9 Screech Owl secrets revealed: fishing, nest boxes, junk food and sex

9 Screech Owl secrets revealed: fishing, nest boxes, junk food and sex

By Rex Graham

Screech Owls have flourished in many urban neighborhoods and parks in North America in the past 30 years, and now the birds have invaded pop culture. And they don’t even hoot.

The nocturnal raptors’ yellow eyes now stare back at us from caps, cards, iPhone cases (Halloween version is now on sale for $56.95), coasters, coffee mugs and even Christmas ornaments ($12.50 each).

However, ornithologists have revealed nine screech Owl secrets that paint Eastern and Western Screech Owls as something other than warm-and-fuzzy caricatures: they are surprisingly entrepreneurial predators thriving in forests to backyards.

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Eastern Screech-Owl (Bill Westheimer) topbirdingtours.com

Eastern Screech-Owl (Bill Westheimer)

1. Most popular prey: mice, birds or bats?

Spotted Towhee chick, birdsnews.com

During nesting, about 68 percent of an Eastern Screech Owls diet is birds. Spotted Towhee fledglings are one of many songbirds on the menu of the Western Screech Owl. (Photo: Amy Shipley)

The quick answer: it depends on the time of year. Screech Owls are as opportunistic as any bargain-seeking shopper at a farmers’ market.

Insects are seasonal snacks, but larger prey makes a real meal. In spring, small songbirds are at the top of the screech Owl menu. During the March-June nesting season, about 68 percent of an Eastern Screech Owls diet is birds. That drops to 30 percent in fall and winter, when mice and other small mammals make up a larger part of the owls’ diet.

In an Ohio study, species of migrating birds killed most often by Eastern Screech Owls were (starting with most taken) Yellow-rumped Warbler, White-throated Sparrow, House Sparrow, Common Starling, Ruby-crowned Kinglet, Song Sparrow and American Robin.

Eastern and Western Screech Owls synchronize the hatching of their young, usually four to five, with the spring migration of songbirds. Opportunistic owls grab the birds roosting in unfamiliar surroundings. When the migrants are gone, Screech Owls turn to the fledglings of local birds.

In a two-year study of 47 nests of Spotted Towhees in a forested park surrounded by residential neighborhoods in Lake Oswego, Oregon, Western Screech Owls were the second-leading predators of post-fledglings towhee chicks. (Domestic cats killed the most.)

“We don’t think fledglings are killed in the middle of the night, because they are good at hiding and don’t move around or vocalize in the middle of the night,” Amy Shipley, then a Ph.D. student at Portland State University, said in an email. “I’m guessing that the owls catch the fledglings at dusk or dawn when the adult towhees are making an early morning or late evening feeding trip, and the fledglings are moving around and vocalizing at that point.”

During the towhee’s breeding season, Shipley said owls killed very few adults, focusing instead on the bumper crop of fledglings.

The young towhees, within 30 days of fledging, were killed most often along the edge of the 48-acre forested-park study area. Shipley speculated that light from houses near the edge of the park enabled Screech Owls to hunt more efficiently.

The Oswego towhees, which often raise two or three broods a year, preferred to nest at the park’s edges where food was more plentiful. Three of Shipley’s towhee fledglings wearing transmitter devices were killed by a single pair of owls, and a fourth was killed by different owl.

Screech Owls also kill bats, rats, snakes, lizards and other small animals. However, birds are their most common prey, and owls are very good at nabbing them.

A pair of adult Screech Owls owls must catch more than 150 birds to feed five chicks from hatching until they’re on their own.

2. Do Screech Owls scavenge?

Road-kill opossum,  birdsnews.com

Western Screech Owls are well known as opportunistic predators, especially of small birds and rodents. But, carrion, too?

Screech Owls are well known as opportunistic predators, especially of small birds and rodents. Beetles are happy-hour hors d’oeuvres, and the owls readily dine on earthworms and burrowing snakes that come to the surface of irrigated urban lawns.

But, carrion?

Scavenging for such junk food was thought to be the realm of vultures and other scavengers, not Screech Owls. However, motion-sensing cameras don’t lie.

A Western Screech Owl was unmasked the night of December 27-28, 2011, in a northern California Douglas fir forest. That’s when ornithologists Max Allen and Preston Taylor, of Victoria University in New Zealand and Humboldt State University in Arcata, Calif., respectively, placed a road-kill opossum in the forest.

The scientists attached a motion-triggered video camera to a nearby tree. An appetizing call of a dying rabbit, a dinner bell for carnivores, was played every five minutes.

They were shocked by what their cameras recorded: the first-ever Western Screech Owl scavenging. Ingesting the road-kill opossum wasn’t just a momentary lapse in judgment.

It was more like a guilty pleasure – finally revealed. During an 11-hour period, a Screech Owl visited the opossum seven times.

“During the visits, the owl actively fed on the carcass,” Allen and Taylor said in a 2013 paper in The Wilson Journal of Ornithology.

The episode was probably not the first time carrion was a main course for a Western Screech Owl. Hair of deer, elk or even bear in owl pellets would be a clue, but the diet of Screech Owls is not intensively studied. (DNA analysis of bird poop can be used to identify the prey that has been consumed.)

In the past, telltale hair of large animals in pellets or at a Screech Owl nest site may have been disregarded or missed. Possibly, the birds may avoid ingesting hair when scavenging.

Actually, Screech Owls don’t always immediately eat what they catch. They hide kills in their nests or other nearby “cache” locations, consuming them one or two days later. Cold weather preserves caches up to five days.

3. Do Screech Owls take fish from trout streams?

Western Screech-Owls catching trout, Bret Harvey, birdsnews.com

Motion-activated cameras photographed Western Screech Owls catching 11 fish at night in two trout streams in northern California. (Photo: Bret Harvey)

In 2011, Bret Harvey, a wildlife biologist with the U.S. Fish and Wildlife Service, used monofilament fishing line to tether rainbow and cutthroat trout at 88 locations in the shallow waters of Jacoby and Little Jones creeks in northwest California.

He wanted to document predators removing the fish as part of a study to assess the effect of predators on trout in the fly-fishing streams.

As expected, River Otters, Raccoons, Belted Kingfishers, Great Blue Herons and Common Mergansers were either seen by researchers catching the 4.5-inch-long fish, or were caught by motion-activated cameras taking the fish. But Western Screech Owls were a major surprise.

The cameras documented Screech Owls catching 11 tethered trout in the two creeks. Belted Kingfishers were the only bird species that caught more fish: 18.

“Our observations suggest that owls can be important nocturnal predators of fish in some streams,” Harvey wrote in his study published in 2013 in Transactions of the American Fisheries Society.

Harvey said Screech Owl predation of trout in the California streams may be more significant in winter when other owl prey is hibernating.

In a study published in 1975, one nest box in northern Ohio, used during the 1973 nesting season by a pair of Eastern Screech Owls contained the remains of several dozen Gizzard Shad, five Green Sunfish, and one unidentified minnow. Other nest boxes smelled of fish, even though none were found.

Another owly fish tale focuses on Florida.

While Vladimir Dinets watched crocodiles at night in May 2006 in Clearwater Slough in Big Cypress National Preserve, about 45 miles west of Miami, he witnessed small Osprey-wannabes.

Dinets, a research assistant professor at the University of Tennessee, said a gray-morph Eastern Screech Owl on three consecutive nights walked along the edge of the slough, “at times wading into water to a depth of about 3 cm.”

  • At 3:12 a.m., on the first night, Dinets watched the owl catch a 3-cm-long fish and immediately fly away.
  • At 4:05 a.m., on the second night, an alligator chased the owl away.
  • At 3:04 a.m., on the third night, the owl caught a 5-cm-long fish, probably a Sailfin Mollie.

“Both fish were caught by the rapid movement of one foot,” Dinets wrote in his study published in 2011 in The Wilson Journal of Ornithology.

Other ornithologists have reported Eastern Screech Owls flying down to catch fish from a perch, and Frederick Gehlbach, a biology professor at Baylor University in Waco, Texas, the world’s leading expert on Eastern Screech Owls, has observed the raptors hopping from shore into shallow water to grab fish.

4. Can Screech Owls survive in the wild with one eye?

One-eyed Eastern Screech Owl, birdsnews.com

Most wildlife rehabilitation centers remove the entire damaged eye of owls via enucleation, but the less severe evisceration, may help owls better survive in the wild.
(Photo: Wildlife Center of Virgina)

Birds of prey suffer a variety of serious injuries, including eye injuries. They come with the territory in the form of collisions with vehicles or buildings or gunshot wounds.

Luckily, injured owls are often found in time. After one is treated for eye injuries it may be considered a candidate for a zoo or captive educational exhibit.

A one-eyed owl released back into the wild wearing a radio transmitter survived, and more could probably be released.

Owls’ ability to successfully hunt with their ears and one eye could lead more wildlife rehabilitation centers to returned more rehabilitated sScreech Owls into the wild. However, the way that wildlife rehabilitators remove an owl’s injured eye can make a big difference in its survival, after release.

Owls’ small skulls are so jammed with eyes and ears that part of the back of an owl’s eyes can be seen through its ear openings. Reshaping of an owl’s symmetrical skull can throw off its stereo hearing.

Most veterinarians perform an enucleation, a surgical procedure in which the entire globe of the eye is removed. However, this highly invasive procedure risks damage to the other eye. In addition, the large defect that remains will disrupt the facial disc, an integral factor in an owl’s hunting precision.

Maureen Murray, a professor in the Tufts Cummings School of Veterinary Medicine in North Grafton, Mass., said a less invasive surgical alternative called evisceration is preferable. With evisceration, Murray said, “There is much less disruption to the structure of the head.”

A study by Murray and a team of Tufts veterinary researchers published in 2013 in the Journal of Avian Medicine and Surgery documented how evisceration results in less-severe disruption of the facial symmetry of nine Eastern Screech Owls, four Barred Owls and one Great Horned Owl.

One-eyed Eastern Screech-Owl chick, birdsnews.com

“Three years ago, one of the owl chicks in my nest box had only one eye,” said Werner Deuser, a retired Falmouth, Mass., scientist who has watched Eastern Screech Owls raise their young for two decades in his backyard. “I don’t know if it was born that way, or perhaps was injured by a sibling. In any case, it fledged, last of four.” (Photo: Werner Deuser)

Murray doesn’t know how often wildlife rehabilitation centers perform the complete enucleation eye removal, but most veterinary studies refer mainly to enucleation as the treatment for severe eye injury in birds of prey, including owls, rather than the less severe technique of evisceration.

“Ocular injury is very common in birds of prey,” Murray said. “So, I hope that this paper in Journal of Avian Medicine and Surgery will present an alternative in those cases that require surgery.”

By being nocturnal, owls avoid their predators, such as hawks, eagles and other raptors. Owls also have an uncanny ability to land on a tree limb or other perch in total darkness.

An owl’s immobile, forward-facing eyes are equipped to give it a wider binocular field of view than birds that move their eyes, or have eyes on the side of their heads.

Binocular vision in owls coupled with the large size of their eyes, relative to body size, form part of a highly efficient sensory system to find prey in extremely low-light conditions.

Even a one-eyed owl can probably spot motionless prey in very dim conditions. How dim?

Barred, Long-eared and Barn Owls (all with two good eyes) were successful in lab tests at finding dead deer-mice with illumination as low as 0.000,000,73 foot-candle. Under the shade of deciduous trees on a clear, moonless night, illumination is about 0.000,004 foot-candle. (Conifers cast darker shade.)

However, if clouds replace the clear sky on moonless nights, illumination falls to about 0.000,000,4 — too dim for an owl to hunt by eyesight alone. That’s when stereoscopic hearing takes over.

True nocturnality in owls, including Screech Owls, is possible because of over-sized outer ears. These hearing devices, packaged with large eyes, amount to a formidable package. But one eye may be sufficient.

“Three years ago, one of the owl chicks in my nest box had only one eye,” said Werner Deuser, a Falmouth, Mass., ornithologists who has chronicled the behavior of Eastern Screech Owls raising young for two decades in his backyard.  “I don’t know if it was born that way, or perhaps was injured by a sibling.  In any case, it fledged, last of four.”

5. Do Screech Owls have long lives?

Western Screech-Owl, Amy Shipley, birdsnews.com

Western Screech Owls increasingly nest in urban areas and hunt along roads, which increases their chances of dying from vehicle collisions. (Photo: Amy Shipley)

Actually, they live fast and die young.

The 21 closely related species of  in the Strigidae pecking order (all found only in North and South America) Screech Owls are themselves preyed upon by larger owls.

Even before hatching, raccoons, red squirrels and other egg thieves snatch Screech Owl eggs. Like many other birds, Screech Owl parents will “double clutch,” laying a second clutch of eggs, but those lost are replaced with fewer additional eggs.

Eastern and Western Screech Owl fledglings are a favorite prey of Great Horned Owls. The total first-year mortality rate for these Screech Owls is about 70 percent.

It’s no picnic for the adults: about 33-39 percent dies annually.

Since 1960, bird-banding records show that while total raptor deaths due to shootings have greatly decreased, road-kills have increased.

Screech Owls commonly nest in urban areas and hunt along roads, which increases their chances of dying from vehicle collisions. Pets, hunters, pollution, poison, building collisions, disease and starvation all take their toll, too.

Indeed, most Screech Owls die young, as do most small raptors. However, many adult Screech Owls are fairly long-lived, with lifespans of 12 to 13 years, based on banding studies.

In comparison, Peregrine Falcons live up to 20 years and Bald Eagles live more than 27 years.

6. Screech Owl nests: tree hollow or wood box?

Eastern Screech-Owl male, birdsnews.com

Eastern Screech Owl males readily accept nest boxes, but the materials, size and placement can enhance their appeal. (Photo: Werner Deuser)

Either. They will accept almost any style of nest box. However, the materials, size and placement of a nest box can enhance its appeal.

Screech Owls flourish at the forest edge, and readily adapt to urban locations where they find plentiful prey. Large individual oak, elm, ash and cedar trees that have rotted hollows are favored by Screech Owls.

Frederick Gehlbach, birdsnews.com

“Suburban heat, bird feeders, bird baths and artificial ponds have benefited our Screech Owl neighbors with more small birds, mice, rats, lizards, snakes and fish to eat on an ever-earlier schedule.”
— Frederick Gehlbach, a biology professor at Baylor University in Waco, Texas, and the world’s leading expert on Eastern Screech Owls.
(Photo: Baylor University)

Cavities created by Pileated Woodpeckers or Northern Flickers are acceptable, but dense understory shrubbery can discourage nesting above it.

Screech Owls will use Wood Duck boxes, Purple Martin houses, mailboxes and even porch columns.

Three-quarter-inch plywood or similar thickness wood is the best material. The floor should be at least 8 by 8 inches and 12 to 15 inches deep. A 2.75-inch diameter hole and no perch are recommended. A couple ventilation holes, one-fourth-inch in diameter are suggested.

The direction the nest box faces doesn’t seem to matter.

Nest boxes close to houses with few shrubs, with a permanent water source nearby are most highly sought by nesting Screech Owls, according to Gehlbach, the Baylor biology professor.

Screech Owls use a U-shaped flight pattern flying to and from their nests, whether man-made or natural. Leaving the nest, the birds drop abruptly and then level off at the height of low tree branches, avoiding impediments. Returning with or without food, they come in low, and rise sharply just before reaching home.

This flight pattern conceals trips to and from the nest.

Each female Screech Owl selects her mate based on the properties of the tree cavity or nest box. Offerings of food inside seal the deal. Successful parents, most are monogamous, will return to the same nest for years.

7. Are forests favored by Screech Owls?

Eastern Screech-Owl male, birdsnews.com

Warmer temperatures in urban environments has led to more prey for Eastern Screech Owls. One study found a phenomenal 93 percent nesting success rate. (Photo: Werner Deuser)

Eastern Screech Owls adjust well to suburban sprawl.

The effect of three decades of growth of a central Texas city on the owls was studied by Gehlbach, who has a regularly occupied nest box in his “yard-forest.”

Balsas Screech Owl, birdsnews.com

The Balsas Screech Owl is the only owl endemic to only Mexico. (Photo: ©Marshall Iliff)

As smaller Texas cities have grown, Gehlbach has chronicled how they also have gotten hotter, by nearly 6 degrees Fahrenheit.

Hotter temperatures have speeded up Screech Owl egg laying by 4.5 days in the spring. The warmer urban environments led to a 31 percent increase in fledgling productivity. Nesting success was a phenomenal 93 percent, according to Gehlbach’s study.

“Suburban heat, bird feeders, bird baths and artificial ponds have benefited our Screech Owl neighbors with more small birds, mice, rats, lizards, snakes and fish to eat on an ever-earlier schedule,” Gehlbach wrote in a column published in the Waco Tribune.

In his book, The Eastern Screech Owl: Life History, Ecology, and Behavior in the Suburbs and Countryside, Gehlbach said lawn watering pushes prey, such as burrowing snakes, June beetles and earthworms, to the surface of lawns where enterprising Screech Owls find them.

However, not all urban settings are favored by Screech Owls. A study of 16 urban parks in the heavily urbanized New York City area found few Eastern Screech Owls in parks with Barred Owls and Great Horned Owls. The two larger species that are predators of Screech Owls.

8. Are Screech Owls vocal during sex?

Western Screech-Owl duet calls, birdsnews.com

In these recordings of duets by Western Screech Owls, (top) a female’s territorial call overlapps a male’s double trill; (bottom) a female territorial call is followed by a male territorial call. (Spectrogram: Tania Tripp master’s thesis)

Despite their wide distribution in Canada, the U.S., Mexico and Central and South America, the sex life of Western and Eastern Screech Owls is not a hot topic of research.

However, a few acoustic studies have cataloged their vocal repertoire, occasionally during noisy trysts in the dark.

Screech Owls are highly vocal, but their dialects vary. For example, territorial calls of the Western Screech Owl can have 12-15 bouncing-ball notes for birds along the Pacific to as few as four notes farther inland.

Eastern Screech Owl trill (Source: Cornell Lab of Ornithology):

 

Western Screech Owl trill (Source: Cornell Lab of Ornithology):

 

Acoustic frequency and spacing of notes also varies. Females have higher pitch than males.

Tania Tripp identified 17 variables in the vocalizations of Western Screech Owls to get to know individuals in southwestern British Columbia, Canada. The “vocal signatures” of individuals allowed Tripp to confirm if an earlier-recorded owl was in the same territory the following year. (Other owl species have vocal signatures, too.)

Reproductively isolated groups of owls may eventually form a subspecies. During courting, females may swoon over males with just the right dialect.

Eavesdropping with her own ears and sound-recording equipment on the romantic lives of Western Screech Owls, Tripp listened to 12 steamy Screech Owl courtships. She recognized a tryst when excited pairs of owls voiced their vows in noisy unison.

“Duets eventually led to copulation, with overlap of calls, increased calling rate and a crescendo of heightened vocal activity (typically overlapping trill calls) followed by mounting of the female, with the male flapping its wings for balance,” Tripp wrote in her master’s thesis. “During one occasion, a gentle, quavering blend of cr-r-oo-oo-oo-oo-oo notes was heard immediately following copulation, but was too quiet to capture on the recording.”

9. Are Screech Owls truly nocturnal?

Eastern Screech-Owl, Bill Westheimer, birdsnews.com

The soft-and-cuddly bird with the yellow eyes now stares back at us from the world of kitsch: caps, cards, coasters, coffee mugs and Christmas ornaments. But it’s also a fierce and adaptable predator. (Photo: Bill Westheimer)

In daylight hours, usually well hidden owls appear to be basking with their eyes closed.

Werner Deuser, a Falmouth, Mass., scientist and birder, documented the nocturnal departure of owls from one nest over 12 years.

Deuser, a retired senior scientist from Woods Hole Oceanographic Institution, attracted the owls by attaching a nest box to one of the many trees in his backyard. Then he pulled up a chair.

He also used an accurate clock and sharp eyes.

He recorded 514 nest-box departures, arriving at statistically significant results. “I could collect the data comfortably sitting in my backyard, or if it got too cold, in my study,” Deuser said in an email.

In a paper published in 2011 in The Wilson Journal of Ornithology, Deuser said male and female Screech Owls departed the nest box, on average, about 21 minutes after sunset (plus or minus 12 minutes, depending on cloud cover) every night they occupied the nests.

“I have no photos of their departures as they leave when the light is quite low, but thought that their regularity made an interesting story,” he said.

Some of the Deuser’s owls were more consistent about departure times than others.

Thick clouds lead to slightly earlier departures, but the phase of the moon doesn’t matter.

Farther south, in Washington, D.C., a 1937 study found that Eastern Screech Owls left their nest 7 minutes and 48 seconds after sunset, and 24 seconds earlier on cloudy or stormy evenings. (Both male and female left the nest earlier during nesting season.)

Even farther south, in central Texas, Gehlbach, the biology professor at Baylor University in Waco, Texas, and the world’s leading expert on Eastern Screech Owls, found that males left nest boxes, on average, 15 minutes and 12 seconds after sunset (plus or minus 4 minutes and six seconds). One female consistently delayed her departure by three to 24 minutes after the male departed.

As males perched outside the nest box during later stages of the nesting season, Deuser noted that females began leaving the nest box earlier and earlier, possibly to feed increasingly demanding chicks.

One might ask how Deuser identified the gender of individual owls. “Distinguishing the individual owls was based on differences in color – gray, brown or rufous, behavior, or position during mating,” he said.

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