Territory Establishment in Red-Winged Blackbirds: Importance of Aggressive Behavior and Experience

Condor, 81:258-264 0 The Cooper Ornithological Society 1979

TERRITORY ESTABLISHMENT IN RED-WINGED BLACKBIRDS: IMPORTANCE OF AGGRESSIVE BEHAVIOR AND EXPERIENCE

KEN YASUKAWA

In this paper I assess the importance of most intense when males attempt to acquire aggression in acquisition of initial breeding their initial territories. Competition be- territory by male Red-winged Blackbirds tween males occurs primarily among birds (Agelaius phoeniceus). The ability to ac- in fully adult plumage, that is, birds who are quire territory is expected to depend upon at least two years old. The existence of con- that portion of male quality determined pri- siderable competition for territories is in- marily by intrasexual selection. dicated by: rapid replacement following re- Darwin (1859, 1871) formulated the the- moval (Orians 1961); high conspecific male ory of sexual selection to account for certain trespass rates (Peek 1971); and defense of apparent exceptions to his concept of natu- suboptimal areas (Orians 1961, Case and ral selection, but evolutionary biologists Hewitt 1963, Robertson 1972, 1973). now realize that sexual selection is a special Male Red-winged Blackbirds establish case of individual selection relating to the territories by taking over unoccupied areas, evolutionary implications of competitive intruding between occupied areas and ex- mating. Most zoologists agree with Darwin panding, or displacing established residents that sexual selection has two phases: intra- (Nero 1956). As defined here, males are sexual, usually in the form of competition “successful” when they possess suitable between males; and intersexual or epigam- nesting sites and are not displaced by other ic, usually in the form of female choice of males prior to the onset of nesting by fe- a mate (Selander 1972, Emlen 1973, Brown males (regardless of whether they acquire 1975, Wilson 1975, but see also Mayr 1972). females themselves). In contrast, males who In practice it is difficult to separate the ef- are unsuccessful: (1) take possession of areas fects of non-sexual and sexual selection lacking suitable nesting sites; (2) are dis- (Mayr 1972) and to distinguish between in- placed from or desert suitable areas prior to tra- and intersexual selection (Selander the onset of nesting; or (3) take possession 1972). While the conceptual distinctions of suitable areas but only as replacements have been valuable, these difficulties have of successful males whose mates have com- limited advances in the study of sexual se- pleted their nesting cycles. lection. I studied the aggressive tendencies of In a polygynous territorial species such as male Red-winged Blackbirds attempting to the Red-winged Blackbird, males control establish initial breeding territories. My hy- access to mates by defending resources es- pothesis that aggression determines the out- sential to females (Emlen and Oring 1977) come of competition for initial territories and control of a territory is necessary for was tested by comparing the sizes, ages, be- male reproductive success. Several features haviors and time budgets of successful and of Red-winged Blackbird reproductive ecol- unsuccessful males that lacked prior repro- ogy support the view that acquisition of ini- ductive experience on my study area. tial breeding territory is mediated primarily I assessed the aggressive tendencies of through the intrasexual phase of sexual se- these birds by using the relation between lection. Male Red-winged Blackbirds return male Red-winged Blackbird behavior and to the breeding grounds approximately aggression, as determined from my previous three weeks before females (Orians 1961, experiments with conspecific song playback Case and Hewitt 1963) and attempt to es- and male-mount presentation within the tablish territories. Returning males with territory (Yasukawa 1978). Aggressiveness prior reproductive experience are ex- is taken to be the tendency to approach, tremely faithful to their previous territories spend time near, and attack the mount. I (Nero 1956, Case and Hewitt 1963), and re- had found these to be positively correlated establishment occurs with little difficulty with rate of more intense Song Spread dis- (Nero 1956, pers. observ.). Fidelity to ter- play (the Song Spread is the most common ritory and the relative ease of re-establish- territorial display given by the male; Nero ment indicate that intrasexual selection is 1956, Orians and Christman 1968), with P581 TERRITORY ESTABLISHMENT IN RED-WINGED BLACKBIRDS 259

mean Song Spread intensity, and with pro- body weight (Power 1969). Exact ages were known for portion of time allocated to singing and de- males captured previously as yearlings. Ages of males fending the territory (Yasukawa 1978). The first captured in the definitive (as opposed to yearling) plumage were unknown, and a minimum age of two rate of less intense Song Spread display and years was assumed. the proportion of time spent foraging on the Differences between successful and unsuccessful territory were negatively correlated with males were tested using t-tests and Mann Whitney U- Red-winged Blackbird aggressive tenden- tests (two-tailed). Statistical significance was accepted at the 0.05 level. The angular transformation was ap- cies. plied to time-budget variables (proportions) and the natural log transformation was applied to wing length (Sokal and Rohlf 1969). Untransformed data and 95% STUDY AREA AND METHODS confidence intervals (CI) of the means are presented I analyzed data collected from a population of Red- where appropriate. winged Blackbirds at Yellowwood Lake (Yellowwood State Forest) in Brown County, Indiana. The birds in- habitated a marsh at the north end of this small (54 ha) RESULTS man-made lake. Cattail (T&a Zati~&olia),bulrush (Scir- Evidence consistent with the hypothesized pus didus) and bur reed (Sparganium eurycarpum) importance of aggression would be the fol- were the dominant emergents of the marsh. About one- half of the territories adjoined a frequently-mowed lowing. When compared with unsuccessful camping area. The remaining territories and the camp- males, successful males would: (1) perform grounds were surrounded by mixed deciduous forest Song Spreads of higher intensities at faster interspersed with large planted stands of pines, in- rates; (2) perform Song Spreads of lower including red (Pinus resinosu), Virginia (I.’ virginiano) tensities at slower rates; (3) produce a Song and Scotch pine (I.’ syluestris). Willows (S&x spp.) and planted bald cypress (Taxodium distichum) were com- Spread of higher mean intensity; (4) allocate mon at the marsh-forest and marsh-campground edges. more time for singing and defending the ter- Data had been gathered on this population during ritory; and (5) spend less time foraging on the six years prior to my study (W. H. Barnard and V. the territory. These predicted differences Nolan, unpubl. data); 54 males (color- and U.S. Fish and Wildlife Service-banded) with no known breeding between the two groups of males were not experience at the study area formed the sample. Of fulfilled and the data offer no support for these, 37 were successful and 17 were unsuccessful. the aggression hypothesis. All were at least two years old and in the definitive I found the following: (1) Successful plumage. males performed medium intensity Song To minimize confounding factors arising from intersexual interactions, analysis was limited to observa- Spreads at a significantly slower rate than tions conducted prior to each males’ first interactions did unsuccessful males (Table 1). (2) Suc- with a female. I conducted 15-min observations of in- cessful males performed incipient (least individuals daily in 1974, 1975 and 1976 between sunrise tense) Song Spreads at a significantly faster and 10:00 h. A random-number table was used to determine the order in which males were observed. Data rate than did unsuccessful males (Table 1). from the three years were pooled. Behavior and time- (3) Successful males tended to produce a budget variables (see below) were registered by hand Song Spread of lower mean intensity than on a flow chart marked in one-second intervals. A stop- did unsuccessful males. This difference was watch and shorthand system allowed both time and near statistical significance (P < 0.08; Table duration of activities to be recorded. Observations were aided by binoculars and a spotting scope. My 1). (4) Successful males spent less time sing- presence, 10 to 20 m distant, did not seem to affect the ing and defending the territory than did un- birds ’ behavior. successful males; however, this difference Terminology for display and behavior follows Orians was not statistically significant (P > 0.2; and Christman (1968). The Song Spread was divided into four intensity categories (incipient, low, medium Fig. 1). (5) Successful males spent signifi- and high) based on the degree of wing extension (Peek cantly more time foraging on territory than 1972, Yasukawa 1978). A rate per minute for each cat- did unsuccessful males (Fig. 1). egory and a mean intensity were calculated for each These results cannot be explained by dif- male, as were rates of Flight Song, total songs, alarm ferences in duration of observation time. calls, and trespass. A trespass occurred when a conspecific male perched within the subject males’ terri- Mean lengths of time the 37 successful and tory for at least one second. The date each male was 17 unsuccessful males spent on territory first sighted on the study area was noted and expressed during observation periods were 86.8 min relatively; the arrival of the first male of the year being k 4.71 (S.E.) and 96.2 2 5.40 min, respec- treated as day 1 of that year. Also determined were the proportion of time spent on the territory and, within tively; these periods did not differ signifi- time on territory, the proportions of time spent singing cantly (t = -0.336, P > 0.7). Nor can the and defending (including silent periods of alert perch- results be explained by differences in total ing), preening, resting (inactivity exceeding 30 s), for- song rate, trespass rate, date first sighted aging, and flying about the territory. (Table l), size (mean wing length for suc- Flattened wing length (Palmer 1976) was used to estimate body size. Geographic variation in Red- cessful males = 126.7 mm, 95% CI 125.8- vinged Blackbird wing length was found to be highly 127.6 mm; unsuccessful males = 126.0 mm, correlated with geographic variation in cube root of 95% CI 124.4-127.5 mm; t = 0.90, P > 0.3), 260 KEN YASUKAWA

100 of their territories, and all but four unsuc- r * cessful males defended areas that fell with- got l-l in territories of successful males in other 60 Successful years or other parts of the same breeding I- II 70 season. The two groups of males did not Unsuccessful differ significantly in size or age, and total 60 I foraging times seem similar. Much of the 50 time male Red-winged Blackbirds spend away from territory is allocated to foraging 40 (Orians 1961). If we assume that all time off 30 territory was spent foraging, then the per- * centages of total time spent foraging by the 20 two groups were quite similar: successful IO males = 26.3%, unsuccessful males = 28.9%. However, it is also possible that a

OT SI FO FL PR RE portion of time away from territory was used to explore and assess other potential terri- Time Budget Category tories (Martin 1974, Nolan 1978). FIGURE 1. Comparison of time budgets of success- As success in initial territory establish- ful and unsuccessful male Red-winged Blackbirds ex- ment does not appear to be related to ag- pressed as percentage of total time spent on territory gressive tendencies, successful males must (OT), and percentages of time on territory spent singing and defending (SI), foraging (FO), flying (FL), possess some other advantages in encoun- preening (PR) and resting (RE). The t-tests are based ters with other males. Observations made at on angular transformations of proportions of time in times other than the 15-min periods de- each category. Significant differences between the two scribed above indicate that prior experience groups (denoted by an asterisk) occurred in proportion of total time spent on territory (t = 2.09, P < 0.05) and (excluding breeding experience at the study proportion of time on territory spent foraging (t = 2.02, area, since all observed males lacked such P < 0.05). experience) can affect the location and success of attempted territory establishment. In comparison with unsuccessful males, a sig- or age (median minimum age for successful nificantly greater proportion of successful males = 2.4 yr; unsuccessful males = 2.3 males had been captured at the study area yr; Mann-Whitney U = 320, P > 0.9). in years prior to those analyzed in this paper Successful males also spent significantly (Table 2). Assuming equal probabilities of more time on territory than did unsuccess- capture, this suggests that successful males ful males (Fig. 1). While it is difficult to ac- had greater opportunities to acquire expe- count for this difference, because data are rience at the study area in earlier years. lacking on territorial food density and male However, since five unsuccessful males had activity while away from territory, it is un- also been previously captured, mere prior likely that differences in food resources or presence at the study area does not seem to energy requirements are responsible for have been sufficient to produce success. this result. All males foraged on at least part Twelve of the previously captured success-

TABLE 1. Comparison of behavior of successful and unsuccessful male Red-winged Blackbirds.”

Sucuessful(N = 37) Unsuccessful (N = 17) Male behaviors i + SE i ? SE t

Incipient Song Spread rate” 2.00 -r- ,187 1.35 + ,204 2.10* Low intensity Song Spread rate 1.67 2 .149 1.97 +- ,221 -1.14 Medium intensity Song Spread rate 0.514 ” .0776 0.817 ? ,134 -2.08* High intensity Song Spread rate 0.132 2 .0358 0.301 2 .0730 -0.90 Mean Song Spread intensityc 0.749 ” .0603 0.945 ” .0871 -1.83 Flight Song rate 0.128 ? .0156 0.116 ? .0276 0.40 Total song rate 4.44 * ,194 4.45 2 ,252 -0.01 Alarm rate 0.268 ? .0515 0.156 & .0551 1.31 Bill-up rated 0.285 + .0792 0.272 ? .0854 0.11 Trespass rate 0.0886 * .0109 0.0826 t .0135 0.32 Date first sighted on territorye 33.2 t 3.73 34.4 & 7.01 -0.17

a Significant differences between the two groups denoted by an asterisk, P < 0.05. b Rate = number per minute. c Sang Spread intensity categories based on degree of wing extension. d Sample sizes are 28 successfid and 13 unsuccessful. e Date expressed relatively; arrival of first male determines day 1. TERRITORY ESTABLISHMENT IN RED-WINGED BLACKBIRDS 261 ful males had first been caught as year-old TABLE 2. Comparison of previous captures of suc- birds one year before establishment, and cessful and unsuccessful male Red-winged Black- seven successful males briefly defended birds.” their eventual territories late in the pre- Capturedb Not captured vious breeding season after the original owners had disappeared. Experience before Successful 23 14 Unsuccessful 5 12 success gained by yearling males combined =x2= 4.97, P < 0.03. with late-season territorial behavior could ’ Captured in any year prior to year of observation. be advantageous in encounters between males in subsequent breeding seasons. ing males restrict their movements to a few DISCUSSION occupied breeding territories and establish Aggressive behavior appears to be an im- linear dominance hierarchies within these, portant element of initial territory establish- but do not reproduce. When a breeding ment in many birds. Social hierarchies in male dies, the vacancy is quickly filled by lekking grouse such as the Greater Prairie the dominant, formerly non-breeding, male. Chicken (Tympanuchus cupido) are the re- Experience within a territory seems neces- sult of differential social aggressiveness sary for its successful acquisition (Smith among the participating males. Robe1 (1972) 1978). Observations and experiments have asserted that selection for aggressiveness shown that location-specific experience has resulted from the reproductive advan- with females is important in determining tage accruing to high-ranking males. Ag- the site on which male Black Grouse at- gressive behavior was apparently important tempt to acquire territories, and in reinforc- in successive territorial shifts toward central ing fidelity of experienced males to their status on leks of male Black Grouse (Lyre- established territories (Kruijt et al. 1972). rus t&-ix; Kruijt et al. 1972). Aggressiveness Experience shortly after leaving the nest of territorial male Red Grouse (Lagopus appears to help determine the area and ter- 1. scoticus) toward neighbors was correlated ritory selected for first breeding in the Reed with territory size (Watson and Miller 1971), Warbler (Acrocephulus scirpaceus) and the and testosterone propionate implantations Sedge Warbler (A. schoenobaenus; Catch- indicated that aggressiveness was important pole 1972). Possibly related to this apparent in territory acquisition and expansion (Wat- link between youthful experience and son and Moss 1971). choice of territory is evidence that winter My results do not support the hypothe- dominance rank and breeding-season terri- sized importance of aggression but rather toriality are associated in Field Sparrows indicate that non-breeding experience ac- (Spizellu pusilk) and Carolina Chickadees quired a year or more before successful ter- (Purus curolinensis). Male Field Sparrows ritory establishment may produce delayed in winter flocks were dominant over their benefits. These benefits may take the form associates when these flocks foraged on of a relatively long-term site-specific advan- breeding territories of such males (Fretwell tage in intrasexual encounters and a reduc- 1968). Further, first-year male chickadees tion in the overt aggression necessary for with high rank in winter flocks acquired ini- success at the site of the experience. tial breeding territories within the flocks’ Experience can produce long-term ef- home ranges (Dixon 1963). fects, a finding that is indicated by data on Experience can also affect the amount several aspects of avian behavior. First, and and result of a males’ overt aggression, as most closely related to my results, location- suggested by experimental studies of grouse specific experience may be an important behavior. Removal of male Sharp-tailed determinant of success in territory estab- Grouse (Pedioecetes phusiunellus) holding lishment in other species. Young, non-ter- central territories on a lek resulted in an or- ritorial, male Carolina Wrens (Thryothorus derly, relatively predictable progression of ludovicianus) often trespass and behave centripetal movements without associated cryptically within territories of breeding changes in levels of aggression (Rippin and males. These young males can eventually Boag 1974). Increases in individual aggres- take over the territories trespassed upon siveness induced by implantation of testos- when vacancies occur (Eugene Morton, terone propionate were not sufficient to pers. comm.). A more complex form of male change the previously established status of territory acquisition has evolved in a Costa male Sharp-tailed Grouse on a stable lek Rican population of the Rufous-collared (Trobec and Oring 1972). Finally, simula- Sparrow (Zonotrichia capensis). Nonbreed- tion of ritualized combat based on game the- 262 KEN YASUKAWA

Year-old males that more closely resembled females were attacked less often than were darker yearlings. The level of aggressiveness exhibited by successful males may be related to the mode by which they established their territories. Of the 37 successful individuals studied, 11 acquired territories of males that had failed to return to the study area in spring, presumably having died during the non-breeding season; seven acquired territories of males that had returned in spring but had unexpectedly disappeared, proba- bly having died during the breeding season; 11 acquired territories that had apparently been deserted by other reproductively in- +-4--tt-2h experienced males, since most of the latter birds were sighted on the study area after leaving their territories (see below); seven Aggression inserted themselves at boundaries of early- FIGURE 2. Graphic model of the relation between established territories and acquired por- experience and aggressive tendencies. The curve rep- tions of these; and one displaced an old, re- resents the minimum levels of experience and aggres- productively experienced male, driving him sive tendencies necessary for successful territory establishment and defense. (1) Experience gained prior away with repeated attacks. The 29 males to initial territory establishment by male Red-winged that occupied vacant areas showed little ini- Blackbirds results in (2) a decrease in aggressive ten- tial overt aggression, as might be expected dencies exhibited during establishment. (3) Experi- since they were not resisted. Males that ob- ence gained after initial establishment results in (4) a further decrease in aggressive tendencies exhibited tained territories by insertion performed a during re-establishment in the second territorial year. behavior called “challenging” (Nero 1956), in which they elicited chase by territory owners. The seven challengers were suc- ory indicates that a previously established cessful simply by withstanding limited at- asymmetry can determine the outcome of a tacks by territorial males rather than by contest regardless of actual fighting abilities their overpowering or conspicuous aggres- (Maynard Smith 1974). siveness. Thus territory establishment un- The foregoing evidence indicates that ex- der circumstances normally accompanied perience prior to acquiring a territory can by fighting (Neros’ category 3) occurred be beneficial. The advantages apparently only once in 37 cases. held by experienced males could result The apparently lower aggressive tenden- from: (1) dominating competitors while cies and greater experience of successful holding late-season territories; (2) acquiring male Red-winged Blackbirds suggests a re- knowledge of specific areas; or (3) establish- lation between levels of overt aggression ing asymmetries (sensu Maynard Smith) im- and amount or quality of experience. Such portant in later confrontations. This sug- a hypothesized relation is shown in Figure gests that the late-season occupation of 2. As the level of experience increases, the deserted territories by male Red-winged minimum amount of overt aggression nec- Blackbirds is adaptive despite the low prob- essary for successful competition (for terri- ability of acquiring a mate at that time. The tories, mates, food, etc.) decreases. This immediate cost of this territorial behavior, graphical model predicts that Red-winged especially when it is performed by year-old Blackbird males returning to defend terri- males in yearling plumage, seems relatively tories established in previous years behave low. Territorial defense by male Red- less aggressively than males establishing winged Blackbirds decreases as the breed- and defending their initial territories. ing season advances (Peek 1971), and ter- Lack of experience could explain the 11 ritorial males frequently tolerate trespasses cases of apparent abandonment of territo- by yearling males (Nero 1956). Further, ex- ries by males that had first attempted to ac- perimental presentations to territorial Red- quire these sites. Variability of the Song winged Blackbirds indicate that the year- Spread display (Orians and Christman 1968) ling plumage reduces the probability of at- and the apparent relation between intensity tack by territory owners (Rohwer 1978). of this display and aggressive tendencies of TERRITORY ESTABLISHMENT IN RED-WINGED BLACKBIRDS 263 displayers (Yasukawa 1978) suggest the ex- liam H. Barnard, William A. Searcy, Donald E. Kroods- ma, Jared Verner, and an anonymous reviewer. The istence of an evolutionarily stable signaling Indiana Department of Natural Resources and the staff system in which undetected cheating is rare at Yellowwood State Forest allowed use of the study (Rohwer 1977). Unsuccessful males who area. This research was conducted in partial fulfillment desert could have been cheaters, i.e., birds of the requirements for the Ph.D. in the Department feigning higher aggression by performing of Zoology, Indiana University, and was funded in part by Grants-in-Aid of Research from Indiana University inflated displays. If these speculations are and Sigma Xi. correct, then lack of experience forced these males to cheat but they deserted their ter- LITERATURE CITED ritories when confronted by more experienced males. The apparent difference in ag- BROWN, J. L. 1975. The evolution of behavior. W. W. Norton, New York. gressive tendencies between successful and CASE, N. A., AND 0. H. HEWITT. 1963. Nesting and unsuccessful males could, therefore, result productivity of the Red-winged Blackbird in rela- from inflated signals produced by cheaters. tion to habitat. Living Bird 2:7-20. This alternate interpretation underscores CATCHPOLE, C. K. 1972. A comparative study of territory in the Reed Warbler (Acrocephulus s&pa- the eclectic and elusive nature of aggression ceus) and the Sedge Warbler (A. schoenobaenus). (see Wilson 1975). Future study of the J, Zool. Lond. 166:213-231. effects of experience should help to deter- DARWIN, C. 1859. On the origin of species by means mine its important variables and its rela- of natural selection. John Murray, London. tionship to aggression, territory establish- DARWIN, C. 1871. The descent of man, and selection in relation to sex. John Murray, London. ment, and sexual selection. DIXON, K. L. 1963. Some aspects of social organization in the Carolina Chickadee. Proc. XIII Int. Omi- SUMMARY thol. Congr. (1962):240-258. The importance of aggression in initial ter- EMLEN, J. M. 1973. Ecology: an evolutionary approach. Addison-Wesley, Reading, Massachusetts. ritory establishment was investigated by EMLEN, S. T., AND L. W. ORING. 1977. Ecology, sexual comparing sizes, ages, behaviors and time selection, and the evolution of mating systems. budgets of two groups of male Red-winged Science 197:215-223. Blackbirds. Fifty-four males believed to be FRETWELL, S. 1968. Habitat distribution and survival seeking their first breeding territories were in the Field Sparrow (Spizellu pusilla). Bird-Band- ing 39:293-306. classed as successful or unsuccessful and KRUIJT. J. P., G. L. DE Vos, AND I. BOSSEMA. 1972. predictions, based on the hypothesized im- The arena system of Black Grouse. Proc. XV Int. portance of aggression, were tested. Results Omithol. Congr. (1970):399-423. did not support a hypothesis that predicts MARTIN, S. G. 1974. Adaptations for polygynous that successful males are more overtly ag- breeding in the Bobolink, Dolichonyx oryzioorus. Am. Zool. 14: 109-l 19. gressive than unsuccessful males. MAYNARD SMITH, J. 1974. The theory of games and Successful males relied on less intense the evolution of animal conflict. 1. Theor. Biol. display and spent more time foraging while 473209-221. on territory than did unsuccessful males. MAYR, E. 1972. Sexual selection and natural selection, D. 87-104. In B. G. Camnbell led.1. Sexual selec- These results were contrary to those pre- Fion and the descent of man i87i-1971. Aldine, dicted by the aggression hypothesis and Chicago. could not be explained by between-group NERO, R. W. 1956. A behavior study of the Red- differences in length of observation, singing winged Blackbird: II territoriality. Wilsoli Bull. frkquency, trespass rate, age or size. In ad- 68: 129-150. NOLAN, V. 1978. Ecology and behavior of the Prairie dition, successful males spent a greater pro- Warbler Dendroicu discolor. Ornithol. Monogr. portion of time on territory than did unsuc- 26. cessful males. ORIAPITS,G. H. 1961. The ecology of blackbird (Age- I suggest that experience gained prior to la&s) social system. Ecol. Monogr. 31:285-312. ORIANS, G. H., AND G. M. CHRISTMAN. 1968. A com- attempted establishment is important. A parative study of the behavior of Red-winged, Tri- greater proportion of successful (than un- colored, and Yellow-headed blackbirds. Univ. Cal- successful) males was captured at the study if. Publ. Zool. 84:1-85. area at least one year before they gained ter- PALMER, R. S. 1976. Handbook of North American ritories and mates. Experience may help birds. Vol. 2. Yale Univ. Press, New Haven. PEEK, F. W. 1971. Seasonal change in the breeding territory acquisition while reducing overt behavior of the male Red-winged Blackbird. Wil- aggression. A potential function for late-season Bull. 83:383-395. son territorial behavior and territoriality of PEEK. F. W. 1972. An exDerimenta1 studv of the ter- yearling males is thus indicated. ritorial function of vocal and visual display in the male Red-winged Blackbird (Ageluius phoeni- ACKNOWLEDGMENTS ceus). Anim. Behav. 20: 112-118. POWER, D. M. 1969. Evolutionary implications of wing and size variation in the Red-winged Black- It is my pleasure to acknowledge the assistance of Val ._ . . . Nolan Jr., J. Merritt Emlen, Cindy B. Patterson, Wil- bird in relation to geographic and climatic factors: 264 KEN YASUKAWA

A multiple regression analysis. Syst. Zool. 18:363- sparrow: Adaptive strategy for floaters. Am. Nat. 373. 112:571-582. RIPPIN, A. B., AND D. A. BOAG. 1974. Social organi- SOKAL, R. R., AND F. J. ROHLF. 1969. Biometry. W. zation among male Sharp-tailed Grouse on arenas. H. Freeman, San Francisco. Can. J. Zool. 52:591-597. TROBEC, R. J., AND L. W. ORING. 1972. Effects of tes- ROBEL, R. J. 1972. Possible function of the lek in reg- tosterone propionate implantation on lek behavior ulating tetraonid populations. Proc. XV Int. Omi- of Sharp-tailed Grouse. Am. Midl. Nat. 87:531- thol. Congr. (1970): 121-133. 536. ROBERTSON, R. J. 1972. Optimal niche space of the WATSON, A., AND G. R. MILLER. 1971. Territory size Redwinged Blackbird (Agelaius phoeniceus) I: and aggression in a fluctuating Red Grouse popu- nesting success in marsh and upland habitat. Can. lation. J. Anim. Ecol. 40:367-383. J. Zool. 50:247-263. WATSON, A., AND R. Moss. 1971. Spacing as affected ROBERTSON, R. J. 1973. Optimal niche space of the by territorial behavior, habitat and nutrition in Red Redwinged Blackbird: spatial and temporal pat- Grouse (Lagopus 1. scoticus), p. 92-111. In A. H. terns of nesting activity and success. Ecology Esser [ed.], Behavior and environment: the use of 54: 1085-1093. space by animals and man. Plenum Press, New ROHWER, S. 1977. Status signaling in Harris Spar- York. rows: some experiments in deception. Behaviour WILSON, E. 0. 1975. Sociobiology: the new synthesis. 61:107-129. Belknap Press of Harvard Univ. Press, Cambridge, ROHWER, S. 1978. Passerine subadult plumages and Massachusetts. the deceptive acquisition of resources: test of a YASUKAWA, K. 1978. Aggressive tendencies and levels critical assumption. Condor 80:173-179. of a graded display: factor analysis of response to SELANDER, R. K. 1972. Sexual selection and dimor- song playback in the Red-winged Blackbird (Age- phism in birds, p. 180-230. In B. G. Campbell laius phoeniceus). Behav. Biol. 233446-459. Led.], Sexual selection and the descent of man 1871-1971. Aldine, Chicago. The Rockefeller University Field Research Center, SMITH, S. M. 1978. The “underworld” in a territorial Tyrrel Roud, Millbrook, New York 12545. Accepted for publication 5 September 1978.