The flag hypothesis predicts that subtle changes in feather shape will tune the frequency of sound produced by feathers. The mechanism is not a whistle, and we propose a flag model to explain the feather's fluttering and accompanying sound. High-speed video shows that fluttering of the trailing vane of the outermost tail feathers produces the sound. Here, we use high-speed video of diving birds, experimental manipulations on wild birds and laboratory experiments on individual feathers to show that the dive sound is made by tail feathers. Production of the sound was originally attributed to the tail, but a more recent study argued that the sound is vocal. The loud, high-frequency chirp emitted by a male Anna's hummingbird ( Calypte anna) during his display dive is a debated example. Few studies have established that these sounds are non-vocal, and the mechanics of how these sounds are produced remains poorly studied. Understanding how these hummingbirds have evolved to communicate using high-pitch songs will teach us more about the ways in which the brain adapts to sense information in the environment and facilitate communication and social interactions in nature.A diverse array of birds apparently make mechanical sounds (called sonations) with their feathers. It is likely, though, that other species of hummingbirds can hear and use them for communication too. The Ecuadorian Hillstar is the only bird known, to this date, for communicating using high-pitch songs. While some owls have specialized ears that allow them to detect high-pitch sounds for hunting, they do not use this ability to communicate with other owls. At the same time, they sing their high-pitch song, which makes the feathers of the head vibrate, enhancing their iridescent purple color. During courtship, Ecuadorian Hillstar males hover in front of females displaying their feathers. Females evaluate these displays and choose their favorite male to mate. Courtship displays are common among animals and consist of a series of behaviors aimed at showing off the attributes and ornaments of males. Eventually, we also found that Ecuadorian Hillstar males sing the high-pitch song to females during courtship. So that, a male controlling a patch of flowers lets other males nearby know that a territory is already taken. Based on the behavior and brain responses, we concluded that the Ecuadorian Hillstar can hear its high-pitch song.īut what are the functions of these sounds? Previously, we observed that the Ecuadorian Hillstar uses these songs to defend its territories. So, we searched for this gene in neurons in the auditory regions of the hummingbird brain, and we found that the auditory regions in hummingbirds exposed to the high-pitch song were more active than the same ones in birds not exposed to any sound. In the past, scientists have seen that a gene called ‘zenk’ is a good indicator that a neuron has been activated. This activation triggers a process known as ‘gene expression’, where information stored in the genes is used to guide the synthesis of functional products, such as proteins. Normally, when the brain detects a relevant cue in the environment, a song of the same species in this case, neurons in several regions of the brain activate to process the information. We also studied the responses to high-pitch songs in the hummingbird brain. We also played environmental noise at a lower pitch, but hummingbirds did not respond to these sounds as they did to the high-pitch song. We found that Ecuadorian Hillstar hummingbirds followed these patterns by tilting their heads, extending their necks and adjusting the position of their body towards the source of the song. They often respond with aggressive calls and displays towards the speaker, which they perceive as an intruder. Moreover, birds react distinctly when they hear the song of a different male inside their territory. Just like humans, when other animals hear a sound that calls their attention, they look around searching for the source of sound. First, we played back a high-pitch song to the hummingbirds in the field. Thus, if the Ecuadorian Hillstar produces high-pitch songs to communicate, we would expect that this hummingbird can detect similarly high sounds coming from other hummingbirds of the same species.īut how do we ask a hummingbird which sounds it can hear? We designed two experiments to test the hearing ability of this remarkable bird. For vocal communication to be effective, the listener needs to hear the sounds that another individual is producing and birds hear best at the same pitch at which they sing. Our previous research suggests that these hummingbirds produce high-pitch sounds to stand out from ambient noise. Among them, the Ecuadorian Hillstar hummingbird produces a song with the highest pitch.
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