Earth Matters: Eastern screech-owls — Why there and not here?

Naturalists are often asked “W” questions: What is this organism? Where is it found? When am I likely to see it? And, most interesting and most difficult to answer, why? Why-questions provoke answers that address natural processes, often multiple, complicated, and incompletely understood processes. Full answers to why-questions include acknowledgment of uncertainty.

I was recently asked such a question by a friend who lives most of the year in a western Massachusetts hilltown and spends part of their summers close to salt water in southeastern Massachusetts. My friend asked why she hears the nocturnal whistled whinnies and tremolos of eastern screech-owls on the coast, but not in the hill country? An answer sometimes required for such questions is randomness: even in continuous habitat in the range of a common species, random events can result in spottiness of distribution. In this case, though, I think that I can offer some plausible, if not easily proven, explanations.

Massachusetts is near the northeastern corner of the eastern screech-owl’s range. Small differences in our coldest winter temperatures and depth of snow cover (which limits access to rodent prey) might limit winter survival for this non-migratory bird. The two locations where my friend lives differ by about 45 miles in latitude, and, probably more important for winter temperature, in proximity to the moderating influence of the Atlantic Ocean and by about 600 feet in elevation. In western Massachusetts this owl is most common along streams, especially close to the Connecticut River, places where water helps to moderate local temperatures and to melt snowpack. My first guess, then, is that the hilltown’s winter temperatures and snow cover are a bit too much for this species.

The eastern screech-owl is a hole-nester that requires pre-existing holes of an adequate size. It cannot live where trees do not have a sufficient diameter to provide such holes, so one possible reason for its absence is lack of large trees. In this case, though, the presence of medium-sized and large woodpeckers near my friend’s house indicates both adequately sized trees and birds that excavate holes the owls can use.

Rodenticides used to kill mice and rats also threaten birds, like the eastern screech-owl, that feed on rodents. Chemicals designed to kill rodents wind up killing rodent-killing birds. Could a difference in the use of pesticide poisons explain the observed difference in the presence of one bird species? Possibly, but the fact that other rodent-eating species of hawk and owl are common in my friend’s hilltown neighborhood suggests that chemicals are not to blame in this instance (although the effects of the poisons on birds are clear in many other cases).

The presence of another owl, though, the barred owl, might be relevant. Barred owls prey on the smaller screech-owls. A study by Olivia Leonard and her colleagues in Indiana indicated that screech-owls are less likely to be found where barred owls are present. A close look at the distribution of the two species across Massachusetts suggests a similar pattern: barred owls are common in the wooded hilltowns and scarce in the commonwealth’s southeastern lowlands. Do barred owls eat enough screech-owls to suppress their population? Do screech-owls avoid areas where they hear barred owls? No one knows the answers to those questions, but the birds’ fine-scale geographical distribution suggests that one or both of those processes might be happening.

My best guess in answer to my friend’s question is that a combination of winter weather and barred owl presence makes eastern screech-owl much less common in her hilltown home than at her seaside summer retreat. The research necessary to have more confidence in this answer would be quite a challenge: I’d want long-term weather records for the two locations and nearby areas (not just for major airports 20 miles away). I’d like measurements of eastern screech-owls’ responses to extreme cold, and experiments on their ability to locate prey under snow. Detailed observations of the prey of many barred owls over long periods and experiments on the responses of eastern screech-owls to the larger species would help in evaluating hypotheses related to the interactions of these species.

For the future? As the climate warms, eastern screech-owls might be expected to extend their range northward (as is currently happening in Maine) and uphill, unless limited by Barred Owl predation. Human-induced warming will harm many species, but some species, possibly including the eastern screech-owl, will benefit from this planet-wide unplanned experiment.

Distribution of birds breeding in Massachusetts can be explored at the Massachusetts Breeding Bird Atlas site.

David Spector (he/him) is a retired biologist and former board president of the Hitchcock Center for the Environment in Amherst.

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