High-frequency hearing in a hummingbird

Abstract

Some hummingbirds produce unique high-frequency vocalizations. It remains unknown whether these hummingbirds can hear these sounds, which are produced at frequencies beyond the range at which most birds can hear. Here, we show behavioral and neural evidence of high-frequency hearing in a hummingbird, the Ecuadorian Hillstar (Oreotrochilus chimborazo). In the field, hummingbirds responded to playback of high-frequency song with changes in body posture and approaching behavior. We assessed neural activation by inducing ZENK expression in the brain auditory areas in response to the high-frequency song. We found higher ZENK expression in the auditory regions of hummingbirds exposed to the high-frequency song compared to controls, while no difference was observed in the hippocampus between groups. The behavioral and neural responses show that this hummingbird can hear sounds at high frequencies. This is the first evidence of the use of high-frequency vocalizations and high-frequency hearing in conspecific communication in a bird.

Fig. 1. HF vocalizations and behavioral responses to conspecific song.

Spectrogram and power spectrum of (A) a representative HF vocalization. (B) Picture of a male O. chimborazo producing the HF song. Feathers on the cheeks flare while singing, and throat inflates eliciting waves of iridescent feathers moving along the purple hood. Photo credit: Fernanda G. Duque, Georgia State University. (C) Hummingbirds in the field that responded to playback of HF song and of ambient noise. When we played HF song, all observed hummingbirds (n = 13) exhibited changes in body posture, head tilts with neck extensions, and approached the area surrounding the playback speaker. Only a few of these hummingbirds also elicited some of these behaviors during playback of ambient noise. Hummingbirds that elicited behaviors during both stimuli (ambient noise and HF song) are shown in blue (approach: 2 hummingbirds; body posture, 3; head/neck, 1), while hummingbirds that only responded to the playback of HF song are shown in pink (approach, 11; body posture, 10; head/neck, 12).

Fig. 2. Induction of ZENK expression in the brains of hummingbirds.

Timeline showing the experimental design for inducing ZENK protein expression in the hummingbird brain in response to a sound stimulus. The control group (n = 4) was exposed to a playback of silence, while the experimental group (n = 4) was exposed to playback of HF song.

Fig. 3. Neural responses to HF song.

Top left panel shows (A) the representative section of the brain showing the secondary auditory areas CMM (a) and NCM (b) and the HP (c) in hummingbirds exposed to HF song. The blue square delineates the area magnified 10 times in the picture below to show the two secondary auditory areas and, as landmark, L2, the primary auditory region, which expresses little to no ZENK (49). Additional panels show representative magnified images of each region from brains exposed to silence and to HF song. (B) ZENK protein expression in the brains of hummingbirds exposed to HF song (n = 4) (pink) compared to hummingbirds exposed to silence (n = 4) (blue). The boxplot was produced using median and interquartile range (IQR) as measures of centrality. Upper and lower whiskers in the graph extended to the highest and lowest value, respectively, with a threshold set at 1.5 × IQR; any values beyond this threshold were plotted as outliers (48). Statistical analysis was done using a two-way ANOVA, Tukey’s post hoc test. ***P < 0.001.