Bird Calendars
NOTES FROM NATURE
By Jerry Toll
Eastern Screen Owls
Eastern Screech Owls, although common, are rarely detected and thus underrepresented or absent in general bird surveys. Breeding Bird Surveys (BBS) conducted in June cannot be used to determine the health of screech owl populations. The BBS is designed to detect species that are easily seen or heard during daylight hours. Screech owls require survey techniques specifically designed to detect them.
For two years I have done Breeding Owl Surveys at DeSoto NWR in March and April and have learned a few things, but did not feel confident in the information gathered. The survey method is in need of refinement this year. I thought I would share some of the interesting tidbits about screech owl behavior that I learned during the research.
Gehlbach's "The Eastern Screech Owl" was a comprehensive 10-year study of a population in Central Texas. In comparing study plots in both rural and suburban areas, he found that the screech owl populations in suburban areas were more dense and stable than in rural areas.
During late winter months when surviving is most precarious due to diminished prey and the higher demands of breeding, the suburbs provided a steady supply of prey birds of the same species. The trees planted in the suburbs also provided more nesting and roosting cavities than in equal-sized plots found in rural areas.
Screech owls have a variety of vocalizations, each with a purpose. The single-tone trill is used primarily by the male, particularly during courtship and nesting in February and March. It is a bonding signal to coordinate family movements.
In February, during courtship, the male will begin the single-note trill shortly before sunset and repeat it about once a minute for the next two hours. By March, when nesting begins, it is reduced to one-half hour but with a 15-second interval.
The female, who is the quieter of the two during courtship, picks up the slack during nesting and fledging. She uses the descending trill that is characteristically a nest-site territoriality defensive signal. She alerts the mate and fledglings to the possibility of danger and advertises to other screech owls that this is her home range.
Loud songs are by individuals of both sexes that are widely separated. Muted versions are by family members that are close to one another. Both trills have variations that can be slurred, fast, slow, long, or short, and can be used in combinations.
Screech owls also hoot, bark, and screech in increasing response to perceived danger.
I have not heard these but have seen bill clapping, a fear response common to all owls.
Gehlbach observed that Eastern Screech Owl calling is related to the gibbous cycle of the moon. The frequency and persistence of calls is greater when the moon is more than half full and greatest when the moon is full. These phenomena have also been documented in Western Screech Owls, Saw-whet Owls and Boreal Owls.
Last month I asked the questions: Which bird has the largest eye, and why is it a bird? Which bird has the smallest eye of any bird?
The answers to these questions will be given in next month's column.
01/24/08