William A. Searcy

Measuring Responses of Female Birds to Male Song

It has long been realised that bird song has two principal functions, one in male-male competition, usually for territories and one in attracting and stimulating females (Howard 1920). It follows that investigations of communication via song have two principal audiences to consider, conspecific adult males and conspecific adult females. In particular species, song may be directed especially at one or the other of these audiences, but across all songbirds the two audiences and functions seem to be of similar overall importance.

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

SummaryLive 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.

Songs of isolation-reared sparrows function in communication, but are significantly less effective than learned songs

SummarySongs of male song sparrows (Melospiza melodia) and swamp sparrows (M. georgiana) isolated before hatching from conspecific song were played to males and females of both species. “Isolate” songs of these sparrows resemble natural conspecific song in several aspects of gross structure, but differ from natural song in note structure. Male territory owners of both species responded more to conspecific isolate song than to heterospecific normal song. Captive females of both species, previously treated with estradiol, courted in response to isolate song but not to heterospecific song. We conclude that there is sufficient speciesspecific information in isolate song of both species to allow a degree of normal function. To assess the importance of the structural refinements added through learning, we compared response to isolate and natural conspecific songs. Male territory owners and captive females responded more to natural than to isolate songs in both species. Learning thus significantly increases the potency of song in both intersexual and intrasexual communication.

Species recognition of song by female red-winged blackbirds

The ability of female red-winged blackbirds, Agelaius phoeniceus, to discriminate between normal songs of local, conspecific males and altered or foreign songs was tested. Females were tested using the solicitation display assay, in which captive females are first treated with oestradiol and then exposed to playback. The number of copulation solicitation displays was the sole response measure. Females responded more strongly to full songs than to songs with all introductory notes removed in tests using nine pairs of stimuli. Females did not discriminate between normal songs and the same songs divided into three parts and reassembled in shuffled orders. Subjects drawn from a Pennsylvania population displayed more in response to local songs than songs recorded in California. Pennsylvania females also responded more strongly to trill portions of Pennsylvania songs than to trill portions of California songs. Overall, the results are compatible with the hypothesis that female red-winged blackbirds are more discriminating than males in species recognition, attending to more acoustic cues than do males.

Pseudoreplication, external validity and the design of playback experiments

Article critique sur les problemes que posent les experiences de restitution demissions sonores principalement en fonction de leur validite

Are female red-winged blackbirds territorial?

Spacing behaviour of female red-winged blackbirds (Agelaius phoeniceus) was observed to determine whether resident females within a herem divide their mates territory into sub-territories. Some results accorded with defence of sub-territories: (1) females were more aggressive (towards decoys and mounts) close to their own nests than further away; and (2) there was little overlap in the use of space betwee residents on a males territory. Other results did not accord with defence of sub-territories: (1) when the nest site was eliminated from the analysis, spatial overlaps were large; (2) areas defended through aggression by different females overlapped greatly; and (3) areas defended through advertisement by different females overlapped greatly. I conclude that the spacing behaviour of female red-winged blackbirds does not meet a strict difinition of territoriality.

Song repertoires and density assessment in red-winged blackbirds: further tests of the Beau Geste hypothesis

SummaryThe Beau Geste hypothesis proposes that song repertoires are advantageous in territory defense because nonterritorial males, who are prospecting for territories, use the number of song types they hear to assess the density of territorial males, and then avoid densely-settled areas. A territorial male can then inflate the apparent density of singers on his territory by singing several distinct song type. This hypothesis assumes (1) a positive correlation between the density of song types and the density of territorial males, and (2) a negative correlation between the rate of trespassing and the densities of both song types and territorial males. We studied the behavior of male red-winged blackbirds (Agelaius phoeniceus) and found support for the first, but not the second, assumption. Results showing a positive correlation between density of song types and density of territorial males show that the proposed method of density assessment is feasible in this species. However, we could find no support for the assumption that nonterritorial male red-winged blackbirds avoid densely settled areas. In contrast to the assumed negative correlation, rate of trespass per territory was not consistently correlated with male density, and trespass rate per unit area was positively correlated with male density. Further, these relationships were retained when we controlled statistically for the effects of territory quality. We conclude that prospecting male red-winged blackbirds to not attempt to avoid densely settled areas, and that, although they do avoid territories defended by many song types, they do not use song type density to assess the density of territorial males. It is thus unlikely that the Beau Geste hypothesis adequately accounts for the evolution of song repertoires in the red-winged blackbird.

Deceptive behavior in pied flycatchers

SummaryOne explanation for polygyny in pied flycatchers (Ficedula hypoleuca) is the deception hypothesis, which proposes that females mate with already-mated males only because they are unaware that such males already have a mate on another territory. Recently this hypothesis has been criticized on the grounds that already-mated and unmated males differ sufficiently in their behavior that human observers can discriminate between the two classes fairly easily. Here we test whether male pied flycatchers change their behavior when visited by a female so as to make this discrimination more difficult. In our experiments we presented a caged female near the nestbox of an advertising male, in order to mimic the situation in which a female investigates the male and his nest site. During control periods, when only an empty cage was presented, we found that already-mated males were present on their secondary territories significantly less and sang significantly less than unmated males on their primary territories, confirming the earlier results. When we presented the stimulus female, all behaviors we measured changed significantly relative to control periods for both mated and unmated males: both classes of males increased the time spent on the territory, decreased singing rates, and increased various courtship behaviors. During the experimental periods there were no significant differences between mated and unmated males on any of the behavioral measures. Discriminant analysis was more successful in classifying males as to mating status using data from the control periods (without female) than using data from the experimental periods (with female). We conclude that when a female is present male pied flycatchers change their behavior in ways that make it more difficult to discriminate mated from unmated males.

An experimental study of song type function in the bobolink (Dolichonyx oryzivorus)

SummaryWe tested the hypothesis that the alpha and beta songs of male bobolinks (Dolichonyx oryzivorus) have separate intrasexual and intersexual functions by presenting caged males and caged females on the territories of male bobolinks. Although all males in our study population share both alpha and beta songs, we found no evidence for a specialized intersexual function of alpha song or a specialized intrasexual function of beta song. Territorial males increased their singing rate of both alpha and beta songs during both the male presentations and the female presentations. The proportions of the two song types did not change between control and experimental periods for either male or female presentations. We found no evidence in support of specialized intrasexual and intersexual functions of note types. Only 10% of the vocalizations initiated by territorial males were complete alpha or beta songs, while 87% were song fragments, and the remaining 3 % were compound songs. We did find that territorial males responded to simulated male intruders by increasing the proportion of fragmented songs and decreasing the proportion of compound songs. Moreover, males included fewer notes per song initiated and sang a higher proportion of songs that included only introductory notes during male presentations. We conclude that the vocal response of territorial male bobolinks to conspecific male intruders is to sing short, simple sequences of either song type. We found no consistent changes in vocal behavior of territorial males in response to female presentations other than increases in song rates.

Function of male courtship vocalizations in red-winged blackbirds

SummaryDuring courtship, male red-winged blackbirds (Agelaius phoeniceus) produce long sequences of simple, single-note precopulatory calls, punctuated with occasional songs. Male song has previously been shown to stimulate copulation solicitation in captive, estradiol-implanted females of this species (Searcy and Brenowitz 1988; Searcy, in press). Here I show that sequences of precopulatory calls also stimulate copulation solicitation in female redwings, but that they are not as stimulatory as song. In tests contrasting mixed bouts of song and precopulatory calls with bouts of song alone, the mixed bouts elicited significantly more female solicitation in one instance, more but not significantly more in a second, and an exactly equal amount as song alone in a third. Neither song nor precopulatory calls could be shown to affect proximity of females to the speaker.