The Butcher Bird by Myles Bakke
The shrew was so tiny that the spike that had been used to impale it had nearly passed through its entire body, pushing a bulge of hide out on the opposite side. It was either a pigmy shrew or a masked shrew, the only shrews this small occurring in this area. These are among the smallest mammals on the planet, seldom weighing more than six or seven grams (depending on when they last had dinner), with head and body length less than two and a half inches. For a weight comparison, a nickel weighs five grams, brand new.
I decided it was likely a masked shrew, Sorex cinereus, based on the length of the tail, its silvery belly, and the few dark hairs in a small tuft on the tail’s tip. A close examination of the cheek teeth would yield a more definitive identification, but I didn’t have a hand lens and was satisfied with the field markings.
This was the cache of a shrike, hung here for later consumption, if needed. Known as “shrike larders,” they may be used habitually if they are near a nest site, or only occasionally, for convenience, near hunting perches spread over a large territory.
Shrikes belong to a large Order of birds, the Passeriformes, often referred to as perching birds, or songbirds. The predatory shrike family, Laniidae, consists of 33 species and four genera most of which are found in Eurasia and Africa. Two are native to North America, the only ones to be found in the western hemisphere, and both are found in Minnesota. The loggerhead and the northern shrikes have ranges that somewhat overlap spatially, but not seasonally. The loggerhead shrike, Lanius ludovicianus, is a southern species, and a few migrate this far north to breed, although in recent years it has become a rare visitor. Its Genus name, Lanius, is the Latin word for “butcher.”
The northern shrike, Lanius excubitor, does not nest here, but visits to enjoy the more balmy Minnesota winter from its breeding range in the Arctic and sub-arctic. By the time one species shifts into the state, the other is shifting out, and the twain is unlikely to meet.
It is likely that few people have ever heard our shrikes sing, and while they do sing, it probably is not for declamation of territory, but rather vocalizations between the mated pair. The songs have been described as being softly sung or whispered, in a rhythmic series of trills and warbled phrases, by both males and females, while on the breeding territory. Since shrikes include other small birds among their prey species, it is not surprising that they are understated about announcing their presence. The male marks his territory in another way, using visual cues.
As part of my responsibilities as the Carleton Arboretum Manager, I also managed McKnight Prairie, a small tract of land owned by Carleton, a few miles east of campus. The prairie is within the boundary of Minnesota’s driftless area, that the last period of glaciation left untouched. Two sandstone hills run east to west, dominating the site, and are home to original prairie remnants that escaped serious damage from surrounding agricultural activities. A pair of loggerhead shrikes nested at McKnight for a number of years, their nest concealed in the branches of a juniper about seven feet above the ground. A barbed wire fence that served as a functional delineation of the male’s breeding territory encircled the prairie. I could always tell when the male had returned to take up residence, because the barbed wire was decorated with impaled grasshoppers, bumble bees, beetles, and the occasional cricket. This was his quiet declaration of ownership to other males. If he were to lose his mate, it would also serve as a potent advertisement to females of a prime breeding habitat. We lost our breeding pair a few years back, possibly the result of clearing brush that was heavily invading the north hill slopes and threatening parts of the native prairie remnant, changing the brushy character of the site too much for their tastes. We also removed the barbed wire, swapping it out for a more visitor- friendly, barbless variety. Possibly the final shrike insult, it is hard to say. Maybe a management accommodation can be reached at some time in the future. In any case, loggerheads have become increasingly rare in their breeding range in southern Minnesota.
Dr. Thomas S. Roberts, writing in his historically important, magnum opus, Birds of Minnesota, calls the loggerhead shrike “an abundant summer resident” and a common “wire bird“ along the highways of southern Minnesota. He recounts a June road-trip, conducting a bird census as he breezed along at an average speed of forty miles per hour (high-speed birding in 1926). His described route, an ugly figure- eight loop from Minneapolis to Wilmer, south to Pipestone, north to Ortonville, southeast to Olivia, and back to Minneapolis. The trip covered, according to Goggle calculation, 499 miles. His tally for loggerheads, northern shrikes having long since vacated the state by June, was 79. That is a shrike spotted from a passing car, once every five and a half miles. These are unimaginable numbers to be seen along today’s highways.
The Minnesota Department of Natural Resources changed its official status for the loggerhead, from threatened to endangered, in 2013. In the last 25 years, nesting territories have been consistently found in only Dakota and Clay Counties, with the highest numbers recorded in Dakota County.
Shrikes are songbirds, but they have the heart of a raptor. Roughly the size of a robin, this predatory bird utilizes a wide variety of prey species. These may include many species of insects, such as beetles and grasshoppers, and small vertebrates from frogs, lizards, and snakes, to birds and small mammals. On occasion, they have been observed attacking prey as large as themselves. No small feat for a predator that has the feet of a perching bird and lacks the killing talons of a raptor. Their primary weapon is a hooked beak, or tomial tooth, formed from the hardened cutting edges (tomia) of the upper beak. Repeated strikes to the head are often used to stun and disable its prey. Then by grasping the neck and vigorously shaking the victim, it creates g-forces strong enough to dislocate the cervical vertebrae.
The shrike then flies the carcass to a suitable location and impales it on a sharp thorn or twig for caching or disassembly. Lacking the powerful muscles and talons for holding and controlling the body while butchering, the impaling larder serves as the bird’s personal abattoir.
Dr. Gary Wagenbach, my friend and colleague on the Valley Grove Preservation Society Board, had spotted the small carcass and brought it to my attention. Gary is a retired Carleton College biology professor, and we had been walking over the blackened area of a recent prescribed prairie burn at Valley Grove, when he noticed it impaled in a small sapling at about eye level.
I have seen several dozen of these caching sites over the years; most of them were ones that I had searched for after seeing the bird sitting atop a hunting perch. Shrikes, like most predators, are opportunistic, and if prey is plentiful will often kill more than their immediate needs require. Caching is a hedge against the vagaries of future hunting success due to bad weather, low prey availability, or just bad luck.
Impaling their prey off the ground makes the carcass more difficult for scavengers to locate, by leaving no ground scent trail to follow, and dispersing the scent higher in the air column. It also avoids the acquisition of the carcass by carrion beetles, which would locate and bury a small animal in a single day, if left on the ground.
I’ve come across several references in the literature of shrikes hanging their prey by the head in a forked branch. This practice being used, presumably, when a good thorn bush for impaling prey, was unavailable in the near vicinity. It is possible, I suppose, but I am skeptical about the assumption of a shrike using this particular technique. I have found many such hangings, and I doubt that shrikes cached them in this manner, for several reasons.
Firstly, shrikes often use sharp spikes on small branches, broken off by the bird to use in place of a thorn. I have found half a dozen such spikes used for just this purpose, including the one hanging the shrew that Gary and I examined. These locations had no apparent thorn bushes within easy flying distance, carrying such a burden. I would suggest this deficiency necessitates improvising a substitute, and the bird simply breaks off a twig, leaving a sharp spike to use as a butchering hook.
Secondly, impaling is a behavior that I believe to be innate, or “hardwired,” in shrikes. The photographer, Larry McPherson, has film of young, inexperienced shrikes practicing their impaling skills with leaves on thorns and twigs, before they learn to hunt. The fledglings don’t need to learn to do the behavior, but they practice and refine the skill without knowing why it is important.
Finally, there is another predator that uses a fork-hanging caching technique, almost exclusively, and often frequents the same habitat. The first time I found a mouse hung in a fork this way, I assumed that a shrike had hanged it, since I had seen a northern shrike in the area several weeks earlier. The deer mouse had been there for a while, its eyes shrunken, dried, and opaque from dehydration, and body, while not frozen, was stiff with rigor mortis. It was mid-December, and we were still wondering if this part of the state would have a white Christmas. I was investigating a wet meadow in the Cannon River bottoms near my home, which was an excellent and often-used shrike habitat. Finding a deer mouse hung by its head was odd, but I had read that shrikes did this occasionally. The recent shrike sighting predisposed me to assume its ownership, but something seemed off, and I wasn’t sure exactly why. When I found the second deer mouse, hung in the same manner only a hundred yards away, I realized what had bothered me with the first mouse. It was the species. A deer mouse would be an excellent prey item for a shrike, and doubtless they nab a few from time to time as opportunity provides. The problem is that deer mice are very nocturnal. They don’t become active until the dark of night, and return to their nests before first light. An early-bird shrike might score an errant deer mouse returning home late, or possibly one that had been disturbed from its digs by some unforeseen event during the day, but finding two hung like this seemed even more unlikely. I had begun to suspect a different predator, and I had one in mind.
For a tracker, it is never a good idea to get too attached to your first reading of a sign, especially one with which you are unfamiliar. I lifted the mouse from its display fork to examine it more closely. It was a very fresh kill–the eyes still clear, and the body relatively relaxed. Killed last night, I thought. The carcass looked undamaged, until I noted a small bloodstain on the back of the head. I grabbed a pinch of fur at the nape of the neck and ripped it forward to expose the flesh of the neck and the top of the skull. Twin puncture wounds were exposed, one on the back of the skull and another on the upper neck region. I looked at the entry wounds on the underside of the skin I had torn loose and decided the wounds were perhaps a little more than a quarter inch apart. It was a little farther apart than the wounds that penetrated the bone and flesh. I was thinking “long tailed weasel,” but I couldn’t rule out damage from multiple beak strikes by a shrike. Biting a rounded surface, like a skull, could allow for the skin to stretch and slip before the second upper canine penetrated to the neck muscle, possibly accounting for the spacing discrepancy. As a forensic mouse pathologist, I was clearly out of my depth here. I had one more thought about the problem. With my pocketknife, I made a cut across the skin at the base of the throat, and two more, one each on either side of the neck. Lifting the flap of skin, I exposed the underside of the mouse’s lower jaws. Examining the skin flap revealed only a single perforation, which matched up with a wound near the mouse’s left mandible. Since carnivores have a second set of canine teeth on their lower mandibles, there should have been a second puncture from the canine tooth located on the weasel’s other jaw. A close inspection of the inside of the skin showed only broken capillaries where the second canine tooth had met little resistance below the mouse’s tongue. Maybe not definitive proof, and probably not convincing enough for a C.S.I. episode, but it was good enough for me. My long tailed weasel theory made the most sense, I decided. I hung the mouse in the crotch where I had found it and wondered what the weasel would make of her mutilated and human scent-tainted prize, upon her return . . . possibly a satanic cult?
Several weeks and four or five inches of snow later, I came across a long tailed weasel track that showed drag marks on either side of its trail. It was evidence of a small body being carried. The trail led to a clump of diamond willow decorated with a meadow vole ornament hung by its neck, with care. It was Christmas after all.