Why do birds flock and fish shoal?
The wingbeats of mute swans produce substantial noise to an earthbound listener, but have you ever thought of the noise from the bird’s point of view, or rather, point of hearing?
”Incidental sounds of locomotion” (ISOL) will be ever-present to all flying birds, and most birds have acute hearing with an inner ear similar to that of humans. What is the amplitude of ISOL at the ear of a flying bird? Hingee and Magrath recorded wing-whistles in the crested pigeon and found that, one metre from the bird, the wing whistle amplitude was over 60 dB, which translates to over 80 dB close to the pigeon's ear.
So how do they perceive anything but their own flapping, for example an approaching hawk? What is the rationale of moving around in big and presumably noisy animal groups? A further question is whether ISOL is merely debris, or do birds and other animal use ISOL as communicative signals?
Since we all are fish descendants it is likely that challenges related to ISOL began in the sea. In filter feeding ancestors, before the octavolateral system (lateral line and inner ear) had developed, problems with ISOL should have been non-existent in a shoal of fish. No ears, no lateral line – no problem. At that time the risk of predation would also have been limited, and safety in numbers was probably not a major reason for fish to gather in a shoal. However, grouping could have been beneficial for other reasons, such as for foraging and mating. Small filter feeders may have ventured near large conspecifics without risk. The development of sensory systems, such as hearing and the lateral line, would have permitted detection of potential prey, leading to an increased potential for cannibalism within the group. On the other hand, enhanced perception of ISOL would give small individuals the possibility of escaping or of never joining a shoal with larger fish. In other words, it seems likely that ISOL may help fish to recognize whether the fish they meet is a potential predator, prey, or partner.
Every breath you take, every move you make – causes vibrations
Moving creates vibrations in the surrounding air or water that may stimulate the sensory system of the animal producing the movement and mask critical environmental signals. This is a classic problem, labelled "sensory reafference". Animals have developed sophisticated internal communication systems to handle sensory reafference problems. When motor neurones send a command to muscles to move, an “afference copy” is concurrently transmitted to parts of the nervous system that might otherwise be stimulated. The message is, “don’t bother about this noise – it’s self-made, filter it out.”
Such mechanisms may also help an individual animal to handle its own ISOL, increasing the perception of crucial signals such as the noise produced by an approaching predator. Travelling in a group raises obvious difficulties. A large group of animals is likely to produce energetic and complex noise, and quiet intervals will be few. Sync may be a strategy to reduce such problems. Birds going into glide flight will reduce their own noise for a while; a group of water fowls that splash down simultaneously will create a lot of noise, but synchronisation will result in a relatively brief period of “landing noise”. Fish in a school ceasing fin movements concurrently may achieve similar advantages. In addition, fish and birds flocks with uniformly-sized individuals, moving fins or wings in almost the same frequency will produce very similar ISOL. When ISOL of the group is uniform, that noise can be grouped together – it will be like one sound source. This may improve the odds of hearing other, more critical sounds such as an approaching hawk or a predatory fish. Yet, ten thousand herring swimming together are likely to stir the water a good deal. What is the benefit of such a noisy environment, why not stay alone instead?
Benefits of being together
In addition to safety in numbers, foraging, and mating advantages, reduction of encounters with predators, watching for predators and reduction of energy expenditure has been proposed as good reasons for fish to form groups. Visual confusion of predators through schooling has been proposed, while other forms of predator confusion have not been discussed. Is it possible that being together creates a form of confusion other than a visual one?
Confusion of a predator's lateral line and electrosensory system
Imagine a single fish in the ocean. The tiniest movement and what happens? The creature will be in the middle of its own pressure gradients, sitting at the centre of a dartboard, an easy catch for a nearby predator with a keen mechanosensory lateral line. A predator with an electrosensory system would find equally easy pickings. Again, our friend would occupy the bull’s eye, a perfect target created by electrical gradients. In a group, on the other hand, the picture changes. Fish tend to school in risky situations, and the larger the school, the lower the vulnerability of the fish. Why? As mentioned, a common explanation is that a highly synchronized school might confuse a predator’s vision. However, schooling is likely to confuse the lateral line as well. Imagine a hungry predator outside a dense school of fish. A potential feast, but the pressure gradients of dozens and dozens of closely-swimming prey fish will now overlap, rendering an individual fish a difficult target. In addition, a symmetrically arranged school might emit a flat wave-front, mimicking the water movements of a much larger fish. Research into the fish electrosensory system has revealed that individual prey must be about five body widths apart to produce separate signals. Hence, schooling ought to blur the electrosensory reception of predators. Fish descendants, such as birds and mammals, are likely to possess anatomical structures, wiring, and processing abilities that may be reused in an ecological niche where synchronised movements of the group is beneficial. The confusion of the electrosensory system of predatory fish will not be relevant to birds. Instead, reduced energy expenditure, group cohesion, optical advantages, a collision risk for predators attacking a cluster formation, and confusion of a raptor’s auditory system have been suggested.
So what about marine mammals?
Humpback whales will often breathe in synchrony when resting. Social cohesion has been suggested to be the rationale; however, reduced masking could be an alternative hypothesis, since an effect will be long periods of silence, which may facilitate detection of critical signals in the surroundings. Analogously, concurrent surface diving of dolphin dyads will result in simultaneous splash down, providing longer periods of relative silence compared with non-synchronised diving. Periods of relative silence (without splash) may favour intra-specific vocal communication, and facilitate perception of approaching danger, such as sharks or humans.
Does ISOL help animals to keep together?
Fish schools and flocks of birds seem to possess no leaders, only local rules. Transmission of information is generally thought to be through visual signals. However, ISOL may provide information about neighbours’ wing- or fin-beat frequency, relative speed, and location. The distance will influence amplitude and timbre, information that may be used to help maintain proper spacing between animals. When fish or birds in a large group see a predator and react by moving away from the threat, the movements will create sound. ISOL may serve to augment vision in informing the group of forthcoming movements, leading to a reaction in one area of the group spreading to the entire school or flock. Studies of ISOL may provide new insights into schooling fish and bird flight behaviour. Ramifications of mammalian ISOL, including human footsteps, are topics for further study.