Although a possible neural substrate for global stereopsis has been described in an owl,
Tyto alba, Tytonidae (van der Willigen, Frost, & Wagner,
1998), and stereopsis was behaviorally demonstrated in a falcon
Falco sparverius, Falconidae, 40 years ago (Fox, Lehmkuhle, & Bush,
1977), these demonstrations have not been replicated or extended more generally among bird species. The presence of global stereopsis in other bird species is based upon conjecture. McFadden (
1994) found some behavioral evidence of local stereopsis in pigeons; however, these results did not support the presence of global stereopsis. Furthermore, although binocular cells have been reported in the Wulst of pigeons (Micheli & Reperant,
1982; Perisic, Mihailovic, & Cuenod,
1971) and chickens (Wilson,
1980), they are relatively scare and are not thought capable of supporting global stereopsis. A deliberate search for disparity sensitive cells in Wulst of pigeon
Columba livia yielded negative results (Frost, Goodale, & Pettigrew,
1983), and while this negative result was originally attributed to anesthetic procedures, a further series of investigations also failed to find such neurons (Frost, Wylie, & Wang,
1994). Despite this conflicting evidence, the assumption that global stereopsis is a primary general function of binocularity in birds seems to be widely held. Thus, Iwaniuk and Wylie (
2006) stated recently, “Although the role of the Wulst in stereopsis has yet to be definitively proven, the available neural evidence from owls suggests that the Wulst mediates stereopsis”…“it is conceivable that Wulst enlargement underlies functional global stereopsis.” Furthermore, based upon extrapolation from owls, a further assumption that binocular vision is associated with nocturnality and predation also appears to be widely held. Thus, Schwab and Hart (
2003) asserted that, “This evolutionary position [of frogmouths Podargidae] between the swifts and the owls, visually, illustrates the binocular imperative for sophisticated nocturnal predation.”