Sunday, January 27, 2008

Life After Dark

Photograph by David Powell

“Whoo-hoo-o-o, whoo-hoo-o-o, who” reverberated from the snow-topped tree, only to be answered a few moments later by a higher-pitched, more elaborate response from the nearby forest. One of the birds flew to the top of a utility pole, where he continued his elaborate duet with the female of his choice. Apparently she returned his affection, as the hooting continued throughout much of the night. Among the earliest of nesters, Great Horned Owls may already be incubating this year’s clutch of eggs. Widespread throughout North America, and occurring locally south to Tierra del Fuego, South America, this highly adaptable species might be considered the supreme generalist among birds. The word variable personifies the species as a whole; as a result the population is reported to be robust without a need for special management in scientific literature.

Great Horns have many advantages that contribute to their overall success. As one of the largest owls in North America (the other is the Snowy Owl of the Arctic tundra), adults have few predators and are long-lived. Some studies have reported non-captive owls 21-28 years old. Monogamous pairs remain on territory year-round, and aggressively defend it from other owls. They do; however, often nest in close proximity to other diurnal (daytime hunting) raptors such as hawks and eagles. In 2003, we documented a Great Horned Owl nest and a Red-tailed Hawk nest on opposite sides of the same promontory in northwestern New Mexico, a total distance between nests of about 25'. Our raptor biologist speculated that only one nest would succeed, and wondered which it would be. In the end, only the owl nest was successful, fledging two young, while the hawk nest failed. There was no evidence of predation by the owls on the other nest, and we speculated the more visible hawk nest might have failed due to human disturbance while the more camouflaged, cryptically colored owls may have escaped detection. The pair successfully nested at the same spot in 2004 and 2005, but the hawk paired move on to another location. Other studies report owl nests in close proximity to other species such as Golden Eagle, Harris Hawk, and in one instance a Great Horned Owl and a Turkey Vulture nested in different cavities in the same tree. There is an apparent benefit to the nesting birds due to additional protection afforded by two separate species, one active during the day and the other at night. At the same time, there is little competition for food.

The prey taken by the Great Horned Owl varies according the occupied habitat. For instance, desert dwelling owls consume a greater number of reptiles than birds living near ponds and marshes that take waterfowl and muskrats. Documented prey includes everything from earthworms, crickets, and scorpions to mice, rabbits, and even skunks. Free-ranging house cats often fall victim to the talons of this owl. Other species of birds are routinely hunted by some owls. Indeed, I once located an owl nest in the cavity of a cottonwood tree after finding the headless body of an adult Red-tailed Hawk at the base of the tree.

Several remarkable biological adaptations make all owls supremely adapted to their inconspicuous nightlife. Their feathers are very soft, with comblike fringes along the leading edge of each feather, and a fuzzy surface that muffles sounds, enabling them to fly almost silently. They have large, forward facing eyes with binocular vision like us. But, unlike humans, they are unable to move them in their eye-sockets. They compensate for this limitation by bobbing their heads up and down and from side to side. Possessing more vertebrae in their necks than humans, they also can rotate their heads about 270 degrees!

The feathered tufts atop their heads, often called ears, are used for communication between owls and to break up the owl’s silhouette when it is roosting during the day. Their true ears are asymmetrically located on their heads within the feathered facial disk that helps direct sound into the ear canal. Some owls, such as the Barn Owl, can hunt in nearly total darkness. Owls use a combination of hearing and vision to locate prey. Cryptic coloration serves this species particularly well, and the birds are often indistinguishable from the tree in which they are sleeping, unless a mobbing actions of a flock of crows, ravens, or jays draws attention to them. Even within New Mexico, the plumages of Great Horned Owls vary by habitat with soft browns and tans characterizing the lowland desert dwellers setting them apart from the charcoal gray and dark brown of those that live in the coniferous forest.

Truly a remarkable species, survivors in a tough world, these owls adapt well to human activities, and often nest close to our dwellings. One might then wonder then, why it is that the bird chooses to nest in the dead of winter when little food is available and keeping eggs warm might present a greater problem. Owls do not build their own nests, but instead occupy nests constructed by other birds or cavities in cliffs or trees. Because they remain on territory year-round, they have ample opportunity to select the best nest site within the area without interference from the original nest builder who may be spending the winter further south. In the Middle Rio Grande bosque, they often choose the largest Cooper’s Hawk nest of the previous year! Their offspring leave the nest about May 1, the time when most of the migrant songbirds are returning to their nesting territories. Fed by their owl parents for another 30-45 days, the young owlets begin learning to hunt and catch their first live food at about the time that young, vulnerable songbirds are just hatching and leaving their nests. It makes perfect sense, from an owl’s point of view!

Birds in Winter Storm Events

Photograph by David Powell

The New Mexico New Year’s Eve snowstorm of 2006-07 dumped record amounts of heavy, wet snow statewide, snarling traffic throughout the state, creating a run on, and subsequent shortage of, snow removal equipment, and emptying the produce shelves of supermarkets. Although most humans were merely inconvenienced, others paid for this storm with their lives. The economic hangover of this storm will not be as easily vanquished as the more traditional headache and queasy stomach that often accompany the Rose Bowl parade.

The Office of the Medical Investigator (OMI) attributed the deaths of 16 individuals to the storm as of January 7, 2007. Ten death were caused by vehicle accidents related to driving on snow and ice; three stemmed from carbon monoxide poisoning; and another three were from exposure to cold. Cattle losses remain difficult to fully quantify because ranchers have still not been able to reach all of their livestock. The true number of livestock deaths may not be known until spring. Costs incurred by the state, local governments, and schools associated with emergency protective measures and snow removal will approach $10 million, and lost economic activity primarily related to highway closures and reduced productivity is estimated to be tens of millions of dollars.

Although the record snows of this winter will provide better habitat for the spring breeding season, the dense snowpack limits the ability of all animals to find forage. The food shortages that ravage pregnant livestock are equally brutal on pronghorn antelope, elk, deer, sheep, and even more so on small mammals such as jackrabbits and cottontails. For land-locked mammals, flight in the face of an approaching storm is an impossibility, and finding shelter in the short grass prairie can be a daunting prospect. Animals that take refuge in underground burrows may be buried beneath several feet of drifted snow. Storm related avian deaths of small birds cane be difficult to quantify; however, mortalities of larger birds that concentrate in limited areas, such as waterfowl and waterbirds, are well documented. One can assume that the smaller, and less detectable songbirds suffer similar mortality.

Storm related trauma was reported in several species following a blizzard that struck the Midwest in late March, 1996. A rainstorm that quickly changed into a snowstorm with winds in excess of 60 mph contributed to the death of 2,000 Sandhill Cranes in Nebraska's Rainwater Basin, 7,000 coots and ducks in northern Iowa, and 9 Canada geese in Wisconsin. There were also unconfirmed reports of storm related mortality of waterbirds and migrating passerines in surrounding states.

The USGS reports the storm-related deaths of 1,900 Eared Grebes in southern Utah in 1997. Apparently, the grebes became disoriented during a snowstorm and mistook snow covered fields and streets as water, and attempted to land. Over 2,700 additional grebes were rescued and returned to a nearby refuge.

In Iowa, there was a 70% reduction in Ring-necked Pheasant following a severe winter blizzard in 2004 that dropped 23" of snow. In 2005, a die-off of birds was reported in Yellowstone National Park in Wyoming following a severe spring snowstorm blew nearly 50,000 birds into the park. Of these, 127 individuals of nine species succumbed and were scavenged by predators including Common Ravens, pine martens, coyotes, Bald Eagles, and Red-tailed Hawks.
So, while storms can be devastating to some birds, others might be spared. This particular New Mexico storm may have caught many birds off guard. The unseasonably warm weather that preceded the storm may have resulted in wildlife that remained in marginal habitat, and then been unable to move to a more desirable location. There is evidence; however, that birds can sense changes in barometric pressure, and use this information to determine when to move to a new location. Numerous radar studies of birds migrating over the Gulf of Mexico have shown that birds are able to determine when winds aloft are favorable for northward or southward migration. Preceding storms from the northwest, Sandhill Cranes flocks often remain stationary, feeding voraciously on grains, until shortly before the storm’s arrival when higher, more favorable winds speed them south to a new location. One of the species known to winter on the open expanses of grassland in northern New Mexico is the Lark Bunting. There is evidence that buntings on the eastern plains of New Mexico also can predict weather, and leave prior to the onset of major storms. The week following the storm, the sky was blackened by thousands of buntings that suddenly appeared on the Armendaris Ranch in the Journada del Muerto, a phenomenon not previously reported in this south-central New Mexico location.

Studies by researcher, Bob Gill, from Anchorage, Alaska, support this theory. Gill attached radio transmitters to Bristle-thighed Curlews. These Arctic nesting shorebirds traditionally fly non-stop over the Pacific Ocean to the French Mariana Islands. The birds wait at staging areas until intuitive knowledge of high pressure from west or east, enables them to determine favorable winds that increase their chances of successful flight. With favorable winds, the curlews can fly the non-stop distance in 3-4 days. One transmittered curlew, started south, then circled around, and returned north. When favorable winds appeared near Tahiti, this intrepid bird overflew its destination before correcting its direction and landing at its traditional wintering site at French Mariana islands, after flying non-stop for 8 days.

Dry feathers are very nearly the best the insulation provided in nature. In cold weather, juncos, sparrows, finches, and other ground feeding birds frequently drop down to the ground, covering their legs and feet with their breast feathers while pausing in their search for food. All birds have the ability to enhance their insulation by fluffing out their feathers to increase the thickness of their coat. However, wet feathers provide little or no insulation, and the unfortunate individuals that are unable to stay dry almost certainly succumb to hypothermia.

Like livestock and wild mammals, the greatest threat to birds during a winter storm event is the inability to find sufficient food supplies. Logically, large animals can survive for longer periods without food and water than small ones, and this is true for birds as well. A time/activity budget study conducted on Baird’s Sparrow, a small grassland dwelling bird, has shown that this species needs one seed every 8-10 seconds to survive, essentially eating throughout the daylight hours. Heavy, wet snow blanketing and crushing fragile grasses and forbs for several days or weeks, prevent small songbirds from reaching vital food sources. Similarly, lack of water, in the frozen aftermath of a storm, can be devastating to birds and other wildlife. Some birds, like the American Robin, have been observed eating snow to obtain moisture following storms.

So, what can you, the compassionate birder do, to aid your feathered friends in situations like this? This is the time to be particularly generous with the seed, suet, and water you provide in your back yard. The extra food you provide may mean the difference between life and death to the birds that flock to feeders following a storm. Large seeds, like black oil sunflower that is high in fat content and has a relatively thin shell, can be easily hulled by small birds. Sunflower seed can be tossed onto the sidewalks and other icy areas where it, combined with the actions of foraging songbirds, help to break down the ice and snow. Rather than purchasing suet cakes, why not try making your own suet! Birds love it, and you can add a wide array of nutritious tidbits, from shelled peanuts to raisins. And, of course, break the ice crust from the top of your bird bath, and fill with fresh water daily, or add a bird bath heater to keep it from freezing in the first place.

No-melt Peanut Butter Suet Recipe

1 cup crunch peanut butter
2 cups quick cooking oatmeal
2 cups corn meal
1 cup lard (no substitutions)
1 cup white flour
1/3 cup sugar

Optional additions: raw shelled peanuts, raisins, cranberries, dried fruit. Be creative!

Melt the lard and the peanut butter, then stir in the remaining ingredients. Line a 9 x 13 cake pan with waxed paper (to ease removal), and pour the mixture into the pan. Refrigerate until cooled. Cut into cakes and store in the freezer.

Clear and Present Danger

It was just a sparrow. Just one. I found its dead body on the sidewalk in front of the gym where I try to coax intransigent inches from my aging waistline. Lying on its back in front of the big, plate glass windows, the cause of death was obvious. I made a mental note to collect the tiny body on my way out, so it could permanently repose in a more dignified location. When I picked up the carcass, I instinctively felt the breast of the bird. This individual was very fit, with a well padded chest. The tiny Dark-eyed Junco, a member of the sparrow family, now lies permanently in my garden.

Up at the cabin, another pile of junco feathers lay scattered across the welcome mat at the front door. A small smudge on the adjacent glass window gave witness to the bird’s final moments. In this case, however, the avian victim may have only been stunned by the window. As it lay wounded on the ground, death may have come at the paws of the feral cats that overrun our small neighborhood in the ponderosa pine forest.

Windows! You gotta love the view, especially in our enchanted Jemez Mountains. However, our panoramic view of the world is a veritable death trap for birds. Daniel Klem is a professor at Muhlenberg College in Allentown, PA, and the acknowledged expert in avian window kills. Klem estimates that between 100 million and 1 billion birds die in glass collisions each year in North America. And, this is a conservative estimate based on an average of one bird a year slamming into each of the approximate 100 million homes, apartment building, office towers, schools and storefronts that cover the American landscape. Klem states that glass is one of the world’s greatest bird killers, rivaled only by habitat destruction and perhaps cats. Depending on the number selected among the wide range of estimates, bird kills due to window collisions represent 0.5% to 5% of the 20 billion birds estimated as the North American total population after the breeding season each year.

It is the reflective property of glass that creates the illusion of sky and vegetation that spells doom for birds. Extensive tests conducted by Klem reveal that birds simply do not recognize glass as a barrier. Mortality studies of window killed birds show that the hazard is indiscriminate, meaning that all birds are at equal risk. Window kills affect common and rare birds, large and small species, occur day or night throughout the year, and under most weather conditions. An Indigo Bunting (which is present in the Jemez Mountains) banded after surviving a window collision in Canada, killed itself striking the same window a year later, direct evidence that individual birds use the same migratory path each year.

As ominous as these numbers might seem, they may indeed be quite conservative. Klem’s studies also have shown that of all bird collisions with windows, approximately 50% directly result in death. For the other 50% of birds that lie stunned beneath the window, death by predation is a substantial threat. The household cat, and other predators have learned to patrol the borders of homes and office buildings in search of injured birds.

One emotion unique to humans is the guilt and anxiety we feel upon discovering that the windows of our homes and work places are killing birds. Personally, it affected my decision when replacing all the doors and windows in my home. Wallet opened wide, all glass here now sports multiple panes with faux wood or metal strips. I haven’t found a single bird body in the two years since they were installed.

The best time to make a decision about bird-friendly glass is when the house is designed. There are a number of options available to the builder such as fritted or patterned glass or visual markers that separate smaller panes. Perhaps the most simple and effective treatment is to angle glass 20%, thereby reflecting the ground rather than the sky. Some cities, notably Toronto, have enacted bird-friendly development guidelines for future construction, particularly those that seek “green” certification for their buildings. The Toronto document can be found online .

So, what can you, the homeowner with windows already in place, do to reduce unnecessary deaths at your home? There are a number of online resources although be wary of sites that are actively trying to sell to you. I found the most extensive information at Fatal Light Awareness Program (FLAP), including the Top 10 Ways to Make Glass Less of a Hazard for Birds. g with windows. When a window at the front and back of your home face each other or when two windows meet at a corner they give the impression of a clear passage.

And, finally, although not endorsed by FLAP, there is some evidence that dirty windows are more effective than clean at reducing strikes. So, for those of you looking for the perfect excuse, you never need to wash those windows again!