Why Do Birds Gang Up and Attack?
It's not uncommon to see smaller birds ganging up on larger birds. Sometimes you'll be driving down the highway and notice a group of American Crows dive-bombing a perched hawk. What's behind this type of behavior, and what the heck are these birds up to?
Mobbing Behavior
When smaller birds harass larger birds (or other animals), they engage in a type of behavior called mobbing. Mobbing functions primarily as an anti-predator behavior. Most predators rely heavily on stealth for successful hunting, so advertisement of their location by potential prey species reduces the likelihood of a predator's hunting success. Mobbing behavior also serves to educate young, inexperienced birds to the identity of predators. Scientist have shown in controlled studies that some young birds can be experimentally tricked into mobbing inanimate objects.
Reasons for Mobbing
Large flocks of birds frequently assemble to torment potential predators. The alarm calls and related vocalizations serve to advertise the location of a predators, and helps recruit other birds to join in the foray. Mixed-species mobbing flocks are not uncommon, especially amongst passerines, where birds will often respond to the alarm calls of other species.
Patterns of Occurrence
Mobbing behavior occurs more frequently in the vicinity of active breeding grounds, especially in the presence of relatively vulnerable nestlings and fledglings. Species that reside on the same territories year-round will display mobbing behavior outside of the breeding season, but migratory species are unlikely to mob outside the breeding grounds.