Today Bruce Lyon, an ecologist and evolutionist at UC Santa Cruz, graces us again with a nice science-and-photo post—his final contribution on owls (Honorary Cats™). Bruce’s words are indented. Don’t miss his remarkable frogmouth video at the bottom!
Here is a third batch of owl photos and natural history to follow on the previous two owl posts (here and here) from a couple of weeks ago. (Note that ‘owl post’ also refers to mail delivery in Harry Potter novels—owls deliver the mail). Today I focus on Spotted Owls, why they are threatened, and why some owls have ear tufts and others do not.
This spring I was also lucky enough to see Spotted Owls (Strix occidentalis) in three different locations in California. Spotted Owls live in old growth coniferous forest in the western part of North America and they are threatened in much their range.
Below: A female Spotted Owl roosting in late afternoon near Yosemite Park in California. The pair’s territory was in a lovely patch of mixed forest with some huge oaks, madrone, cedars and Douglas firs. I was able to sex the owls by differences in their calls.
Below: The male was roosting about 30 feet from the female.
Below: As evening approached, the male started waking up and he did several wing stretches. Many birds do these wing stretches prior to activity when they have been sitting for a long period of time—I assume it is like an athlete warming up.
Below. The photos of the male owl revealed the asymmetry of plumage markings on his face, particularly the brown facial disk and spots that separate the sides of the disk above the eyes. I find this asymmetry interesting because the ear openings are also asymmetric in some owls—I wondered if these two asymmetries are linked. I found a paper with measures of the degree of ear asymmetry for a few owl species but when I compared these measures to photographs of owl faces from the Internet, I could not see any link between ear and plumage asymmetry. The top photo is a normal photo that shows the right-left asymmetry in plumage markings; the lower photo is a Photoshopped fake that shows what perfect symmetry would look like for comparison (a mirror image composite of the same side of the face duplicated and flipped).
Spotted Owls require old-growth forest with very large trees for breeding. This habitat requirement made them famous not only as an icon for habitat conservation but also as a hated species in some camps because of the bitter dispute between conservation and the timber industry. The owls competed with timber interests in the Pacific Northwest because owls and clear cutting cannot coexist, resulting in a pitched battle over the fate of old-growth forests. Russ Lande, a former colleague of Jerry’s at Chicago (and still a close friend it turns out) wrote an influential academic paper that used population growth models to explore the relation between habitat integrity and the stability of Spotted Owl populations. Lande used life table data—estimates of birth and death rates of different life stages or age classes—to determine the expected lifetime number of surviving offspring for the average female in a population. Any population is stable if the average female replaces herself with exactly one surviving female offspring—any less than that and the population will decline.
Lande determined that the population was stable under the conditions at the time. He also used the data to explore which ‘life table’ variables were particularly important to population growth. He found that the annual survival rate of adults is most important (these are long-lived birds) followed by the survival rate of independent young when they disperse from their birth territory to find a territory of their own. Lande realized that forestry often creates a patchy mosaic of suitable habitat islands (patches of forest) in a sea of unsuitable habitat (clearcut) so he used a second modeling approach to examine this aspect (technically a ‘metapopulation’ approach). In mosaics of suitable and unsuitable habitat like this, global population persistence is determined both by the probability that populations within individual habitat patches go extinct, and the probability of successful dispersal between habitat patches by youngsters. Too little dispersal relative to extinction, and the entire population slowly dwindles to extinction. When Lande factored in the negative impact of habitat loss on the dispersal phase of the owl’s life cycle, his model predicted the ultimate extinction of the spotted owl based on the proposed management plans of the USDA Forest Service for the region. My memory is that Lande’s paper played an important role in getting the owl listed [JAC: as a “threatened” species] and putting large swaths of forest out of reach of the chainsaw, but perhaps there are readers more familiar with the details of this conservation story who can weigh in on this.
Below: While this fight was going on, the possibility that large swaths of old growth forest might be placed beyond the reach of the chainsaw enraged the logging community, who rightly saw their livelihoods as threatened. This resulted in some now amusing examples anti-owl vitriol, such as the famous bumper sticker “Save a logger, eat an owl” shown on the left, and a barrel painted like a tin of cream of owl soup on the right (photos from the web)
Below: The owls featured here and in previous posts are all round-headed owls that lack visible ear tufts. However, about a third of all species of owls have ear tufts. The Long-eared Owl (Asio otus) has particularly nice ear tufts. I photographed this bird at Mercy Hot Springs in central California.
Below: Baby Long-eared Owls at Mercy Hot Springs. Owls can rotate their faces to a remarkable degree and the slow shutter speed I was using nicely captured the motion of this rotation.
Why do some owls have horns while others lack them? Several hypotheses have been proposed, but one seems most compelling to me: ear tufts make the owl more cryptic, which reduces the risk of being mobbed and, for small owls, perhaps even being preyed on by larger predators. When disturbed, many roosting owls change their shape and become tall and skinny. The idea is that the skinny posture, the bark-like plumage pattern and ear tufts all combine to make the owl resemble a broken-off branch. This idea is supported by the observation that owl species that are active during the day tend to lack ear tufts (they are active, not hiding, so there is no benefit to hiding). In contrast, nocturnal species that roost during the day tend to have tufts, especially those that roost in forests.
Below: A fun study with a captive Northern Pygmy Owl (Glaucidium gnoma), a diurnal species that usually lacks ear tufts, provides convincing evidence that ear tufts serve an anti-predation function. When humans approached the owl it never changed its appearance. However, when the owl was exposed to two types of predators, a cat and a peregrine falcon, it invariable changed its posture—it became skinny and extended its eyebrows to create prominent ear tufts. It also exposed a few different white plumage patches, which the authors suggest might act as disruptive coloration—bold or contrasting coloration that breaks up the outline of an animal and makes it harder to detect. The authors thanked Elwood the cat for his cooperation in the study. Can’t beat a study that combines honorary cats and real cats.
Below: The Tawny Frogmouth (Podargus strigoides), a large and nocturnal nightjar relative, also mimics a broken-off stick. I found this bird roosting in a patch of forest near Melbourne, Australia a couple of years ago. Frogmouths can be very difficult to spot in this pose because they really do look like a branch.
Below: In the same large patch of forest, someone found a female frogmouth and her chick sitting together on the top of a wired in enclosure. I suspect the chick had fallen out of its nest and then somehow clambered to safety up on railing. The video shows that frogmouths undergo the same posture change as owls when a predator approaches. I put my camera on a tripod far enough away so that the birds assumed a relaxed pose but as I approached they got skinny and pointed their beaks up at an angle. They do not have ear tufts but instead use their beak to resemble a broken off branch stub; they also have a feather tuft that points in the same direction as the beak that may work like an owl ear tuft.
Great photos and video! Especially enjoyed the commentary. Thank you for posting.
Great photos and fun for those of us supporting raptors
Spectacular photos. I don’t have time to read the whole narrative right now, but I will.
Great pics!
Thank you! Fascinating explanations and great photos.
Darling, Dr Lyon ! Sw e e e e t Honorary Cats™ !
and lovely words thereabout ! My thanks for all !
Blue
Lovely photos and great information! Love the frogmouth!
I did not notice that was a photoshopped owl. Once I read the description it was obvious.
Coincidentally I heard my first own in almost fifteen years last night. Maybe I will spot him/her at some point. Maybe they are eyeing our cats.
That was some fascinating information about owls. Wow.
The owl is one of the coolest animals out there. If someone was going to sit down and design a fucking awesome animal, it would have a good chance of turning out to be an owl.
Great stuff, thanks Bruce!
Very nice article.
Bruce. There is so much to think about in your posts!
re owl skull asymmetry: Owl ear asymmetry for improved sound location – does that asymmetry effect the eye socket positions? I think the eye pairs in some of your photographs aren’t quite on the level, but maybe the owl is holding its head tilted to one side.
I think eyes are uncoupled from ears in this case. Eyes are for binocular vision and as far as I know, there is nothing equivalent to the three dimensional aspects of ears. That said, I am not an expert in this stuff.
I was unclear, sorry. I meant that the asymmetrical ear hole positions that presumably occur during skull development/growth might cause the eye sockets to be asymmetrical too.
Super cool stuff. And I especially love frogmouths, as the look like a Jim Henson Muppet.
Great post, Bruce! Thanks.
Owls are a big favorite. I’m on the lookout for them and see them once in a while. Thanks very much for the terrific photos and commentary.
Love your owl posts Bruce. Thanks…the video was especially cool.
Excellent information. I liked much the asymmetry and the wing-stretching owl.