How do birds interpret sounds around them and what does this mean for pest control efforts?
To interpret the world around them, birds primarily rely on their vision, followed by their hearing. Our knowledge about avian hearing has advanced in recent years, giving us a greater understanding of how they process and respond to auditory cues.
Hearing frequencies and ear structure
Unlike humans, birds are most sensitive to sounds within a fairly narrow frequency range. For most bird species this is 1-4 kHz, with the upper threshold being around 10 kHz. By comparison, the human ear can hear a much wider range of frequencies but is most sensitive to sounds between 3-4 kHz, which is the frequency of human voices and musical instruments. Large, nocturnal owls are the exception in that they can hear well over a wide frequency range. No species of bird has shown sensitivity to ultrasonic frequencies (>20 kHz).1
Birds and humans both have an inner ear and a middle ear. However, birds differ from humans in that they lack an external ear structure. While humans have an outer ear, birds have a funnel-shaped opening (often hidden under feathers) that functions as their outer ear, located on each side of their head. The inner ear of birds serves two functions: equilibrium and hearing. Hearing takes place in the cochlea, as it does in mammals. Birds are interesting as different species have cochlea of different sizes e.g. in pigeons (Columba livia) it is about 5 mm long, while in barn owls it is 1 mm long, with this size difference relating to the different frequencies the birds are able to hear.
Interpreting sound
One problem for birds is determining the direction of a sound source. In mammals, the outer ears play an important role in helping the animal identify sounds coming from different elevations. Mammals identify sound sources in the vertical plane using their external ears, which absorb, reflect or diffract the sound waves because of their special structure. Their sense of hearing uses this information to determine the elevation of the sound source. But without external ears to respond to sound waves, how can birds identify the source of sounds?
Because their ears are close together, birds have a different mechanism for localising these sounds – they use their entire head. Researchers have discovered that their slightly oval-shaped head transforms sound waves in a similar way to external ears. Depending on where the sound waves hit the head, they are reflected, absorbed or diffracted. The avian brain determines whether a sound is coming from above or below from the different sound volumes in both ears. The bird then combines these two pieces of information to determine the source of the sound, with a high level of accuracy.2
Loud noises
One problem that birds suffer in the same way as humans is ear damage caused by very loud noises. More specifically, loud noises can damage their tiny hair cells (auditory receptors). Birds that nest in the active areas of airports might be constantly subjected to sound pressure levels that damage their hearing in the same way that a builder using a jackhammer might damage their hearing if not wearing proper ear protection.
Taking this into account, can we use this understanding of avian hearing to improve bird control outcomes?
The use of acoustic deterrents
Acoustic devices have been employed by the agricultural and aviation sectors for many years, with the aim of displacing birds to minimise damage to crops or the danger posed to aircraft. To give any benefit, a sound signal must be loud enough to be heard by the birds, be within the frequency range the birds’ ears can detect, and provide a biologically relevant message, so the birds’ natural instinct is to fly away from the location. ‘Biologically relevant’ sounds usually refer to distress and alarm calls made by members of their same species, or alternatively the sound made by a predator.
While it sounds straightforward enough, the effects of using acoustic mimics alone are almost always short term. Birds are dispersed by the sound for a short time, but constant signals that don’t change frequency or intensity – even distress calls or alarm sounds from members of their own species – are generally ignored by birds when they quickly learn there is no real threat. The unreinforced warnings are ignored, and sometimes within a matter of days, sometimes in a couple of weeks, the birds no longer seek to avoid the area.
Humans are predators of birds, at least in biological terms. Fireworks, or pyrotechnics such as bangers and poppers are ‘biologically relevant’ sounds because they provide the acoustic information generated by a (human) predator without the actual predatory attack. These have been shown to be used with some limited success. When such sounds are reinforced by a real threat, such as shooting, the behavioural avoidance lasts much longer. However, shooting is an expensive control measure that is only used in specific, controlled circumstances.
Interestingly, some bird scaring devices that are commercially available use ultrasonic frequencies, i.e. frequencies above 20 kHz. Because no species of bird has shown behavioural or neurophysiological responses to ultrasonic frequencies, so there is little evidence that these ultrasonic devices have any effect on the behaviour of birds.
Several in-depth reviews of bird control practices have concluded that acoustic devices must be combined with other control techniques as part of an integrated management program for any successful control outcome to be achieved.
References
1 Beason, Robert. (2004). What Can Birds Hear?. U.S. Department of Agriculture National Wildlife Research Center- Staff Publications. 78.
2 Schnyder, Hans & Vanderelst, Dieter & Bartenstein, Sophia & Firzlaff, Uwe & Luksch, Harald. (2014). The Avian Head Induces Cues for Sound Localization in Elevation. PloS one. 9. e112178. 10.1371/journal.pone.0112178.
