Field-of-view:
Small perching birds can easily become prey to larger birds, snakes, and mammals. As such, they need to be on the look-out in all directions while perching, hopping along the ground, and in flight. They have evolved a wide field-of-view extending between 300-320 degrees across the horizon (Moore et al., 2015).
Focal field:
While their total horizontal field-of-view is almost as wide as other prey species like frogs, perching birds require a wider field of binocular overlap to allow for targeted foraging. The overlapping range of vision spans 20-40 degrees, just under that of humans. However, birds like sparrows have a very high receptor density fovea, almost twice the density of the human fovea, yielding high acuity in the central focal area (Ensminger and Fernández-Juricic, 2014).
Image: Fig. 3. from Moore et al. 2015: Orthographic projection of the boundaries of the two retinal fields around the head of an animal while the eyes are in a resting position. (A) American tree sparrow, (B) chipping sparrow, (C) dark-eyed junco, (D) Eastern towhee, (E) field sparrow, (F) song sparrow and (G) white-throated sparrow. Values are averaged across all individuals measured per species. A latitude and longitude coordinate system was used with the head of the animal at the center of the globe. The grid is set at 20 deg intervals and the equator aligned vertically in the median sagittal plane (the horizontal plane, 90–270 deg). The projections of the pecten produce a blind spot in the upper, frontal field. Projections of the bill tips are presented for orientation purposes.
Spectral sensitivity:
Image: This animated image compares the same view in the human visible spectrum and false-color ultraviolet. Figure by A.Z. Andis.
Sparrows, like most avians, are tetrachromats, perceiving all frequencies from deep red to ultraviolet. The ability to perceive ultraviolet opens an entirely different world. Flowers, insects, and birds exhibits patterns in the ultraviolet range completely hidden from the human visual spectrum, and seeing in UV improves bird vision in the dark forest under the canopy.