Into the Nest: Home, home on the range (northern harrier style)
Northern Harriers are my favorite raptor. Their graceful, floating flight above the grass is one of the first images that comes to mind when I think of Wisconsin prairies. I love that they are sexually dimorphic, which is unusual for raptors: the males are gray with black wing tips (hence their nickname “gray ghost”) and the females are generally brown. The more I learn about their behavior, the more interesting they become.
Harriers are usually socially monogamous, with one male and female to each pair. Sometimes, however, they can be polygamous with one male maintaining pair bonds with several females. In Wisconsin, about 11-14% of the males are polygamously mated and about 20-30% of the females were in harems (Hamerstrom et al. 1985). Their nests are placed closer together than most raptors, both because habitat is limited and also because they are sometimes polygamous. Nests of harem-members are usually closer together than nests of monogamous pairs, and are typically more than 100 m apart (Burke 1979).
Harriers usually only defend small areas around their nest, and defense is strongest while establishing pair bonds and incubating eggs. When defending their nest area, females lower their legs as they fly towards an intruder - sometimes grappling with their talons- and escort the intruder away (Barnard and Simmons 1986). Males will use similar behavior to remove other males from their territory. They also engage in sky dancing both to defend territory boundaries and to attract mates (Simmons et al. 1987).
Sky dancing Harriers make deep, undulating, dips while flying. They may flip over to fly upside-down, while calling “kek-kek-kek”. The sky dance often ends with the male landing in habitat suitable for a nest site. Usually the male performs the sky dance to attract mates, but females will sometimes join the male in dancing.
Once birds are pair-bonded, they perform food-transfers while in flight (Simmons et al. 1987). While flying together, one bird will drop a food item. The other member of the pair will catch the food mid-air, sometimes flipping over in the process.
Hamerstrom, F., F. N. Hamerstrom and C. J. Burke. 1985. Effect of voles on mating systems in a central Wisconsin population of harriers. Wilson Bull. no. 97:332-346.
Burke, C. J. 1979. Effect of prey and land use on mating systems of harriers. Master's Thesis, Univ. of Wisconsin, Stevens Point.
Barnard, P. and R. Simmons. 1986. The functions of leg-lowering in territorial predatory birds. Ostrich no. 57:107-109.
Simmons, R., P. Barnard and P. C. Smith. 1987. Reproductive behavior of Circus cyaneus in North America and Europe: a comparison. Ornis Scand. no. 18:33-41.Simmons et al. 1987