Eastern Screech-owls (Megascops asio) are a small species of owl found throughout eastern North America. They are known for their cryptic plumage that allows them to blend into their surroundings. But when it comes to their coloration, Eastern Screech-owls can actually display quite a bit of variation.
Plumage Variations
There are two main color morphs of the Eastern Screech-owl:
Grey Morph
The grey morph is the most common coloration. As the name suggests, these birds have predominantly grey plumage with little to no rufous coloring. Their underparts are lightly barred and they often have dark streaking on the breast. The grey morph provides excellent camouflage amongst tree bark.
Rufous Morph
The rufous morph exhibits a much warmer, redder plumage. They have heavy rufous barring on the underparts and fine streaking on the breast. Some rufous morphs can appear almost entirely reddish-brown. This coloration likely helps them blend in with reddish tree bark.
The rufous and grey morphs are not considered separate subspecies. Eastern Screech-owls are unique among North American owls in exhibiting this degree of polymorphism within the same species. The plumage variations appear to be inherited as simple Mendelian traits.
Geographic Distribution of Color Morphs
Research has shown that the distribution and frequency of the two color morphs varies geographically:
Northeastern Populations
In the northeastern United States, around 90-98% of Eastern Screech-owls are the grey morph. Rufous morphs are relatively rare.
Southeastern Populations
In the southeastern United States, rufous morphs are much more common, comprising 20-50% of some populations. There appears to be a north-south cline in morph frequencies.
Midwestern Populations
Midwestern populations generally have intermediate frequencies with a mix of both morphs. Exact ratios can vary locally. Urban areas tend to have lower frequencies of rufous morphs.
Western Populations
In the western extent of their range, rufous morphs become rare again, similar to northeastern birds. Western screech-owls are predominantly grey.
Advantages of Color Morphs
Researchers believe the striking geographic variation in Eastern Screech-owl color morphs relates to differences in local habitats:
Camouflage
The rufous and grey color variations provide camouflage matching either deciduous or coniferous tree bark. This helps the owls ambush prey and evade predators.
Thermoregulation
The plumage color may also aid thermoregulation. The darker gray morph may absorb more solar radiation in northern areas, while the rufous morph provides better camouflage in the south.
Mate Selection
Some research suggests females may preferentially select mates with plumage that better matches local habitats. This could reinforce localized color morph frequencies.
Changes Over Time
The balance of rufous and grey Eastern Screech-owls may shift over time in a given location as the habitat changes:
Forest Succession
As forests mature, bark may lighten in color. Grey morphs would gain a camouflage advantage and increase in frequency.
Urbanization
In urbanized areas, the loss of large deciduous trees could reduce habitat for rufous morphs leading to a decline in their numbers.
Climate Change
Changing climate could alter forest composition in some regions. Resulting habitat shifts could impact relative frequencies of the two color morphs.
Juvenile Plumage
While adult Eastern Screech-owls display rather static plumage patterns, their juvenile plumage shows much more variability:
Downy Young
Hatchlings of both morphs are covered in white down. At this stage they are essentially indistinguishable.
Fledglings
As feathers grow in, juvenile birds display odd patterns of grey, brown, and rufous. There is significant variation among individuals.
Immatures
By four months of age, juveniles start to resemble adults, but may still show atypical barring and streaking compared to mature plumage.
Definitive Adult Plumage
By one year of age, Eastern Screech-owls attain their mature, definitive plumage – either grey or rufous morph.
Exceptional Variants
While most Eastern Screech-owls conform to the typical grey and rufous morphs, exceptional color variants are sometimes observed:
Dark Morph
An extremely dark grey morph lacking rufous tones sometimes occurs. These almost blackish owls are likely the result of melanistic mutations.
Rufous-Grey Intermediates
Owls showing an unusual mix of rufous and grey feathers may arise between morphs. Their plumage appears intermediate between the two typical morphs.
Albinistic and Leucistic
Very rarely, screech-owls with albinistic or leucistic plumage have been documented. These birds exhibit a near complete lack of melanin pigmentation.
Conclusion
In summary, the variable coloration of Eastern Screech-owls relates to complex evolutionary factors including habitat camouflage, thermoregulation, mate selection, and genetics. Their plumage polymorphism is unique among North American owls. While most individuals can be classified as either grey or rufous morphs, exceptional color variants sometimes occur, revealing the flexibility of screech-owl coloration. Whatever their hue, these fascinating little raptors continue to thrive across eastern forests and woodlands. Their ability to adapt their plumage to local environments is one key to their success.