Tuesday, January 19, 2010

Butcher Bird

"Saw a shrike pecking to pieces a small bird, apparently a snowbird. At length he took him up in his bill, almost half as big as himself, and flew slowly off with his prey dangling from his beak. I find that I had not associated such actions with my idea of birds. It was not birdlike."

Henry David Thoreau Dec. 24 1850

I came across my first shrike larder about ten years ago as I walked a long a span of barbed wire that bordered a sprawling cow pasture. To tell you the truth at the time I had no idea what I had come across. I began thinking I had discovered the morbid collection of some weird cult.

A bird of mostly open country this passerine raptor at first glimpse resembles a miniature mockingbird with the bill of a hawk. It’s our only predatory songbird, is diurnal, very aggressive and bold for its miniature stature. It does most of its hunting from elevated perches, most times along power lines making use of its phenomenal vision to spot the tiniest of prey. I’ve seen them dive from 40-50 foot perches to the ground to seize something as miniscule as a cricket. Their feet are powerful for grabbing prey, but lack the large flesh piercing talons of raptors (e.g. hawks, eagles, etc). The tip of its upper bill is hooked and has a feature near the tip known as a “tomial tooth”. It's this tooth the shrike uses to dispatch large prey (birds, small mammals, etc.) by biting through the vertebra of the neck, severing the spinal column. Prey selection is mostly insects, small mice, reptiles and amphibians, but it also will take small birds on the wing. Sometimes birds twice their size. I once spotted one in an oak tree with a magnolia warbler in its talons. What’s really interesting about these birds is their caching behavior, which involves the gruesome task of impaling prey, which by the way paved the way to its nickname “butcher bird”.

They have been reported using several types of impaling substrate such as:

Honey locust thorns
Barbed wire
Blackbrush thorns
Agave plant spines
Saguaro spines
Hercules' club thorns
Sharp tree stems
Will also sometimes wedge prey in narrow branch forks

So why do they perform this ghastly act? Here's a list of possible uses of a cache or "larder":

1) Food hoarding for times of prey shortage or increased food demand (e.g. during nesting season to supplement prey brought to nestlings), during periods of stress and/or inclement weather. (1), (2), (3).

2) Impalement used to aid in food manipulation (food tearing) due to lack of talon strength. (2)

3) Large larders could impress females, which may offer some indication of the male's hunting prowess and ability to supply food. It was found there was a higher reproductive success in males with larger larders. (2), (3)

4) Used as a method of marking territory ownership and boundaries. (2), (3)

5) Chemically defended prey (e.g. monarch butterflies, lubber grasshoppers, eastern narrow-mouthed frog.) are impaled enabling toxins to degrade and allowing these to be used as food sources. (2), (3), (4), (5) .

6) Used to feed female to conserve energy during brooding. (6)

Several years back I surveyed a number of birders asking of their experiences with larder finds and observations of shrikes taking prey. Here's what I got:

Birds seen captured or impaled:

Carolina chickadee, English sparrow, Field sparrow, Northern Cardinal, Cedar waxwing, Bewick’s wren, Chimney swift, Cassin’s vireo, Ruby-crowned kinglet, Pine siskin, Chipping sparrow, American goldfinch, Painted bunting, Grey gnatcatcher, Black-capped chickadee, Yellow-rumped warbler, Ruby-throated hummingbird, Eastern phoebe, Chestnut-sided warbler, Indigo bunting, Downy woodpecker, Summer tanager, Wilson’s warbler (see photo impaled on yucca- courtesy of Brush Freeman), Tennessee warbler, House finch, Mourning dove, Red-eyed vireo, Savannah sparrow, Cerulean warbler, Broad-tailed hummingbird, Goldfinch, Bluebird, Eastern meadowlark, Mockingbird, Vermillion flycatcher, Red-bellied woodpecker, Hairy woodpeckers, American robin, Prothonotary warbler, Dark-eyed junco, European starling, White-crowned sparrow, Henslow sparrow, Inca dove, Common redpoll, Barn swallow, Virginia rail, Dunlin (northern shrike), Grasshopper sparrow, Harris’ sparrow, Zebra finch (pet store escapee), Lincoln’s sparrow, Pyrrhuloxia, Slate-colored junco, Gambel’s quail (juvenile).

Insects found impaled:

Grasshoppers, Crickets, Moths, Caterpillars, Cattle ticks (witness saw shrike removing them from a cow and then impaling them on barbed wire) (8) , Cicadas, Bees, Wasps, Beetles.

Other items found impaled:

Field mice, bats(see photo courtesy Of David Sarkozi), Northern leopard frog, crawfish, turtles, spiders, 13-lined ground squirrel (Toronto, Ontario), Lizards, Skinks, Green anoles.

Other interesting tidbits of shrike lore:

~ I had someone tell me they had seen one on the Texas coast near a wooded area where migrating passerines stopped to eat and rest following their long trip across the Gulf of Mexico. It was hypothesized that it was taking advantage of and attacking tired migrating birds.

~ They’ve also been known to impale inedible items such as fecal sacs, paper wasp nests, wool, eggshells, and snail shells. (3). They have been seen impaling twine on a barbed wire fence aiding them in breaking it into smaller pieces for nest construction. (3)

~ It is also known as French mockingbird by some.

~ A birdbander reported seeing a shrike attack a banded bird following its release. (7)

~ A group of shrikes is known as a "watch" of shrikes.

~ Chippewa Indians called this bird "big cannibal bird".

~ "Audubon (1842) quotes the Rev. John Bachman as saying: "I have seen one [shrike] occupy himself for hours in sticking up [on thorns]....a number of small fishes that the fishermen had thrown on the shore....." (8)

~ "Carrion is sometimes eaten. Prof. F.E.L. Beal while at Ames, Iowa, in January, 1880, saw a butcherbird fly over the brown frozen prairie to a carcass of a cow, where it lit on one of the ribs and greedily tore off shreds of the flesh." (8)

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