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

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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 in- cluding 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 inter- sexual 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 in- dividuals 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 de- termine 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 con- specific 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.
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