Monday, November 16, 2009

On Great Horned Owls

Photo at left by Lloyd Spitalnik
Great Horned Owl - 1/10/06

The report on eBirds today was what everyone hoped to hear:

Great Horned Owl spotted by Susan Loesser in Ramble near Azalea Bridge, 1:15 pm.

The odds are increasing that this owl may be in the park for a long, or at least longish visit.

In Central Park in the Dark I wrote about the last time a Great Horned Owl settled into Central Park for a lengthy stay. That was in 2006 and the owl stayed for 40 days.

Below, an excerpt from the chapter entitled Little Red and the Big Bad Owl, with the first of several notes from our favorite Red-tailed Hawk expert, John Blakeman.

from Central Park in the Dark, pp 215-217.

{Previously, Chris Wood, a brand new birdwatcher, discovers a Great Horned Owl in a willow near Willow Rock during the 2006 Christmas Bird Count)

In the history of Central Park birdwatching, no great horned owl had ever visited the park for more than a couple of days. But Chris’s owl didn’t leave after the first day, or the next or the next. Soon crowds gathered every night to see that rare event, a great horned owl fly-out. As the days turned into weeks, some of us began to worry: What if the bird had moved in to stay, the way Pale Male had done some fifteen years earlier? Would that make trouble for our resident red-tailed hawk pair?

The hawk expert John Blakeman seemed to know as much about nocturnal raptors as he did about redtails and other daytime hawks. In answer to our questions, he wrote:

Red-tails and great horneds in the East, South, and Midwest have a remarkable relationship, an ecological détente, as it were. Here’s how it usually works:


The crucial fact is that great horned owls don’t build nests.They simply expropriate (read that as steal) existing ones. No one—even if they could see at night—will ever observe an owl bringing twigs to a nest site. Owls don’t do that. The great horneds just perch themselves on an existing red-tail’s nest in December or January and claim it. GHOs [a common abbreviation among birders] are larger and more muscular than red-tails, and at night redtails can see about as well as we can. Red-tails will intelligently abandon a nest claimed by a great horned owl. The hawks will frequently go just a short distance and build a new nest, sometimes even in the same woodlot, perhaps only a few hundred yards from the old nest.

At night a great horned owl could easily drop down upon an incubating red-tail and have her for a midnight snack. Conversely, in broad daylight, when a female owl is hunkered down over her owlets, a red-tail could do a classic red-tail stoop, dive, and take the head off the owl before it knew what hit her.

But these things seldom happen. There seems to be an understanding between the species that the owls will do their things at night, and the hawks theirs in daylight. By this arrangement, the owls find beautifully formed, existing nests to claim each December (long before the redtails resume nesting activities). As for the red-tails, they don’t get killed by the larger, more aggressive owls This is a raptorial version of MAD, mutually assured destruction, the tenuous but effective arrangement between American and Soviet thermonuclear powers that used to be talked about during the Cold War.

So far there have been no MAD things in Central Park. So far there has been only one ecological superpower, the red-tailed hawks. But might that change? I think it could. Great horned owls hunt for and kill the same prey as red-tails, rodents. They use exactly the same habitat, only at night instead of in the hawks’ day-hunting periods. And clearly there is an abundance of available food in Central Park. Because they are nocturnal hunters, great horneds aren’t going to take many pigeons. But at night there will be a surfeit of rats that owls could thrive upon, the same population of rats the red-tails catch by day.

Blakeman’s letter allayed our anxieties about the red-tailed hawks. Obviously Central Park has more than enough rats to go around, enough for Pale Male’s family and for a great horned owl family. But as we studied various accounts of GHOs and their behavior, we began to understand what Blakeman meant when he called it a new superpower. This was one big savage hunter! Maybe the powerful red-tailed hawk had worked out a cordial détente with the GHO over the centuries. But what about smaller creatures? What about the screech owls we had come to be so fond of? According to A. C. Bent’s Life History of North American Birds, the great horned owl “is not particular as to what it kills for food and will take what is most available and most easily caught.” It does on occasion eat birds, even other owls. Bent’s long paragraph about the avian preferences of Bubo virginianus itemizes the various owl species on the big owl’s carte du jour—barred owls, saw-whet owls, long-eared owls, and short-eared owls. But the eastern screech owl leads the list. It is the great horned’s favorite dinner-owl.