Birds can be aggressive and attack other birds for a variety of reasons. Understanding what motivates avian aggression can help people better understand bird behavior and biology.
Territoriality
One of the most common reasons for birds attacking each other is to defend their territory. Many bird species are highly territorial and will aggressively defend their nesting areas, food sources, and other resources from intruders. Territoriality helps ensure breeding birds have adequate food, nest sites, and other necessities for raising young. Some examples of territorial bird species include:
- Hummingbirds
- Songbirds such as warblers and sparrows
- Birds of prey such as hawks and falcons
- Seabirds such as gulls and terns
Territorial birds will often attack and chase away intruders entering their turf. They may dive bomb, vocalize warnings, or even make physical contact by pecking or clawing. Birds generally only attack intruders during the breeding season when territories are established and defended.
Competition for Resources
Competition over limited resources is another common cause of aggression in birds. Birds may attack others when defending food, water, nesting materials, or other vital resources that are in short supply. Some examples include:
- Birds at a feeder may peck and displace each other to gain access to food
- Gulls may mob and steal fish from each other
- Hummingbirds defend flower patches as feeding territories
- Seabirds like boobies may fight over nesting materials like sticks or seaweed
A lack of resources can lead birds to become more aggressive towards competitors. Attacking others is a way for them to secure whatever resources they need to survive.
Mate Guarding and Sexual Competition
Male birds often attack others when defending a mate or competing for mating opportunities during breeding season. Mate guarding helps ensure paternity by preventing other males from copulating with a male’s chosen female. Sexual competition weeds out weaker males and allows prime birds to be highly successful breeders. Some examples include:
- Male songbirds chasing competitors from their mate’s territory
- Pheasants and grouse fighting over dominance and breeding rights
- Ducks forcing themselves onto females despite violent resistance from other males
The breeding drive leads male birds to be very aggressive against those seen as sexual rivals. Females may also attack each other when defending resources needed for nesting or defending their chicks after hatching.
Nest and Brood Defense
Parent birds, especially females, may attack approaching animals that appear to threaten their nest or young. Nest defense helps ensure eggs and vulnerable chicks are protected from predators. Some examples include:
- Geese hissing and chasing humans or dogs near their nest
- Killdeer performing distraction displays to lure predators away from the nest
- Hummingbirds dive bombing humans who get too close
- Owls swooping at pets, people, or other birds that encroach on their nest
Such aggressive nest defense normally occurs only during breeding season. Once the young fledge and leave the nest, adults become less territorial and aggressive.
Predation
Birds of prey and scavenging species often violently attack and kill other birds for food. Raptors including eagles, hawks, falcons, and owls prey upon other avian species. Scavengers like gulls and corvids also attack weak, injured, or vulnerable birds. Some examples include:
- Peregrine falcons diving at over 200 mph to knock birds out of the sky
- Eagles snatching birds off branches
- Flocks of gulls pecking and killing weaker injured birds
- Great horned owls eating ducks and pigeons
For predatory birds, attacking and killing other avian species is simply a means of obtaining food. It is an innate, natural behavior rather than aggression in the same sense as defending territories or mates.
Dominance
Fighting with other birds is one way some species establish a social hierarchy or pecking order within flocks. Dominant birds may attack subordinates to maintain their higher status. Some examples include:
- Chickens using their pecking order to keep subordinates in line
- Crows mobbing less dominant individuals that get out of line
- Seagulls fighting amongst themselves to determine flock status
Birds higher up in the pecking order enjoy better access to food, nesting sites, and mates. Aggression helps define rank and keep lower status birds from enjoying the same privileges as their superiors. Once the hierarchy stabilizes, overt fighting is less frequent.
Irritation and Fear
Birds may sometimes attack out of irritation or fear. Some examples include:
- Wrens mobbing owls or snakes that roost too close to their nest
- Swans hissing at boats that invade their space
- Hummingbirds attacking bright clothes that startle them
- Chickadees mobbing perched hawks
While such attacks are generally harmless, they reflect a bird feeling threatened and responding defensively. The aggression ceases once the offending irritation is gone. Birds may also unintentionally provoke attacks by invading another bird’s territory, nest, or food source.
Redirected Aggression
Sometimes birds redirect aggression towards another target when they cannot attack the original source of their irritation. For example:
- A territorial bird attacks an intruding bird that flies away, then turns its aggression on a different nearby bird
- A provoked bird trapped in a cage attacks its companion instead of the actual provocateur
- A predatory bird misses grabbing prey and grasps at a different animal instead
Misdirected aggression often occurs when a bird is breeding, territorial, or frustrated by failed predation attempts. The secondary target becomes an outlet for residual aggressive energy.
Brood Parasitism
Brood parasites lay their eggs in the nests of other bird species. The hosts often attack parasitic birds like cowbirds and cuckoos that threaten their parental investment. Some examples include:
- Warblers pecking at cowbird eggs or killing cowbird chicks in their nest
- Robins attacking cuckoos that invade their nests
- Foster parents driving away adult brood parasites near the nest
Such aggression towards brood parasites is an adaptive response to protect their own eggs and raise their own young. Birds discern foreign eggs and chicks and attack accordingly.
Mobbing Predators
Some birds collectively mob predators and dangerous intruders near their nesting area. They harass predators to drive them away or distract them from finding nests. For example:
- Crows converging to scold and dive bomb hawks, owls, or other predators
- Small songbirds mobbing cats that threaten their nests
- Seabirds swarming and defecating on predators near their colony like foxes
Mobbing provides protection against predators, even if no single bird could fend off the predator alone. It is a cooperative nest defense strategy. The more birds participating, the more effective the mobbing.
Innate Aggression
Some birds are more aggressive than others simply due to innate tendencies within their species. Traits like territoriality, dominance, and boldness vary across species. Some examples of birds known for aggressive behavior include:
- Geese
- Raptors like hawks, eagles, and owls
- Gamebirds like pheasants, grouse, and turkeys
- Corvids like crows, ravens, and jays
- Seabirds like gulls, boobies, and pelicans
- Hummingbirds
The degree of aggression depends on factors like breeding season, personality, lack of resources, and presence of threats. But some species are hard-wired to behave more aggressively on average.
Unusual Stressors
Birds may attack other birds in abnormal situations that induce stress, fear, or disorientation. Examples include:
- Birds trapped together during storms or accidents attacking each other
- Captive birds overcrowded in cages or pens attacking weaker flock mates
- Disoriented fledglings attacking others near them
- Caregiver birds like parrots redirecting aggression towards humans during hormonal surges
Stressful, unnatural conditions can lead birds to act more violent than normal. Their natural behaviors and tendencies get amplified.
Disease
Sick birds may attack others more readily. Some examples include:
- Birds with dementia attacking in confusion
- Encephalitis causing abnormal aggression in birds
- Parasites like avian malaria altering bird brains and behavior
- Toxins and poisons causing disorientation and lashing out
Any illness affecting a bird’s brain or cognition can lower its inhibition towards aggression. Sick birds may attack much more readily when they would normally avoid contact or conflict.
Conclusion
Birds attack each other for diverse reasons based largely on securing resources, territories, mates, and protection from threats. While such aggression seems negative, it is an innate natural behavior evolved to help birds survive and reproduce. Some reasons are more justifiable than others. Nonetheless, virtually all aggression has an underlying motivation related to self-preservation and biological success. Understanding the root causes can shed light on bird behaviors that may otherwise seem senseless or surprising.