We want to be able to measure the relative differences in eye size and "ear" tuft length between species of owls. Hence the eyes of the Great Grey Owl Strix nebulosa are relatively smaller than those of the Barking Owl Ninox connivens. These can be measured from photos although how can we derive a relative size so we can compare between species? If we compare with the width of the head or the width of the facial disc, these values vary depending upon how the feathers or disc are being held. Similarly when measuring the "ear" tufts they need to be compared some other feature, which themselves may be variable. The only facial features that owls show that are stable in their size are the eyes and bill, and these vary in proportions between species.
Hello Carl; If either an eye dimension or ear tuft dimension is known empirically, couldn't you produce indices for each comparison? For instance, eye width / tuft length might produce a stable ratio for each species. These could then be compared. Tricks like this are commonly used in ant taxonomy. I see them often in keys. Best regards, Jim Des Lauriers
Many thanks James, we wish to be able to measure the different signaling features on an owl's face and head and to be able to compare these features between species. We can use qualitative measures "relatively longer (or shorter) than in species A....." although wish to be able to use quantitative measures when possible. The problem we have is that all the features on the owl's face can be moderated by changing postures and the degree of feather erection. We will look at ratios and thanks for this thought.
Carl; Ah!, tuned resonators. Are museum collections accessible to you? In an earlier life I was interested in bat echolocation and, like you, needed these kinds of dimensions. Several museum curators were welcoming and cooperative. Cheers, Jim Des Lauriers
Hi Carl. Could you use the distance between the centres of each eye as your stable relative factor? I have attached an image of what I mean. If you are doing this from photos (and the owls are all forward-looking) it would be very easy using photo editing software or similar.
Thanks Jim, together with some colleagues we want to quantify the proportions and extent of various facial features of owls and compare species and look for ecological correlates. For example ear tufts tend to be longer in forest dwelling owls in the genera Asio and Bubo. We are looking at museum skins, and will be observing captive birds. However skins show considerable shrinkage and distortion of facial features, depending upon how well they are prepared. Good information can be derived from photos of owls looking directly at the camera and then we can measure the length of features and derive relative values for comparisons between species. However owls may fluff out their feathers or sleek them down so comparing tuft length with head width becomes problematic. With large samples we can use mean or modal values. This clearly is a common type of problem when measuring biological samples and it is good to know how other tackle these and similar issues.
Dear Aleks, Thrilled to see you have an interest in another group of predators other than sharks. Your illustration clearly shows the issue we face. We can measure the distance from eye to eye, and the eye dimensions although if we want to compare relative eye size compared with the width of the head, or the distance across the facial disc, then we hit problems. A quick search of google images of specific owls will show how they can modulate the shape of their facial disc and fluff out or sleek their head feathers. The two attached photos of Long-eared Owls illustrates my point.
Dear Carl, Sharks will always be king, but I do appreciate a nice raptor!
My thinking was along the lines of using inter-eye distance as a proxy for either head width or disc width. I believe I am correct in thinking that the ocular orbit is fixed and therefore would be a good constant (within a species) to use in estimating the size of other features in photographs. You could look at museum specimens etc to work out the relationship between inter-eye distance and head width, for example, then use those data to estimate head width from inter-eye distance in the photos. Does that make sense? The attached might be of some interest to you in terms of methods.
Thanks Aleks,
Interesting paper, and good to see the measurements they are using. Eye size and position is a good within species constant. Head width however is more problematic for the reasons I have given. In Ninox owls it is more reliable than in some other species that have more fluffy feathers, such as Strix and Bubo owls. Eye size however varies between species.
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