612. 38 125-128ß Fretwell, S., Margaret Francis, and T. Shane. --Division of Biology, 1974 Dickcissels nesting in Mesquiteß Texasß Kansas State Univ , Om•th Soc Bull 7'5-6 Manhattan, KS 66502 932 American B•rds, September,1977