By Keith Bildstein, Ph.D.
Sarkis Acopian Director of Conservation Science
13 August 2014
Striated caracaras, or Johnny Rooks as the Falkland Islanders call them, by far are the most curious birds I know. Inquisitive beyond initial belief Johnny Rooks will walk more than a kilometer to learn what you are up to, and many appear to thrive on dismantling and flying off with anything new–and small enough to carry–that they encounter. Famed 20th Century evolutionary biologist Ernst Mayr, once classified such birds as being an “open programmed” species, or in layman’s terms, a species constantly curious about their environments. Island-dwelling birds in particular appear this way. By all accounts the small dark parrots called Keas that inhabit many of New Zealand’s more remote areas are every bit as inquisitive as Johnny Rooks, and in many ways, so too are Galapagos hawks.
The Johnny Rook’s extraordinary curiosity is thought to increase feeding opportunities on the Falklands and, indeed, curiosity appears to work quite well in this regard, as Johnny Rooks appear willing to feed upon all varieties of meat they encounter, both fresh and rotting, and even processed and extruded, as when they consume Sea Lion feces.
That said there does remain one mysterious exception in what they are willing to eat: small rodents. Rats and mice, both of which were introduced to the islands by humans, do not appear as items on the caracaras’ dinner plate, even though they occur on many of the islands in the archipelago. Why this is so continues to baffle those who study the birds, including me and my coworkers.
First hearing about this dietary prohibition tends to engender disbelief. Surely the birds can be coaxed into eating rats and mice. At least that is what I thought. I had the chance to test this hypothesis when I visited tiny Steeple Jason Island at the northwestern tip of the archipelago in August 2012, in the middle of the region’s austral winter. The site is home to an estimated 85 nesting pairs of rooks and surely some of them would be willing to consume a few mice. I was traveling with a team of biologists that were trying to assess the possibility of “de-mousing” the island with a rodenticide to reduce mouse predation on nestling seabirds there, and had access to plenty of freshly killed, snap-trapped house mice that were part of an ongoing distribution and abundance study of the pests.
Offering the mice to the rooks was easy enough. Within hours of my arrival the birds had learned that I was trying to trap and band them using mutton as bait, and I quickly became a rook magnet. Tossing small “mouse-size” bits of mutton to peak their interest, a crowd of several dozen birds all-but-immediately appeared at my feet. But a funny thing happened when I tossed the birds the real thing. At first they backed off a bit before returning to look them over and, in a few instances, picked them up and nibbled them. At least one rook cached, or stored, a mouse in some tussac grass, and eventually one pulled on a mouse it was holding in a seemingly, but somewhat tentative, attempt to prepare it for consumption. None, however, actually ate a mouse and most definitely, none of the birds gobbled them up as they did with the mouse-size bits of mutton I was tossing at the same time. Similar experiments by others have yielded similar results… occasional caching, investigation, and nibbling, but nothing that could be called even regular feeding.
Why would a bird so darned curious as about it environment be so cautious, albeit almost fearful of this perfectly harmless potential prey item, an item the islands variable hawks routinely kill and consume. Is it because these prey are “hairy?” This doesn’t seem likely, as rooks frequently scavenge the remains of rabbits that variable hawks have killed and partially eaten. And Johnny Rooks are not put off by the wool of dead sheep.
That said, the answer to this question–and I don’t believe I know it quite yet–may lie in the similarly cautious behavior that many avian scavengers exhibit when approaching seemingly fine carcasses. The large Gyps vultures of Africa are said to ignore the carcasses of carnivores including lions, as well as those of primates, including baboons, while ravenously consuming those of zebras, wildebeests, and dozens of other species of ungulates. Northern ravens, which routinely feed alongside wolves that have killed deer and elk, as well as upon gut piles left behind by hunters, often appear fearful of intact deer carcasses that have been slit from abdomen to throat by researchers wishing to study the species scavenging behavior. And, I have seen similar behavior at road-killed carcasses of squirrels and ground hogs that I have used as bait while trapping black and turkey vultures who have spotted and directly passed over and looked at these offerings before turning way and flying off beyond the horizon.
Some researchers have concluded that when it comes to carcasses, novel food items, particularly when offered in novel setting, no matter how appealing they may be to humans, are just too suspicious to the scavengers involved to merit more than a passing glance.
For well over two years human settlers on the Falklands have been butchering marine mammals, seabirds and their young, and, more recently, livestock, for their own consumption, and in so doing have attracted Johnny Rooks to their actions. How long it took for the first starving rook to feed upon this new human largess is unknown, but certainly it happened a long, long time ago.
Apparently other rooks eventually caught on, and what had once been a suspicious carcass situation became common place and attractive. How many times, by comparison, have humans offered rats and mice to rooks? Aside of the inquisitive researcher, probably not very often… at least not often enough. This is my working hypothesis.
I am not alone in this regard. More than a few researchers working with northern ravens have reached a similar conclusion. The relationship between wolf killed carcasses and a safe food base goes way back, presumably hundreds of thousands of years. And the relationship between hunters and their gut piles of deer and elk, at least several hundred years. The relationship between human researchers and the carcasses they put out–slit open or not–remains too novel to be accepted as safe.
One of my students is deeply immersed in trying to solve this puzzle, and I am convinced, that given sufficient time, eventually she will solve this mystery. In the mean time I wonder what the rooks would do if we offered them an ice cream sundae or a banana split?
To help support our efforts to better understand and protect the near threatened striated caracara email me at Bildstein@hawkmtn.org