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 A  Collusion of Harriers

The winter of 2012-2013 brings an unusual number of Northern Harriers to Virginia's Shenandoah Valley. We see them in strong numbers and in expeditions; not just individuals quartering separately but pairs and trios sectioning the fields at staggered altitudes and in close proximity. A juvenile Harrier crosses a pasture flying low, straight, as if to a destination. A second bird, an adult male, follows 100 feet behind and 100' above. They recede from view in that echelon formation. On a friend's farm two Harriers circle low over her stand of native warm-season grasses, again an adult and a hatch-year bird. They tangle and the adult drives the young bird into the tree line, but two more approach from different directions to hunt the 30-acre grass stand, and the gray bird gives up his swaggering. A minute later they all leave. Out my kitchen window a juvenile sits on  a fence post scanning the winter-brown berry patch. Egad, three posts away sits another!  Then a third juvenile makes a run up the row of raspberry canes and both the perched birds turn to watch.

The close proximity of these hounds of the grasslands is not likely the result of chance; there are hundreds -- OK, thousands -- of acres of pasture in the Shenandoah Valley near Buffalo Gap
and if the Harriers so desired each could have a private hunting reserve. Instead they sometimes cluster, to the extent that if you see one they have trained you to look for another nearby.

So what's going on here? You don't suppose this is... cooperative hunting? The two perched juveniles watching a third make a  pass nearby behave similarly to a squad of Harris' Hawks I saw in Cochise County, Arizona; some perching to watch as others maneuvered. Harris' Hawks are accomplished cooperators, purposefully chasing and flushing while teammates wait to strike. But perhaps there is also a less coordinated, if still intentional, mode of gang hunting. Short-eared Owls do it, wheel and scan for grassland rodents in grumpy proximity,  tangling when their circles overlap. But  chummy or not, they bark at one another and cluster-hunt and you have to suspect there is a net benefit. They could as easily hunt singly. Perhaps it's not essential that everybody agree on an attack plan to bring off an ambush -- something less elaborate than the old  "You wait here while I circle around and flush him your way" trick might work occasionally. It may be sufficient that the adult Harrier who was dogging the young bird in echelon will nab a rodent fleeing the lead bird but unaware of the second. It may be that adjacent-hunting raptors would just as soon make a meal of one another as snatch a vole, but the vole is an easier snack and likely more rattled by two Harriers than by one. It may be that experienced adults clepto-hunt by keeping an eye on juveniles. Other raptors and scavengers, too -- a Common Raven for example -- might be caught on camera confiscating a young Harrier's catch.

Or the explanation may be as simple as young Harriers in casual collusion as a means of making it through their first winter.

Update: Researcher Amy Johnson sends her photos of a pair and a trio of Harriers hunting together.