IT'S NOT JUST THE SONG: Male Visual Displays Enhance Female Sexual Responses to Song in Brown-Headed Cowbirds

Abstract

Resumen. Aunque se reconoce ampliamente que la selección de pareja por parte de las hembras de las aves canoras probablemente se basa en señales multimodales como aquellas contenidas en los despliegues audiovisuales de canto, algunas dificultades de procedimiento han restringido los estudios sobre la selección de pareja en estas aves principalmente a las señales acüsticas. En el presente estudio, utilizamos grabaciones audiovisuales de Molothrus ater para evaluar las respuestas sexuales de las hembras a los despliegues de canto de los machos. Usando los despliegues de solicitud de cópula de hembras tratadas con estradiol (n = 9), encontramos que la reproducción de audiovisuales que mostraban machos realizando despliegues de canto con las alas abiertas fueron mucho más estimulantes para las hembras que la presentación de cantos acompañados de videos de machos que no estaban desplegando y que la presentación de sólo cantos. Los resultados de este estudio demostraron claramente que el estímulo visual del despliegue de alas abiertas en particular, y no sólo el estímulo visual de un macho coespecífico en general, estimula la máxima respuesta observada entre las hembras. En M. ater, las preferencias sexuales de las hembras por los cantos reveladas por estudios de los despliegues de solicitud de cópula varían en paralelo con el éxito de apareamiento de los machos, de modo que nuestros resultados probablemente reflejan un efecto de los despliegues visuales sobre el éxito de apareamiento de los machos silvestres. IT'S NOT JUST THE SONG: MALE VISUAL DISPLAyS ENHANCE FEMALE SExUAL RESPONSES TO SONG IN BROwN-HEADED COwBIRDS Los Despliegues Visuales de los Machos Promueven el Grado de Respuesta Sexual a los Cantos en Molothrus aterAbstract. Even though it is widely acknowledged that mate choice in female songbirds is likely to be based on multimodal cues such as those encoded in audiovisual song displays, procedural difficulties have limited studies of mate choice in songbirds mainly to acoustic signals. In the current study, we used audiovisual recordings of male Brown-headed Cowbirds (Molothrus ater) to test the sexual responses of females to males' song displays. Using the copulation-solicitation displays of estradiol-treated females (n = 9), we found that audiovisual playbacks of males performing wing-spread song displays were significantly more sexually stimulating for females than presentations of song accompanied by video of nondisplaying males, or song alone. Results from this study clearly showed that the visual stimulus of the wing-spread display in particular, not just the visual stimulus of a conspecific male in general, elicits the maximum observed response from females. In the Brown-headed Cowbird, females' sexual preferences for song as revealed by studies of copulation-solicitation displays have been shown to parallel males' mating success, so our results likely reflect an effect of visual displays on the mating success of wild males.1

When Less Is Best: Female Brown-Headed Cowbirds Prefer Less Intense Male Displays

“…We recently developed experimental CSD procedures that demonstrate that female cowbirds extract meaningful visual information from AV recordings played on a LCD monitor [25] and we used these procedures in the current study. In Experiment 1, we recorded and measured the duration of female CSD responses to playback of AV recordings showing the same males performing both high intensity male-directed and low intensity female-directed wing-spread displays.…”

“…Females were tested in two cohorts of four birds from 3 rd to 30 th May 2010. Each female had a Silastic tubing implant (outer diameter 1.96 mm) packed with 10 mm of estradiol (Sigma Chemical Co., St. Louis, MO) and sealed with Silastic adhesive, inserted subcutaneously in her chest to increase her sensitivity to playback stimuli [6], [20], [21], [22], [25]. Levels of circulating estradiol in implanted female songbirds are generally within natural physiological bounds and comparable to normal peaks that occur in passerine birds during the period when they are laying eggs [26].…”

“…There is no evidence that the increased levels of estradiol from implant procedures affect discrimination in CSD tests of female song preferences [27]. After this procedure, females were transferred to individual cages (46×27×27 cm), which were placed in separate acoustic-isolation chambers (inner dimensions, 61×33×38 cm) as described elsewhere in detail [25]. On the 11 th day after receiving her implant, a female was temporarily transferred in her cage to a large sound-attenuating chamber (inner dimensions, 52×57×56 cm), equipped for AV recording and playback where she was presented with a single playback and her response was recorded on digital video.…”

“…duration; low intensity 3.35±0.27 s, high intensity, 3.37±0.27 s). Because the original acoustic recordings of the perched songs on the videos were of poor quality, we stripped the audio tracks from all the videos and added a new higher quality sound track (44.1 kHz, 16 bit, PCM) to each one [25]. The same audio file of a single perched song was added to each of the two videos in a pair and a different perched song type was used for each of the seven video pairs.…”

“…Numerous previous studies have demonstrated that audio playback of perched songs is an extremely reliable stimulus for eliciting CSDs from female cowbirds [6], [20], [21], [22], [23], [24], and results from a recent study demonstrated that the addition of the visual component of the wing-spread display to the acoustic component enhances female sexual responses [25]. The evidence that the audio component of wing-spread song displays alone can elicit CSDs from females, prompts the question of whether the visual component alone also elicit CSDs?…”