A territory is defined as an area defended by an animal or group of animals against others of the same sex or species: a more accurate definition should include that it is when an individual or group defends an area to allow for the continued use of the resources in that area which are likely to increase the individual’s fitness. Fitness is a measure of reproductive success and any behaviour that increases fitness also increases the chances that an individual will pass on their genes to the next generation.
Defending a territory is an energy demanding process therefore there must be a benefit to the territory holder. Jerram Brown, a renowned ornithologist and evolutionary biologist stated that territoriality likely came about due to the competition between members of the same species for limited resources (intraspecific competition) and then sustained itself in populations through aggression. This makes sense as territorial behaviours are displayed mostly through aggression. If we factor in individual selection which states that all individuals will behave selfishly in an effort to increase their chances of passing on their genes to the next generation, there might have been individuals who were able to increase their fitness by behaving selfishly and defending a limited resource e.g. mates, food or nesting sites. If they were successful, they would have passed on their genes and as a result this “trait” for territorial behaviour would be selected for (or favoured). In the same population, the individuals that did not display territoriality would be selected against and not pass on their genes.
Territoriality in birds has been studied extensively and currently there are 6 recognised types of territories from Type A to Type F. Type A territories are territories used for pretty much everything birds do - mating, nesting and feeding, for example the African fish eagle could be classified as holding a type A territory. Type B territories are used for everything except feeding - they are primarily a display ground, Hen harriers hold type B territories. Type F territories are for roosting only and types C - E are everything in between, but as with most things in the natural world, these are not fixed “rules”; there are some grey areas where territories can be Type A and B together…you get the gist.
Territorial Signalling
Territorial species must be able to defend their territories and one way that many species use to defend their territory is through territorial signalling. As fighting is a costly procedure and can lead to serious injuries or death, many species will only escalate to a fight as their last option.
Territorial signalling is commonly done by displaying either through calling or performing a visual display. The African fish eagle is an extremely territorial species and they fiercely defend their territories. They will call regularly in duet to advertise their territory and if an intruder flies into their territory they will almost always chase them away.
Unlike the African fish eagle, the Verreaux’s eagle is usually quite silent and will not call as often. One of the most comprehensive studies on the Verreaux’s eagle in Africa was conducted in the Matopos hills in Zimbabwe by Valerie Gargett and other members of the Rhodesian Ornithological Society in the 1970s. The Matopos hills have a high density of Verreaux’s eagles and part of the study looked at their territorial behaviours towards each other. These raptors were never seen to be overly aggressive towards neighbouring pairs but would engage in fights especially with intruding single adult individuals who were not recognised. They also had exposed perches at different parts of their territories - being able to keep their neighbours in sight seemed to be important in maintaining their territorial boundaries i.e. possibly checking that their neighbours are not trespassing. Most territory intrusions resulted in the territorial pair giving chase to the intruder, they would circle to gain height and then swoop at the intruder from above and sometimes lock talons and cartwheel down in what is a relatively common aggressive behaviour in eagles. After an intrusion, pairs or individual territory owners were commonly seen to display - their displays involve a series of swoops with wings held quite close to their body, they then rise to gain height and at the top of their curve they somersault or roll. They also display in a pendulum form.
A recent study published in the Journal of Zoology in May 2024 proved that Bonelli’s eagle territories remain relatively stable in size over time even when the territory owners change. The study looked at 22 territories in Spain between 2015 and 2021. It presents a novel way of looking at territories whereby the territory remains fixed and different individuals will compete for these patches of “real estate”. It would be interesting to see if this is how some of our African raptors behave and what kind of underlying factors are at play to help maintain these territories.
Another method of territorial signalling is done by pairs during their breeding seasons. There is a trend in the frequency of raptor copulation (with some exceptions of course) whereby there are two peaks in the number of copulations performed - the first peak is just after a pair has formed or reunited for the breeding season and the second at the start of egg laying. In many species the first peak in copulation frequency is far out of the period where the female is actually fertile and therefore the copulations during this period don’t seem to serve any purpose in fertilisation. Many raptors, when copulating, will call and vocalise. In addition, they sometimes copulate in an exposed area. If the female isn’t fertile, how does all this activity help them in any way as this is probably going to attract a lot of attention? Luckily for them, evolution is quite smart - raptors are known to false copulate and this is thought to be a form of territorial signalling. When you are at the top of the food chain and your only threat comes from members of your own species then advertising that you and your partner are a pair and hold this territory might help you keep the others at bay. It is hypothesised that raptors will false copulate as a territorial signal to warn other members that they are occupying that territory. There is evidence to support this theory - when raptors false copulate their cloacas will often not even make contact.
Diurnal Raptors
Diurnal raptors are raptors that are active during the day like eagles, falcons and buzzards. Their primary method of hunting is by sight and the members in this group have some of the best, if not the best vision in the animal kingdom. These raptors come in different sizes and therefore they can only hunt certain sized prey for a given body size - body mass and territory size has been found to be positively correlated. The map below shows estimated territory boundaries of GPS tagged Martial eagles in the Maasai Mara between 2016 and 2018. This species, like many raptors, displays reverse sexual size dimorphism whereby females are larger than males. Females being bigger generally hunt bigger prey compared to males and this would then imply that their territories are larger too. Larger prey is naturally less abundant than smaller prey due to the way energy flows in ecosystems; hunting larger prey would require more space and this is consistent with what is seen in the map below - female Martial eagle territories are generally larger than male territories. So if you hunt larger prey it would make sense to have a larger territory and vice-versa for small prey - remember, holding a territory requires energy and therefore you need to balance your costs.
Another factor that is hypothesised to play a role in determining territory size and nest spacing in diurnal raptors is the agility of the prey hunted. Raptors that hunt agile prey e.g. birds and mammals tend to have larger territories than those that hunt less agile prey like reptiles and amphibians. This makes sense if you think about how agile prey could cover more ground when being chased and therefore the distance covered for a successful hunt would be much greater for raptors hunting agile prey. If they had small territories their prey could escape into their neighbour’s territory where the raptor would most likely be chased off by the territory owner before it could capture a meal. Hence, according to this hypothesis, in habitats where raptors hunt agile prey you would expect each territory to be larger and nests to be more spaced out as this would reduce the competition between individuals - each individual would interfere less with its neighbour. Red-shouldered hawks have a high number of reptiles and amphibians in their diet compared to mammals and have an average territory size of about 1.2 square kilometres whereas the similarly sized (slightly bigger) Red-tailed hawks which have a diet that consists mainly of small mammals have an average territory size of about 5.7 square kilometres.
In a study that investigated 74 raptor species, territoriality was shown to have the highest probability of occurring in raptors that hunt very agile prey and nest in concealed sites. Nest predation poses a serious problem to raptors and it is not uncommon for nesting attempts to fail because a clutch was found by Baboons or other nest predators. Species that mark their territories by calling and displaying could potentially attract nest predators and therefore having a nest in a concealed area would make it harder for the chicks/eggs to be predated on. These types of raptors would also benefit from a concealed nest site if they spend a lot of time away from their nest foraging. Being territorial would confer an added advantage to the individual/pair as they can protect their clutch from predators which in many cases include their own species.
Colonial Nesting
Colonial nesting raptors like the Rüppell’s vultures nest together in large groups on cliff faces. Colonial nesters are different from most other raptors because they don't hold large territories. Their territories are their nests and sometimes a few feet out from their nest (this would be a type D territory). Colonial behaviour was found to have the highest chances of occurring in raptors that breed in exposed nest sites, hunt less agile prey and have unpredictable food sources e.g. carrion. An unpredictable food source means that you cannot be absolutely certain where you will find your next meal. For example, if you are a vulture and scavenge on dead animals and these animals tend to move around a lot then it makes no sense trying to defend a patch because you cannot guarantee that these dead animals will show up in your patch of territory. Instead, it would be better to follow these herds or groups as they move and wait for some to die or get killed.
Nocturnal Raptors
Owls are a fascinating group of birds, perfectly adapted to hunting at night. Being quite secretive and largely inactive during the daytime they are difficult to study and therefore territoriality in African owls has not been studied extensively. Being predominantly nocturnal, owls rely more on auditory signalling to maintain territories - individuals will call to announce their presence and territory to other individuals or mates. A large proportion of the studies on African owls were done by inferring individuals or pairs from calls heard, this is the easiest way to study owls but the limitation (in terms of studying territoriality) is that you cannot form a comprehensive map of territories and territory spacing just from calls alone. Calls play an important role in helping to relay information to other individuals about the presence, condition, and reproductive status of the territory holder. Communication via calls depends on certain factors such as the risk of predation, breeding seasons, environmental conditions and largely on the intent of the individual. A study done in Spain on Eurasian eagle owls showed that the owls will call more frequently on full moonlit nights than on dark nights and the reasoning behind this is quite interesting. Eurasian eagle owls have a white throat patch which they use to display when calling (as they vocalise their throat enlarges making the white patch appear larger). Bright nights would mean that their throat patch would become more conspicuous and they are able to convey their message better. They would also perch higher up when displaying on moonlit nights than on dark nights. Another study conducted on twelve species of African owls in Malawi and The Gambia also noted an increase in calling during full moon nights in the African barred owlet, Pearl-spotted owlet and the Southern white-faced owl. The effect of predation risk was also observed in the vocal activity of some of these owls - African scops owls would stop or call less when Barn owls were active, as Barn owls are known to attack African scops owls.
A different study showed that Eurasian eagle owls, at the start of their breeding season, may mark their territory with their faeces and feathers from the prey they hunt. Their faeces are bright white and were seen to be deposited on posts and plucking sites in a very conspicuous manner around their nests. This behaviour may also have a role in communicating their reproductive status to other individuals.
When reading about territorial behaviour, it can sometimes give the impression that large raptors are highly aggressive birds that could rip your arm off. In all animals behaviours are only performed if the benefit of performing that behaviour outweighs the costs. Raptors are an incredible group of powerful birds and territoriality is an integral part of their survival and fitness. They don’t always behave aggressively, only when they have to - if you still don’t believe me then pay a visit to the Naivasha Raptor Centre and ask for Phil. And no, they don’t fly around ripping people’s arms off (although some could).
About Pranav Peshavaria
Pranav was brought up in Kenya and his fascination with birds and wildlife pushed him to pursue a
degree in Wildlife Conservation. In 2021 he volunteered with the Kenya Bird of Prey Trust to conduct
a survey of African fish eagles at Lake Naivasha as part of his undergraduate thesis. Pranav is also a
member of the Nairobi Ringing Group and takes a keen interest in raptors.
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Kenya Bird of Prey Trust