Signs of Spring 10: The Eastern Screech Owl!

Eastern screech owl. Photo by W.Wander, Wikimedia Commons

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One of the great benefits of warmer weather is sleeping with the windows open. The sounds of the night (at least out here in semi-rural Pennsylvania) are usually quite restful. One of my favorite spring and summer night sounds is the extended, mournful trilling of the eastern screech owl.

Screech owls are very abundant in rural, suburban and urban environments all across the United States east of the Rocky Mountains. They are medium-sized owls (7 to 10 inches long) with prominent, feathered ear tufts and large, yellow eyes.

Eastern screech owls are active, nocturnal hunters. Their primary prey are shrews and deer mice, but they also opportunistically take insects (especially large moths, beetles, and grasshoppers), earthworms, snails, spiders, crayfish, other owls, chipmunks, squirrels, song birds, snakes, frogs, toads, salamanders, rats, and rabbits. Small prey are swallowed whole while large prey may be torn into bite-sized pieces. In a single night these 121 to 244 gram (4 to 8 ounces) birds may eat up to one third of their body weights. Prey is located both by vision and by the owl’s excellent sense of hearing.

Eastern screech owls are, in turn, preyed upon by a wide range of other species including larger owls and hawks, and its nestlings and eggs may be eaten by weasels, mink, raccoons, skunks, snakes, opossums, crows, and blue jays. Humans are also significant causes of eastern screech owl mortality.

Nest cavities may become infested with ants and other insects. Eastern screech owls have been observed in Texas carrying in live, slender, blind snakes to their nest cavities where the snakes will live in a mutualistic symbiosis with the owl (snakes eat ants and other insects, owls provide the snakes a protected habitat). The owls are also subject to a variety of parasites and diseases including malaria and avian pox.

Eastern screech owls mate between mid-March and late May. Owls tend to form monogamous pairs which may, if the pairing successfully produces offspring, be lifelong. In years of rich prey availability a male may mate with two females. Success of these two matings, though, depends upon the abundance of prey and the energy of the male who must simultaneously feed both females throughout their incubation periods.

Screech owl in nesting box (photo taken through telescope). Photo by B.L.Drake

A screech owl’s brood nest is basically an unadorned tree cavity or nest box. The owls utilize whatever materials that are present in the cavity for nesting substrate and add little or no nest materials of their own.  The mated female will lay 2 to 7 eggs (4 is the most common), one egg per day. If there is a delay in laying the clutch the first laid egg may hatch well before the later eggs. This “first born” owlet, then, may develop sufficiently to be able to kill later hatching siblings. Overall success of the clutch, then, depends on precisely timed egg laying and development.

Eggs are incubated entirely by the female for 30 days. During this period the female is fed by the male (again, if a male is keeping “two households” he will be a very busy owl!). Nestlings are initially fed only by the male, but eventually, as the owlets become larger more feathered and, thus, better able to thermoregulate, the female is able to leave the nest and begin hunting for both herself and the clutch. Owlets fledge after 28 days, and the fledges may stay with the parents for the next 8 to 10 weeks.

Mortality in owlets is very high. Abundant nest predators (listed above), injury due to accidents (in one study 33% of a screech owl cohort died due to collisions with cars and trucks and a smaller, but quite significant number, died from injuries sustained after striking walls and windows), poisoning from a wide range of agricultural chemicals (pesticides and vermicides), and active human predation (hunting and trapping). Only half of a season’s owlets survive their first year of life. After this first year, though, an eastern screech owl may live anywhere from 14 (the wild owl record) to 20 years (the captivity record).

Screech owls. Photo by H. Aboundader Wikimedia Commons

Eastern screech owls are quite tolerant of the presence of people, but you are much more likely to hear one of these owls than to see one. Their visual appearance, though, is not only quite striking but also genetically, ecologically and maybe even evolutionarily significant.

The eastern screech owl has three distinct color morphs: gray, brown and red. These colors are determined genetically although the exact genetic mechanism is not known. Probably there are several genes that work together to generate the color of an individual. Inheritance of color, then, does not follow any classical type of simple Mendelian, “true breeding” patterns. Screech owl nestling cohorts may display two or sometimes even all three of the color morphs regardless of the coloration of the parents. Mate selection does not seem to be affected by color morph recognition nor is color morph in any way related to gender or age  (individuals display consistent color patterns throughout their lives).

Over the screech owl’s very broad, North American distribution the gray morph is more common in the north and west, and the red morph is more common in the southeast. The brown morph (which is considered to be intermediate between red and gray) is more common only in Florida.

There are some obvious benefits to the two extreme morphs. The gray morph is particularly well camouflaged against the bark of deciduous trees, and, not surprisingly, these gray morphs are most abundant in the deciduous forests of the north and west. The red morph, on the other hand, is best concealed against the reddish bark and wood of coniferous trees, and these red screech owls are most abundant in the coniferous forests of the southeast.

Eastern screech owl. Photo by P. K. Burian. Wikimedia Commons

Further, the feathers of the gray morph have been shown in laboratory studies to be more effective at reducing body heat loss than the feathers of the red morph. Also, the legs of the gray morph are much more densely feathered than are the legs of the red morph. This increased body and leg thermal insulation would be of great advantage to birds living in cold, northern climates (and, possibly, of great disadvantage to birds living in warmer, southern climates!). In cold winters red morph screech owls spend more time in the shelter of their tree holes than gray morphs. This sheltering could make food acquisition by and , possibly, survival of these red morphs very difficult especially in years of low prey availability.

The scrambling of the multiple color morph genes in each new screech owl generation conveys great survival and success advantages on the species. The owls are able to utilize a wider variety of habitats and climate zones via the camouflage and thermal insulation properties of the different plumages. The absence of any mate selection based on color morph features insures that the color and plumage varieties will continue to be features of this abundant owl species. Maybe this innate variability is one of the reasons that this species has done so well in the human-altered environments of North America.