Flight without Horizon References in European Starlings

Abstract

Many birds fly at night, or in conditions in which a horizon reference is obscured or missing (Berthold 1993). Nonmigratory species such as European Starlings (Sturnus vulgaris) will fly near their roosts in large flocks long after the sun has set (Eastwood 1967). Nocturnal migration and flocking after dusk occur under a variety of meteorological conditions, including fog and heavy cloud cover (Evans 1972, Elkins 1983). These behaviors suggest that for some bird species, visual references are not always necessary to maintain straight and level flight. Radar studies (Griffin 1972) suggest that some birds can maintain straight and level flight under completely overcast conditions, but birds flying within clouds may produce erratic tracks, suggesting disorientation in the absence of visual cues. Able (1982) used a tracking radar to examine the behavior of nocturnal migrants under overcast conditions and concluded that overcast skies did not result in changes in flight behavior. However, he acknowledged that some degree of disorientation occurred when birds seemed to be flying within or between cloud layers. Williams and Teal (1973) blindfolded individuals of six species of birds (Song Sparrow [Melospiza melodia], White-throated Sparrow [Zonotrichia albicollis], Dark-eyed Junco [Junco hyemalis], House Sparrow [Passer domesticus], Rock Dove [Columba livia], and Herring Gull [Larus argentatus]) to observe their flight in the absence of visual cues. One out of every six birds tested tried to fly upside down and crashed, and one-third of the birds dropped immediately to the ground, often using fluttering flight. Only Her-