For the Birds Radio Program: Mobbing and Piracy
Laura talks about birds ganging up on bigger birds, and birds stealing from one another. (3:52) Date confirmed.
Last week, I received a letter from a North Branch, Minnesota, listener who asks, “Why do I so often see several smaller birds touting a hawk? Are they trying to steal its quarry? I sometimes hear a lot of hollering going on with it.”
That kind of behavior is called mobbing. From the viewpoint of most small birds, hawks and owls are inherently up to no good, and the mere sight of a raptor elicits this mobbing behavior. The small birds all join forces to harass, swear at, and occasionally even strike the predator. Crows and small hawks mob Red-tailed Hawks, ravens, and especially Great Horned Owls; blackbirds and Blue Jays mob crows; robins mob Blue Jays; even chickadees mob saw-whet owls.
I don’t know what kind of evidence there is that this behavior actually drives away predators, but mobbing birds invest enough energy in their sport that it must have some value. Great Horned Owls are well known to watch where crows, ravens, and hawks go to roost and then to sneak up on them at nightfall to eat them. These owls are currently the Number One threat to the young Peregrine Falcons being reintroduced into Wisconsin and Minnesota. So it’s little wonder these birds harass the owls by day.
By far the easiest way to find owls is to follow the angry calls of mobbing birds to their source. At Hawk Ridge Nature Reserve, we capitalize on mobbing behavior by setting out a Great Horned Owl decoy near the main overlook. It frequently brings in Merlins and other rare migrants so our visitors can have a better look.
“Mobbing” behavior is exactly what the name implies. The small attackers always outnumber their target. But Ruby-throated Hummingbirds and Eastern Kingbirds are so feisty that they’ll take on a predator as large as a Bald Eagle single handedly (wingedly?).
Another behavior which is similar in a way to mobbing is piracy, when a bird steals the food from a smaller bird. This behavior is also called kleptoparasitism. It’s rare in songbirds, and is found in only one duck, the baldpate or American Wigeon. But piracy is very common in some hawks, gulls, and jaegers. All of these birds are perfectly capable of catching their own food, but often watch for a smaller bird to make a catch and then steal it away. Bald Eagles frequently divebomb Ospreys until the Osprey drops its fish. Herring Gulls chase other gulls, including the young of their own species, to get them to regurgitate their fish. Jaegers, oceanic relatives of gulls, specialize in this kind of piracy over the high seas and occasionally over to the Great Lakes.
Sometimes the thieves themselves are robbed. There is one record of a falcon, a Merlin, stealing a catch from a European accipiter. The Merlin itself was robbed by a Honey Buzzard, and then the buzzard was robbed by a Peregrine Falcon. This incident or one like it may have provided the original inspiration for the axiom that there is no honor among thieves.
House Sparrows have been recorded stealing katydids from female digger wasps. And the most specialized of all avian pirates are the frigatebirds. These oceanic fish eaters have so reduced their oil gland that their dry, light feathers give them the greatest wingspan-to-body-weight ratio of all birds. This may enhance their aerobic flight to make them the fittest of all pirates, but it comes at a cost. Those unoiled feathers prevent them from ever entering the water to catch their own food or even to rest.