Thursday, February 21, 2013

Barn Owls at Eaglecrest

The barn owls at Eaglecrest laid their first egg on 2/19/13. They are nesting in a hole in the tree just below the nest tray that the geese are using. The Countess (also called 'Tess) will most likely lay four to six small eggs that should begin hatching after roughly 33 days, or on March 24. Unlike the geese, barn owls are asynchronous layers - that is, they don't delay incubation to assure that all of the eggs hatch at roughly the same time. The eggs will hatch in  the order they were laid, and there may be an age difference of up to three weeks between the youngest and the oldest nestling. The young birds are brooded for about two weeks and fledge in 50 to 55 days.

Here is a look at egg #1

Barn owls are one of two living lineages of owls. Most owls are classified as Strigidae, or typical owls - a name that appears to have derived from the Greek word strix, meaning screecher. Barn owls are Tytonidae, derived from tyto, a word that means either owl or honored. I've already done a blog post on the features and adaptations that all owls share: facial disks, large forward facing eyes, 'soft' feathers, round heads, distinctive facial markings and/or ear tufts, talons and a 'hawk-like' beak. However, there are also some differences between the two.

Tytonidae have...
Heart-shaped faces
Large heads and long legs
Hunt at night
Are cavity nesters
No feathered ear tufts
Strigidae have...
Round faces
Smaller heads and shorter legs
Some kinds hunt during the day
Nest in a variety of ways and places
Feathered ear tufts

The heart-shaped facial disk of a barn owl has different directional properties than that of its round-faced cousins. If it were a microphone, we'd call it cardioid. The heart shape isolates sound sources and concentrates sound in front of the owl, obscuring sound from the sides and rear. Like its uneven ear holes, this helps it pinpoint prey. Its large head also means a larger facial disk with which to pick up sound. These specialized adaptations help barn owls hunt in very low light to complete darkness: a critical skill for barn owls, since many nocturnal animals curtail their activities to dens and burrows when the moon is full.

A barn owl's long, sparsely feathered legs help it catch mice, shrews, and voles in deep vegetation and underneath snow. Its third toe has a split talon that can be used as a comb. Perhaps their long legs aid in grooming, or help them avoid bites and scratches from prey. Or maybe tytonidae owls really like tall, long-legged mates. We know that dark spots on a female barn owl attract males - the larger and darker her spots, the more interested male owls get. Could long legs serve a similar function?

The lack of feathered tufts is also a mystery. Since some but not all Strigidae have ear tufts, I'm guessing - and this is a guess - that the common ancestor of all Tytonidae and all Strigidae did not have ear tufts (which are not at all related to ears or hearing). Therefore, the question should be 'why did some Strigidae develop ear tufts?'. Researchers have proposed camouflage, species recognition, and signaling under low-light conditions, but no one knows for sure. Barn owls are at the very low end of the avian acuity and contrast sensitivity spectrum. Perhaps they never developed ear tufts because they aren't likely to benefit from whatever visual cues, sexual attractiveness, or camouflage ear tufts provide to owls that have them.

Eaglecrest makes it possible for us to watch great horned owls and barn owls in the same territory. You can see why barn owls have been called 'ghost owls' and 'night owls' - their silent flight and vocalizations can seem a little eerie. They are very different from their louder, showier Strigidae cousins. Barn owls nest in cavities, returning to the same cavity year after year to lay eggs and raise young. Great horned owls commonly usurp the stick nests of other birds, while a barn owl makes a simple nest of her own regurgitated pellets, shredded with her feet and arranged into a cup. I'll be watching for more differences as they incubate eggs and raise young.

Barn owls are more effective than poison and traps at controlling rodent populations. If you have suitable habitat and are interesting in building a barn owl box, click here for directions from the Missouri Department of Conservation.

Things that helped me learn more about barn owls:

No comments: