Territoriality and hierarchical behaviour
Two ways of settling competition without constant fighting
- Direct fighting over every resource would be enormously costly — energy expended, injury risk, time lost from feeding.
- Territoriality and dominance hierarchies are both ways of converting scramble competition into contest competition, so that access is settled once rather than renegotiated at every encounter.
Territoriality
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A territory is an area that an individual or group defends against others of the same species.
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It differs from a home range, which is simply the area an animal uses. A home range is not defended; a territory is.
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Territories are defended for one or more resources:
- Food, nest or den sites, shelter, mates, or a combination.
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How territories are advertised and defended. Defence is mostly signalling, not fighting:
- Song and calls — a bird singing from a perch advertises occupancy over a wide area without any physical contact.
- Visual displays — postures, plumage, size displays.
- Scent marking — long-lasting, works in the owner's absence and in darkness.
- Physical attack — the last resort, used only if signals fail.
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Signalling is favoured because it is cheap and safe relative to fighting. Both animals benefit from settling the contest without injury, which is why displays are so often exaggerated but harmless.
The economics of territory size
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Defending a territory has costs — energy for patrolling and displaying, time not spent feeding, risk of injury — and benefits — exclusive access to resources.
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A territory is therefore only worth holding when benefit exceeds cost, and this predicts its size:
- Where food is abundant, a small territory supplies enough, and defending more would waste energy for no gain.
- Where food is scarce, a larger territory is needed to supply the same requirement.
- If food is extremely scarce or very patchy and unpredictable, territoriality is abandoned altogether — the area needed would be too large to defend, so the cost exceeds any possible benefit.
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Economic defendability is the key idea: a resource is worth defending only when it is predictable, concentrated in space, and not too abundant. This explains why nectar-feeding birds defend flowering trees but seabirds do not defend the open ocean.
Dominance hierarchies
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A dominance hierarchy is a ranked social order within a group, in which higher-ranking individuals gain priority of access to resources.
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It is often called a pecking order. In its simplest form, individual A dominates all others, B dominates all but A, and so on.
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How rank is established and maintained:
- Rank is settled by initial contests — displays and occasional fights — when the group forms or a new individual joins.
- Once established, it is maintained by recognition and signalling. A subordinate that recognises a dominant defers without a contest.
- This requires individual recognition, which is why hierarchies occur in stable groups rather than anonymous crowds.
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Why subordinates accept low rank. This is the question examiners ask, and the answer is a comparison of alternatives:
- Fighting a stronger individual risks serious injury or death, and would probably be lost anyway.
- Remaining in the group at low rank still provides some food and the group's anti-predator benefits.
- Subordinates may inherit higher rank as older individuals die, so accepting low rank now preserves a future opportunity.
- Leaving the group means facing predators alone and finding an unoccupied area — often worse than being subordinate.
- So subordination is the best available option, not an absence of competition.
Advantages of a hierarchy over repeated fighting
- Less energy and injury. Once rank is known, contests are settled by recognition, so both individuals avoid the cost.
- Predictable access. Dominants get reliable resources; subordinates get less but survive.
- Group stability. The group stays together, so its collective anti-predator and foraging benefits are retained.
- Population regulation. In poor conditions, the lowest ranks fail to breed or die first — so mortality falls on a defined subset rather than all individuals starving equally. This is contest competition, and it is why hierarchical populations are more stable.
Selective advantage
- Territoriality.
- The holder gains exclusive access to food, so it obtains more energy per unit of foraging effort and does not have to search areas already depleted by rivals.
- It secures a nest site and shelter, and a familiar area where it knows escape routes and food locations.
- More energy and greater safety mean more resources for egg production and provisioning young, so more offspring survive.
- Because territory holding is heritable in its underlying traits, the alleles increase in frequency.
- Dominance.
- High rank means priority access to food and mates, so dominants are in better condition and father or bear more offspring.
- Subordination.
- The advantage is survival. A subordinate that defers avoids injury and stays in the group, retaining the chance to rise in rank or breed later. Its lifetime reproductive success is lower than a dominant's but far higher than that of an individual killed in a hopeless fight.
Worked Example
Worked Example
A nectar-feeding native bird is studied at two sites.
- Site A: flowering trees are densely clustered and flower steadily for six weeks. Birds hold territories of about 0.2 ha and chase intruders.
- Site B: flowering trees are widely scattered and each flowers unpredictably for only a few days. Birds hold no territories and feed in loose flocks.
- At site A, territory holders spend about 15% of daylight on defence.
- At site A, when flowering peaks and nectar becomes superabundant, birds stop defending and tolerate intruders.
Explain why the birds are territorial at site A but not at site B, and why they abandon defence at peak flowering.
Answer:
The whole pattern follows from one principle: a territory is held only when the benefit of exclusive access exceeds the cost of defending it. Defence costs energy and time — here 15% of daylight, which is time not spent feeding — so it must be repaid in extra nectar obtained.
Why territories at site A. The resource here is economically defendable, because it has three features:
- Concentrated in space. Densely clustered trees mean the area needed to supply the bird is small (0.2 ha), so it can be patrolled and defended at modest cost.
- Predictable in time. Steady flowering for six weeks means the territory will still be worth holding tomorrow, so the investment in establishing it is repaid over many days.
- Not superabundant. There is enough that exclusive access matters, but not so much that intruders take nothing of value.
Under these conditions a holder gains substantially more nectar than it would sharing the trees, and the gain exceeds the 15% cost. Individuals that defend therefore obtain more energy, are in better condition, and produce more offspring than those that do not.
Why no territories at site B. Here the resource fails the same tests:
- Widely scattered, so a bird would have to defend a very large area to meet its requirements — the patrolling cost would rise enormously.
- Unpredictable and brief, flowering for only a few days, so a territory established today may be worthless tomorrow. The cost of establishing it could never be repaid.
The cost of defence would therefore exceed the benefit. A bird that attempted to defend would spend energy patrolling instead of feeding and would end up with less energy than one that simply moved between patches as they opened. Selection therefore favours not defending, and flocking instead — which brings the additional benefit that more individuals locate scattered patches sooner.
Why defence stops at peak flowering. When nectar becomes superabundant, the resource exceeds what the birds can consume. An intruder now takes nectar the holder would not have used anyway, so exclusion gains it nothing — the benefit of defence has fallen to almost zero while the cost has not. Continuing to defend would mean paying 15% of daylight for no return.
This is strong evidence that the behaviour is a flexible response to current conditions rather than a fixed trait. The birds are not territorial by nature; they are territorial when it pays. Individuals that adjust their behaviour to the economics obtain more energy across the whole season than those that defend rigidly, so the capacity to adjust is itself what selection has favoured.