The role of hearing in detecting foraging birds has received little consideration

‘If a bird flies in the forest, does an insect hear it?’

That's the title of a paper from Jayne E. Yack and her team at Carleton University in Canada. “Birds are major predators of many eared insects including moths, butterflies, crickets and cicadas,” they point out in the paper, which has just been published in Biology Letters.

Insects, for their part, have evolved strategies to avoid being eaten. However, “the role of hearing in detecting foraging birds has received little consideration,” the scientists noted in the paper.

An insect’s ‘ear’ consists of a thinned portion of its external skeleton stretched over an air cavity. This tympanal membrane vibrates in response to sound waves and those vibrations are then picked up by nerve cells. Depending on the species of insect, the ears can be situated at diverse places on its body, including the wings, legs and even mouth parts.

Some insects have been shown to be capable of detecting the cries emitted by echo-locating bats, which prey on them, and to then take evasive action.

Whether capable

For their study, Dr. Yack and her team examined whether a moth, Trichoplusia ni, and a butterfly, Morpho peleides, were capable of hearing birds in flight. To do that, they first recorded the sounds produced by the wing beats of an insect-eating bird, the Eastern Phoebe (Sayornis phoebe). The bird-flight sounds were then played back to the two insects and the impulses produced by their auditory nerves recorded.

The team found that the insects’ auditory nerves responded to the flight sounds with a burst of impulses. In the case of the moths, the pattern of nerve impulses was similar to the one produced when these insects hear a bat homing in on them and which is thought to trigger evasive action.

“Since we see similar bursting patterns in response to bird flight sounds, it could be that they [the moths] can evade a bird attacking as they would a bat attacking, using a similar neural mechanism,” said Dr. Yack in an email.

The frequency range of moth hearing overlapped with the high frequency component of bird flight and that of butterfly hearing with the lower frequency components. The scientists estimated that the two species of insects could both detect a bird at distances of at least 2.5 metres.

That figure could be a conservative estimate of the detection distance, said Dr. Yack in her email. “We have been doing trials with live birds, and moth ears can detect a live bird flying from several meters away.”

“Future experiments should explore how eared insects respond behaviourally to bird flight, and whether birds use counter strategies, such as sound-reducing feather modifications or foraging tactics, to avoid detection,” the scientists observed in the paper.

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