What Is a Territorial Call? Advertising a Claim Instead of Defending It
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Defending an area by fighting every intruder would be exhausting and dangerous, so most animals advertise instead. A call that carries far and is expensive to produce does most of the work that fighting would otherwise require.
What the call has to do
A territorial signal must accomplish several things at once. It must reach potential intruders, which sets requirements on frequency and timing, since low frequencies travel further in dense vegetation while higher ones carry better in the open, and dawn and dusk offer the quietest conditions with the most stable air. It must identify the caller as a member of the species and as a resident rather than a passing individual, which requires enough structure to be distinctive. It must be difficult to fake, since a signal any weak individual could produce would convey nothing, so the cost of producing it is what makes it informative. And it must be repeated, since a claim stated once has expired by the time an intruder arrives.
What it communicates
Listeners extract more from a call than the fact of occupation:
- •Identity, since many species produce individually distinctive calls and neighbours recognise each other
- •Condition, since sustained loud calling is expensive and a poorly fed or unwell animal cannot maintain it
- •Size in some species, where the frequency relates to body dimensions and cannot easily be faked
- •Position, since a listener can locate the caller and therefore the boundary
- •Motivation, since the rate and intensity of calling changes with how strongly the resident will respond
- •Population information, since an area with many callers signals that vacancies are unlikely
The neighbours problem
Territory holders face a distinction between established neighbours and strangers, and they treat the two very differently. Responding aggressively to a neighbour whose boundary is already settled wastes energy on a dispute that was resolved, so residents in many species learn their neighbours' calls and respond weakly to them while responding strongly to unfamiliar ones, which has been demonstrated repeatedly by playing recordings from different locations. That arrangement is called the dear enemy effect and it saves both parties considerable effort. It has a limit, since a neighbour calling from the wrong place provokes a full response, indicating that residents track position as well as identity. Where territories are contested rather than settled the pattern reverses, with neighbours treated as the greater threat, which fits the underlying logic that the response should match the actual risk.
The dawn chorus
The concentration of calling at dawn is conspicuous enough to have generated a substantial literature and several explanations, none of which alone accounts for it. Sound travels further at dawn, since the air is still and temperature layering reduces the scattering that daylight turbulence produces, which has been measured and is a real advantage. Light is too poor for efficient feeding, so the time has a low opportunity cost. Overnight losses mean territories may have become vacant and a resident benefits from announcing occupation before anyone tests it. Females in several species are receptive early. And a bird that survived the night and can sing vigorously at first light is demonstrating condition, since the night is when reserves are depleted. The explanations are not exclusive and the current view is that several operate together, with the balance differing between species.
The cost of being heard
Signalling loudly and repeatedly carries risks that shape when and how it is done. Predators locate prey by sound, and calling animals are demonstrably more vulnerable, with several predator species specialising in hunting callers, which is why calling frequently stops when a predator is detected and why some species call from positions offering escape routes. Parasites use calls to find hosts, documented in flies that locate calling crickets and frogs. Energy expenditure is substantial in species that call continuously. Time spent calling is not spent feeding. And human noise interferes directly, with documented shifts in the frequency and timing of calls in urban populations of several species, some of which appear to reduce the effectiveness of the signal, which is a measurable consequence of environmental noise on animal behaviour.
The takeaway
Advertising a claim is cheaper than fighting for it, and the cost of producing a loud sustained call is what makes it informative about the caller's condition. Listeners extract identity, condition, size and position. Residents learn their neighbours' calls and respond weakly to familiar ones from the right place, which saves both parties effort. Calling attracts predators and parasites, which is the price.