By Keith L. Bildstein, Ph.D.
Sarkis Acopian Director of Conservation Science
Hawk Mountain Sanctuary
24 August 2016
With an extensive open-country distribution throughout much of southern South America, Red-backed Hawks are the functional equivalent of North America’s widespread Red-tailed Hawk. The mid-sized Buteo—red-backs weigh in at just over one kilogram, or roughly the same body mass of a Red-tailed Hawk—feeds mainly on small birds and small mammals across much of its continental range. But on the Falkland Islands, where the species also is common, red-backs focus on Upland Geese, a large terrestrial goose that weighs 3.5 to 4.0 kilograms, or more twice the body mass of a Red-backed Hawk. Given this rather unusual predator-prey size relationship, red-backs, which are non-migratory on the Falklands and remain paired-bonded throughout the year, hunt geese together in male-female duos with one hawk distracting the goose while other surprises and “takes down” the un-suspecting waterfowl. In many instances, both members of the pair kill the goose before feeding on it simultaneously, with both gorging themselves and often returning to the carcass for a day or more as they strip every last piece of edible tissue from it.
This cooperative feeding routine works well for the red-backs on the main islands of East and West Falkland, where there are few if any Striated Caracaras to worry about, but such is not the case where the two species co-occur on the smaller, peripheral islands. Our principal Striated Caracara (aka Johnny Rook) study site, Saunders Island, which is inhabited by several dozen Red-backed Hawks, as well as over 80 Striated Caracaras, is a case in point. On Saunders Island as elsewhere, caracaras may be incapable of subduing and killing Upland Geese on their own, but they are not above competing for a dead goose once a pair of red-backs have killed it.
Such was the case on Saunders last week when my colleague Katie Harrington and I came upon a pair of Red-backed Hawks feeding upon a recently killed Upland Goose in a sheep meadow at the eastern end of the island one morning earlier this month. Both hawks, but especially the female, had already gorged themselves on the carcass while cooperatively fending off several dozen, mostly juvenile caracaras that were attempting to partake in the feast. As we approached the group to read the bands on the caracaras—we have fitted more than 1,100 of Johnny Rooks with individually numbered rings as part of our long-term studies of the species—the male red-back took off. The female tried to do the same but was unable to do so given an enormously over-filled crop, which made it impossible while she was being attacked by more than a dozen caracaras that had pinned her down on her back and were feverishly “footing” and pecking at her. Sensing that she was not long for this world, I jumped off my ATV and ran her down after she broke free from the swarming caracaras.
My decision was instantaneous, our initial approach, which had spawned the male’s successful departure, had left the female vulnerable, and although interfering in nature is not something I typically do, in this case our approach had tipped the competitive balance in this ongoing interaction, and my rescue attempt was aimed at minimizing the consequences. The carcass was a little more than a kilometer from our cottage at our farm-settlement headquarters, and I remounted my ATV, cradling the hawk in my left hand while steering and thumb-throttling the ATV with my right. We reached the settlement several minutes later where I placed the female in a dog kennel to give here time to digest her crop peacefully.
Four hours later I released the hawk, who by then had digested about half of food in her crop, but to no avail, as a group for more than a dozen caracaras appeared as out of nowhere and proceeded once again to pin her down in what appeared to be a death grip. Once more I sped toward the hawk, ran her down, and re-rescued her, placing her back in the kennel with a plan to release her the following morning.
By the time I had grabbed her from the kennel the next day morning, the distended crop was no longer visible, and although half-a-dozen caracaras again initially pursued her, this time they kept their distance, as the lack of a crop most likely signaled them as to the danger in approaching too closely. After flying off and perching on a fence post for about 5 minutes, the hawk flew off un-pursued in the direction of the goose carcass and her mate. She was re-sighted at the settlement several days later holding her own against a group of caracaras fighting over a goose carcass that had been fed to the farmer’s pigs.
The sharper talons of the predatory Red-back Hawk make them formidable opponents to the less predatory and less well-armed caracaras—at least when not encumbered by an over-filled crop. However, when they do have a large crop, the interspecies-competitive relationship changes, and in mid-winter (austral-winter August is the equivalent of boreal-winter February) when both species are hard-pressed for food, it becomes something of a raptor-eat-raptor world on the Falklands Islands where things can change rapidly for individual birds.
Stay tuned… next week I will blog about yet another potential dietary item for food-stressed Johnny Rooks: kelp maggots.