Rigor Vitae: Life Unyielding

Thursday, July 17, 2008


Whenever Darren Naish blogs about owls, I end up with far more to say than can be politely typed into a comment space, and once again a short post from him inspires a lengthy one over here, and a chance to post some really old paintings. As usual, Darren's brief essay about the function of owl ear tufts was packed with information as well as interesting comments from knowledgeable people arguing for or against one of three possible tuft functions. The first of these suggests that they lend owls a mammal-like visage that is more likely to intimidate mammalian predators. The second possible function concerns species recognition and other intraspecific signaling, and the third is camouflage. Like Darren, I find the last suggestion the most plausible, but let's examine why.When mentioning feathered ear tufts, owls are the first birds to come to mind, but they're hardly alone. Paired head tufts have developed independently in cormorants, pheasants, puffins and others, including numerous groups of perching birds, culminating in the outrageous head plumes of the King of Saxony Bird of Paradise. Their uses vary from taxon to taxon; the function of the King of Saxony's plumes is obvious. Those of the two above, the Horned Lark (Eremophila alpestris) and Inca Tern (Larosterna inca), are harder to explain.

Ear tufts adorn the heads of several related owl genera, including the screech and scops owls (Otus*) and the eagle owls (Bubo**). Practically all of these owls have ear tufts, while members of all other genera lack them, with the exception of the Asio owls, all of which are eared, and not closely related to the other eared owls. The roots of three odd horned owls are unsure, but the Jamaican Owl (Pseudoscops grammicus) is probably related to Asio, while the Neotropical Crested Owl (Lophostrix cristata) and the African Maned Owl (Jubula lettii) are possibly allied to Bubo, as are the oddly tuftless African fishing owls (Scotopelia spp.). In his post, Darren mentioned a new paper describing ear tuft erection as an alarm response in Ferruginous Pygmy Owls (Glaucidium brasilianum)!! The tuftless barn and bay owls of the family Tytonidae differ so much from typical owls in form and behavior as to limit their relevance to this discussion. I'll only mention that the odd facial disc apices of the Asian Bay Owl (Phodilus badius) look rather like proto-ear tufts to me. A few nightjars and owlet-nightjars also sport tiny ear tufts. So what do they do?

First, let's consider the case for the mammal-mimicking threat. My own observations throw some doubt on this one. The Harpy Eagle (Harpia harpyja - above) is the only bird I can think of whose behavior conforms to this theory. Its cat-like double crest is erected fully when the bird threatens, but why this adaptation would manifest itself most strongly in the one bird species best equipped to defend against mammal attacks, I can't imagine. As far as I know, ear tufts are never important in owl threat displays. These displays are most valuable to fledglings, most of which leave the nest some time before they can fly, and it must be noted that all tufted owls start out as tufted as juveniles as well. Owls' first plumage differs substantially in structure from the adult plumage, which typically comes in before the bird is one year old, and juvenile “horns” are often morphologically very different things from their adult analogs, so they're obviously important to young birds. Still I've never seen them used to threaten. The two photos (below) of a young Great Horned Owl (B. virginianus) show typical behavior. At left, the fledgling erects his horns and adopts a cryptic pose when startled by my approach. Once I get very close, his attitude becomes defensive, and the horns begin to lower (center), and continue to do so as the threat intensifies. The painting at right shows two young birds in similar poses. Within the genera of typically tufted owls, it's instructive to look at the tuftless ones. All members of the genus Bubo have well-developed ear tufts (above, left), except for the Snowy Owl (B. scandiacus - above, right), the only one that lives mostly in open, treeless areas, roosts on the ground and is diurnal. Even juvenile Snowies show no vestiges of ear tufts. It's interesting that Great Horned Owls vary greatly across their range in form. In moist, heavily forested regions they're big and dark, with large feet and narrow wings, while desert birds are small and pale with broad wings and tiny vole-catching feet, and arctic birds are massive and whitish. Nowhere, though, are their ear tufts reduced in the least. Several Otus species have greatly reduced ear tufts, though, and most of these occur on islands or high mountaintops with few or no mammalian predators, giving a bit of credence to the Mammalian threat theory. The tiny ear tufts of the peculiar little Flammulated Owl (O. flammeolus - below) of the Rocky Mountains are usually not visible in either juveniles (left) or adults (right). It's only when the birds are roosting in cover that they're elevated into visibility. It's the Asio owls, though, that best illustrate tuft development trends. The seven members of this genus are pretty uniform, but the ear tufts of two species are reduced to stubs. The Long-eared Owl (A. otus - below, left) and Short-eared Owl (A. flammeus - below, right) are well-known and widespread. Their habits are similar, but the Shortear is more diurnal and normally roosts on the ground. It seems likely that the same factors oversaw the loss of ear tufts in both the Shortear and the Snowy Owl.
Even though its diminutive horns barely affect its silhouette (left), the Shortear moves them in the same fashion as its better-endowed kin, erecting them to their full puniness at a minor disturbance. Unlike the Bubo and Otus owls, members of the genus Asio erect their ear tufts when in full threat display. Their effect is normally obscured, though, by the bird's wings (more on this later). It should be noted that juvenile Shortears have well-developed horns; it's hard to distinguish one from a young Longear. The other earless Asio species, the African Marsh Owl (A. capensis) is a little more problematic. In habits, it lies between the Shortear and the Longear, flying by day and night and exploiting a number of habitats, some of them rather well wooded. There is some controversy regarding the African Marsh Owl's phylogeny, and it's not clear whether it lost its ear tufts along with the Shortear or independently of it. If this could be established it would shed some light on the puzzle of tuft development. In fact, working out general owl phylogeny will be an important step in understanding this issue. For instance, there's fairly good molecular evidence that Bubo is a more basal genus than genera like Strix and Ciccaba (usually conflated, nowadays). Demonstrating that these nocturnal, deep-woods birds were derived from tufted owls would force some intense re-thinking on many of us.The second hypothesis of tuft function suggests that they lend owls a characteristic silhouette identifiable to conspecifics. Since owls are mostly nocturnal and recognize one another by vocalizing, such lavish adaptations seem unlikely, and since there appears to be a correlation between diurnal habits and loss of horns, I tend to dismiss this idea. It's also been suggested that ear tufts are employed in more complex intraspecific communication. When interacting with a horned owl, it's tempting to ascribe emotive intentions to tuft movements, but this is probably more a case of projection than discovery. Owls are mostly solitary, and the most gregarious species like the Burrowing Owl (Athene cunicularia - above) tend to be earless. Burrowing Owls do communicate visually, though, by expanding and contracting the two white fields beneath the facial discs. The previously mentioned Flammulated Owl is one of the most social Otus owls -- also with reduced ear tufts. In certain cases, Long-eared Owls are colonial. It would be interesting to spend a spring in one of these colonies with a pair of night-vision goggles. Most owls have an extended dependency on their parents, and the possibility persists of tuft use in parent-fledgling signaling, not to mention courtship displaying. Again we're limited by our diurnal eyes.
It seems clear to me that whatever other uses owls put their tufts to are secondary to hypothetical function #3: camouflage. When erected, they disrupt the bird's outline, making it resemble a broken snag. The sketches above show typical roosting postures of eared owls. Figure a. shows the typical posture of an owl roosting in moderate cover. In heavier cover, tuft erection tends to be more extreme. The base of a limb is often selected (b.), and the bird often leans into the trunk, extending body and ear tufts to their fullest, as if merging with the tree. When roosting in the open, the tufts are more likely to be appressed (c.). A disturbance will often induce a roosting owl to stretch its body and raise its ears, exaggerating its cryptic aspect (d.). As a perceived danger intensifies, the posture shifts from crypsis to threat. The ear tufts begin to lower as the body feathers are erected, increasing the bird's apparent size (e.). As far as I know, all eared owls lower the ear tufts at this stage. Various vocalizations, bill-clapping and swaying may begin at this stage. In full threatening mode, the wings are spread and tilted forward, displaying the greatest possible surface area to the opponent(f.). The erect ear tufts in the sketch are typical of Asio (note how they wind up flattened against the wings). In Bubo and Otus they are normally (but not always) appressed in this display. Lifting the feathers of the facial discs so as to increase their area appears to be a more important part of threat display than anything done with the ear tufts.

*Some authorities like to split this genus. I choose to ignore them.
**Including the former Nyctea and Ketupa
upper: NDUK EAGLE OWL (1995) acrylic 11" x 7"
second: HARPY EAGLE & TAMANDUA (1999) acrylic 20" x 15"
third: LONG-EARED OWL PORTRAIT (2006) acrylic 15" x 7"
fourth: SHORT-EARED OWL NEST (1983) watercolor 14" x 19"
fifth: BURROWING OWL & BADGER (1988) oil 30" x 18"
sixth: OWL ROOSTING POSTURES (2008) pencil sketch 11" x 8"
All photographs by CPBvK


Blogger John said...

I really like your explanation of how ear tufts functions (and the accompanying drawings). I think that your conclusion is persuasive. When I have seen tufted owls in daylight, one of the biggest challenges in finding them was that their tufts blended with the surrounding branches and the owl shape was not as obvious.

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OpenID Kiggavik said...

Fascinating stuff. Thanks for the thought provoking analysis.

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