The owl Ecology and behavior • Solitary hunters. • Will prey on anything within the appropriate size range (including small cats and dogs). • Hunt along a route. • Typically will have a set of perches where they will sit, watch and listen then pounce. • Hunts at dusk midnight & dawn. Barn owl
Yes owls can see very well • Eye is irregularly shaped and locked in forward position • Forward facing eyes (typical of predators) • Very large (1-5% of total body weight) • Specialized for low light vision (Tapetum) • Nictitating membrane
…But you can’t always see prey: hearing alone is sufficient to accurately locate prey! • Another adaptation of the owl is wing design • Large and broad allows gliding flight at low speeds. • Flight feathers have a fine down coating to reduce noise. Experimental evidence: owls consistently go for the paper
Binaural fusion: the process by which the brain combines and compares the sounds received by each ear producing the perception of sound location in space. • This requires the comparison of two features of a given sound stimulus: • Sound intensity at each ear • Sound arrival delay between ears. Mark Konishi
Barn owl ears • Skull opening is considerably larger than other birds • Feathers of the crown (or ruff) act as a the pinnae of the mammalian ear. • Removal of the ruff results in a loss in ability to resolve elevation of sounds.
Barn owl ears are asymmetrically aimed Left ear points down (below midpoint of eye). Right ear points up (above midpoint of eye
Experimental evidence that: • differential intensity indicates elevation • differential delay (linear phase lag) indicates position on azimuth • Animals are: • Equipped with a search coil. • Positioned within the electromagnetic fields of two induction coils. • Animals are trained to face forward when cued (zeroing speaker). • Stimulated with a target speaker that can be positioned anywhere in around the owl. • The X and Y coordinate of the owls gaze can be calculated by the change in magnetic field as the search coil moves within the fields induced by the induction coils.
Initial analysis of orientation acuity: low orientation error particularly for stimuli directly in front of animal
Experiment 1: plugging ears (affects interaural intensity) causes a specific pattern of errors: • Plugging your ears disrupts azimuth acuity. • Plugging owls ears also (more profoundly) affects elevation acuity. • Plug left ear (down facing) biases orientation upward. • Plug right ear (upward facing) biases orientation downward. • Degree of effect related to % hearing blocked. • Experiment 2: removing the ruff • Completely disrupts animals ability to orient with respect elevation • Azimuth acuity unaffected
Start lag End lag • Experiment 3: Interaural timing differences (delay between ears). • Animals equipped with tiny earphones that presented sounds: • Intensity (held constant) • Start/end delays eliminated (nonlinear phase lag or transient disparity) • Continuous delays varied (linear phase lag or ongoing disparity)
Experiment 3 Results: Animal turns to correct the linear phase lag not start/stop delays