Territorial Male Red-winged Blackbirds Research Articles

Avian visual ecologists should consider UV absorbance and all sensory modalities: A response to Wisenden et al. (2020)

In a recent publication, Wisenden et al. (2020) examined responses of territorial male Red-winged Blackbirds (Agelaius phoeniceus) to models constructed with ultraviolet (UV)-reflective red epaulets for the purpose of determining if the addition of UV reflectance to epaulets (“UV+”) changed the effectiveness of signals to receivers relative to “control” epaulets under field conditions. The authors hypothesized that “UV+epaulet coloration represents a visual signal with increased efficacy in territorial interactions.” They presented behavioral data but no visual modeling data. Our aims in this commentary are to suggest alternative terms to those used by the authors, to express concern about the use of sunscreen to manipulate the UV condition of surfaces, and to make a plea for additional data collection in future studies of avian visual ecology. The terms UV+ and UV– should be reserved for studies that create environments free from UV radiation for comparison with environments that include UV radiation. We believe that commercial sunscreens are not an appropriate choice for altering the UV conditions of surfaces presented during behavioral trials because they potentially introduce confounding influences from other sensory inputs or irritation of peripheral nerves. Wisenden et al. altered the UV absorbance of their sunscreen-treated models but did not present absorbance spectra and may not have collected those data. We acknowledge that the lack of absorbance spectra is not unusual. We implore any such future studies to collect absorbance spectra of treated and control surfaces so that those data may be used to improve visual models for UV-sensitive animals.

Microsatellite variation in red-winged blackbirds (Agelaius phoeniceus).

Territorial male red-winged blackbirds from five locations in the United States and Canada were genotyped using a suite of six microsatellite loci. Each population possessed unique alleles, but numbers of alleles per locus (range = 7.3-8.8) and expected multilocus heterozygosities (range = 0.76-0.80) were similar in all populations. Significant overall allele frequency differences were detected between some population pairs, and some pairwise Fst values were significant (but small). However, Fst among populations, although significant, was also small (0.009). Despite revealing low levels of population structure, the high multilocus polymorphism indicates these loci will be valuable in the genetic analysis of behavior and reproductive strategies in this species.

An Experimental Test of the Contrasting-Color Hypothesis of Red-Band Effects in Red-Winged Blackbirds

We performed an experiment to investigate effects of red leg bands on the behavior of male Red-winged Blackbirds (Agelaius phoeniceus). We presented models having either black, blue, or red leg bands to territorial male Red-winged Blackbirds to test the hypothesis that any color of band that contrasts with the color of the legs makes an individual appear abnormal or unhealthy and therefore subject to attack. Overall, territorial males responded equally aggressively to the black- and blue-banded models, but spent more time at distances greater than 10 m, displayed at lower intensities, and took longer to attack when the model was given red bands. Thus, red bands appeared to make the model initially more threatening to territorial male Red-winged Blackbirds. These results do not support the contrasting-color hypothesis and suggest that the effect of red bands is attributable to the bands matching the color of the male's epaulets. The failure of the contrasting-color hypothesis also leaves unresolved the different outcomes of one previous experiment showing a negative effect of red bands, and two analyses of long-term banding studies that detected no effect of red bands.

Year-to-year patterns of circulating levels of testosterone and corticosterone in relation to breeding density, experience, and reproductive success of the polygynous red-winged blackbird

To determine across-year patterns in plasma testosterone (T) and corticosterone (B) levels in free-living birds, we took blood samples in the same 2-week breeding period during 4 consecutive years from territorial male red-winged blackbirds. We used our data to test predictions of the “challenge hypothesis” of T secretion and also to examine hormonal correlations with age, breeding experience, breeding density, and reproductive success. Average T and B levels across years were not significantly different. Within individuals, T levels between years were, in general, highly variable. T levels of males with territories in high-density breeding areas were significantly higher than those of males with territories in low-density areas. T levels were positively correlated with harem size and there was a trend for males with high T levels to fledge more offspring. We found no relationships between T and B levels and male age or breeding experience. Some results of the study were consistent with the predictions of the challenge hypothesis. This study constitutes one of the few examinations of across-year patterns in circulating hormone levels in wild populations.

Seeing red: uncovering coverable badges in red-winged blackbirds

The coverable badge hypothesis predicts that non-territorial floaters should conceal their badges and that territory owners should expose their badges. Evidence is presented that suggests owners also benefit by concealing their badges at certain times. Forty territorial male red-winged blackbirds, Agelaius phoeniceus, were randomly assigned to one of two clipping treatments. The scapular feathers that cover the epaulettes of experimental males were clipped so that the epaulettes were continuously exposed (clipped males). Contour feathers near the epaulettes were clipped on control males. These contour feathers are not used to cover the epaulettes. Clipped males spent more time chasing intruders (particularly neighbours) from their territories than control males. Clipped males also trespassed on the territories of neighbours less often than control males. These results suggest that owners that conceal their coverable badges enhance their ability to trespass on the territories of other males. Owners may also benefit from being able to conceal their coverable badges while on their own territories because continuous display of aggressive badges may inappropriately signal combativeness, resulting in more aggressive interactions with other males, particularly neighbours.

Color bands function as secondary sexual traits in male red-winged blackbirds

We present the first experiment to assess band color effects in a natural bird population. 38 territorial male red-winged blackbirds (Agelaius phoeniceus) were randomly assigned to 1 of 2 banding treatments. They received either all red bands (to match their epaulets) or all black bands (controls). Over half the red-banded males lost their territories while all black-banded males retained their territories. Red and black-banded males did not differ morphologically. However, among red-banded males, those that lost their territories had larger epaulets and were in poorer condition than those retaining their territories. Red-banded males suffered much higher intrusion rates, particularly by neighbors, than black-banded males. We propose that red color bands exaggerated the males' natural aggressive signal beyond the point where the signal was reliable. Our results suggest that signal reliability is maintained by regular testing, particularly of those males most likely to be signalling dishonestly.

Use of the song repertoire in intersexual and intrasexual contexts by male red-winged blackbirds

Live and stuffed male and female red-winged blackbirds (Agelaius phoeniceus) were presented to territorial males in order to study the repertoire organization of males during intersexual and intrasexual encounters. Territorial male red-winged blackbirds switched song types more frequently and sang more song types in response to presentation of a female stimulus than during control periods, but decreased their switching frequency when a male stimulus was presented. Switching frequency in response to the female stimulus was three times that in response to the male stimulus. These results support the hypothesis that song repertoires of male red-winged blackbirds evolved primarily in response to intersexual rather than intrasexual selection. They also suggest that male red-winged blackbirds have been selected to de-emphasize their song repertoires during encounters with conspecific males as a result of some as yet unidentified cost.

Influence of Population Size on Habitat Use by Territorial Male Red-Winged Blackbirds in Agricultural Landscapes

Abstract Fretwell and Lucas's (1970) model relating settling patterns and habitat suitability was tested by examining the distribution among habitats of male Red-winged Blackbirds (Agelaius phoeniceus) at relatively low (26 males/km2 in 1978)a nd high (45 males/km2 in 1979)overall population densities. Wetlands and hayfields were identified as the most suitable breeding habitats on the basis of habitat selection patterns of territorial males and other evidence, whereas forest, cropland, and human-occupied areas generally were avoided. Some "packing" of males into hayfields occurred when population level increased, but the greatest changes were recorded from less suitable habitats. Occupancy of wetlands and hayfields remained relatively stable. These results were in general accordance with predictions from Fretwell and Lucas's model, but their model did not account for some aspects of spacing behavior. This is because habitat imprinting, site fidelity, and chance events, in addition to territorial behavior, likely contributed also to the spacing patterns of male Red-winged Blackbirds observed.

Breeding and Nonbreeding Season Mortality of Territorial Male Red-Winged Blackbirds (Agelaius phoeniceus)

-The mortality of territorial male Red-winged Blackbirds (Agelaius phoeniceus) was determined in a banded population during the breeding seasons of 1973-1978. I used these data to test the hypothesis that sexual selection for characteristics that are advantageous in territory defense or mate acquisition, or both, occurs at the expense of survivorship. Observations of males that returned to the study area to reestablish territories, and of the tenures of these males on their territories, enabled me to partition annual mortality into components that estimate mortality during the and breeding seasons. The mean annual mortality of territorial male Red-wings was 52%, with 29% mortality occurring during the nonbreeding season (1 June to approximately 1 March) and 22% mortality occurring during the breeding season before 1 June. The effect of body size on survivorship was investigated by correlating male wing lengths measured in the third year of life with their ages at death. The correlation between wing length and survivorship was negative (-0.438) and statistically significant (one-tailed P < 0.05). I conclude that mortality during the breeding season is a potentially important selective force in this species, and that sexual selection may have occurred at the expense of survivorship. Sexual selection for large males appears to be opposed by survival selection for small males. Received 13 January 1986, accepted 15 July 1986. DARWIN (1859, 1871) viewed natural and sexual selection as separate but complementary mechanisms of evolution. According to Darwin, natural selection favors adaptations that increase survival, while sexual selection favors adaptations that increase the ability to acquire mates but decrease survival. In contrast, contemporary evolutionary biologists consider both selection for increased survival (survival selection) and selection for increased mating ability (sexual selection) as components of natural selection (see Mayr 1972, Selander 1972). Sexual selection, however, is still thought to occur at the cost of decreased survival (e.g. Selander 1965, Searcy 1979, O'Donald 1980, Lande 1981, Searcy and Yasukawa 1981, Kirkpatrick 1982). Thus, when sexual selection produces sexual dimorphism, the larger and more conspicuous sex is presumed to pay a cost in higher mortality (see Andersson 1982, Searcy and Yasukawa 1983, Payne 1984). Survival selection therefore would act to limit the effects of sexual selection because it would favor smaller, less conspicuous individuals of the sexually selected sex. Although the opposition of sexual and survival selection has been presumed since Darwin (1859), there have been relatively few attempts to document the reduced survival that should accompany sexual selection. The Red-winged Blackbird (Agelaius phoeniceus) is a species in which sexual dimorphism is thought to be the product of sexual selection (see Searcy and Yasukawa 1983). Male Redwings are larger and more conspicuous than are females (Nero 1956a, b; Orians and Christman 1968), and sexual selection is a potentially important evolutionary force because the variance in mating success is high (Payne 1979). However, although sexual selection is thought to act more strongly on male than on female Red-winged Blackbirds (Payne 1979), male and female Red-wing survivorship is virtually identical (Fankhauser 1971, Searcy and Yasukawa 1981). Thus, there is no evidence that males pay a cost in increased mortality. Furthermore, there is little evidence that larger male Red-winged Blackbirds suffer higher mortality than do smaller males (Searcy 1979, Johnson et al. 1980). These observations are inconsistent with the generally accepted view that sexual selection is primarily responsible for the sexual dimorphism in size, plumage, and behavior in Red-winged Blackbirds (see Searcy and Yasukawa 1983). I tested the hypothesis that sexual selection has favored characteristics that enhance a male's ability to acquire a territory or mates, but at the expense of his survival (Selander 1965, Searcy 56 The Auk 104: 56-62. January 1987 This content downloaded from 207.46.13.105 on Wed, 25 May 2016 05:23:12 UTC All use subject to http://about.jstor.org/terms January 1987] Mortality of Male Red-winged Blackbirds 57 1979). This sexual-selection hypothesis predicts that the mortality of males during the breeding season should represent a considerable portion of annual mortality because males pay a cost that results from their conspicuous and energetically expensive attempts to defend their territories and attract mates. I tested this prediction by partitioning the annual mortality of male Red-winged Blackbirds into components that estimate mortality in the breeding and seasons. The hypothesis also predicts that counter-balancing survival selection will favor small males. I tested this prediction by examining the relationship between size and survival of male Red-winged Blackbirds.

Communication by changing signals: call switching in red-winged blackbirds

Territorial male red-winged blackbirds use many different “alert” calls which overlap broadly in the context of their use and which are often given in continuous, repetitive fashion. Males give one call type many times before switching to another type, and call during all daily activities. In this field study we demonstrate that males tend to give the same call types as their neighbors and change types to match the changes of their neighbors. Individual males change call types in response to the appearance of a mounted hawk and also match call type changes broadcast from a loudspeaker. The various call types are not associated with particular behavioral contexts. We suggest that red-winged blackbirds operate a general acoustic alert system by calling repetitiously and changing call types when detecting environmental changes such as appearances of predators. Evidence is presented that communication is acheived primarily during switching among different call signals, and not by specific calls that refer to particular stimuli or states of alertness.

Playback and speaker-replacement experiments on song-based neighbor, stranger, and self discrimination in male Red-winged Blackbirds

Two experiments were conducted to deterine whether territorial male Red-winged Blackbirds discriminate among neighbor, stranger, and self songs. In the first experiment, recordings of the three classes of songs were played to territorial males. Responses to playback of stranger song, as measured by song rate, intensity of Song Spread display, and closest approach, were significantly stronger than to playback of neighbor song. In addition, stranger song elicited significantly more intense Song Spread displays than did self song. In the second experiment, territorial males were removed from their territories and each was replaced with speakers broadcasting recorded songs of a stranger, and of the removed male. Broadcasts of the removed male's own songs were more effective in discouraging trespassing by neighboring male redwings than were broadcasts of songs of a stranger. However, the two broadcasts did not differ in their ability to deter nonneighboring male redwings from flying through or trespassing within the speaker territory. We suggest that neighbor-stranger discrimination in the Red-winged Blackbird develops as a consequence of associative learing.

Male Singing Ability and Territory Integrity in Red-Winged Blackbirds (Ageliaus Phoeniceus)

AbstractDuring the breeding seasons of 1974 and 1976 three territorial male Red-winged Blackbirds were devocalized utilizing a technique which involved cutting the membrane of the interclavicular air sac. As a result, males were devocalized or muted for a period ranging between 14 and 21 days, after which time they regained their ability to sing typical song. One sham operated male and 10 "undisturbed" neighbouring residents served as control males. The frequency and length of intrusions by a devocalized male's neighbours increased markedly during a male's mute period. Sidling, a highly visual and apparently aggressive behavior, increased dramatically during the time a male was muted. After the experimental males regained their ability to sing species typical song they were successful in regaining territory boundaries which approached or exceeded the pre-operative dimension. It was concluded that 1) under the conditions of this study muted males were not as effective in excluding potential or actual intruders from their territories as when they could produce song; 2) visual dispalys (sidling) are utilized more frequently by devocalized males because of the persistence of intruding males on devocalized males' territories.

Aggressive tendencies and levels of a graded display: Factor analysis of response to song playback in the redwinged blackbird ( Agelaius phoeniceus)

The aggressive tendencies of territorial male redwinged blackbirds were tested using playback of conspecific songs and presentation of a male redwing mount. Factor analysis of overt aggressive responses generated three orthogonal indices of aggressive tendencies. These indices were named (1) Distant Approach (rate of flights over, and proportion of time spent within 5 m of, the mount), (2) Proximity (proportions of time spent within radii of 5, 3, and 1 m of the mount), and (3) Attack Latency (latency to perch beside and attack the mount), and were tested for correlation with the threat displays and other behavior of territorial males to assess their aggressive motivation. In addition, the behavior and time budget variables were used in a linear discriminant function analysis of males that attacked the mount during the testing periods and males that did not attack. The Distant Approach factor was negatively correlated with the proportion of time foraging. The Proximity factor was positively correlated with mean Song Spread display intensity and the proportion of time singing and defending, and negatively correlated with proportion of time foraging. Attack Latency was positively correlated with low Song Spread rate and negatively correlated with medium Song Spread rate given by the territorial males when more than 5 m from the mount. Attacking the nonattacking males could be discriminated on the basis of their (1) proportions of time spent foraging and singing and defending the territory; (2) mean Song Spread (intensity) and incipient Song Spread rate when less than 5 m from the mount; and (3) low Song Spread rate when more than 5 m from the mount. These results suggest that the extent or intensity of the Song Spread indicates the level of aggressive motivation in situations where attack is inhibited, and that threat display and other behavior have signal value and indicate to an intruder the probability of attack.