Spread-winged posture of turkey vultures: single or multiple functions?

The behavior and spread-winged postures of Turkey Vultures (Cathartes aura) were examined at a communal roost near Superior, Arizona, from April-June, 1979. The temporal occurrence of wing-spreading during the morning pre-departure period was positively correlated with the intensity of the sun's rays, but was independent of the ambient low overnight temperature. Wing-spreading was more common when vultures were wet than when they were dry. Turkey Vultures appear to spread their wings for at least two reasons: (1) to dry feathers, and (2) to ameliorate the thermal gradient between themselves and their environment, although the two functions are not mutually exclusive. Spreading the wings to realign feathers or to increase the mobility ofectoparasites seems unlikely. Nocturnal body temperature (Tb) of captive Turkey Vultures (Cathartes aura) varies by as much as 4? C (Heath 1962), and from 0.4 to 2.6? C (Hatch 1970), below normothermic levels (390C) for periods of many days. Diurnal fluctuations in Tb are apparently unrelated to food consumption, night-time low temperature, size or weight of the bird, age, or photoperiod (Hatch 1970). Because these vultures exhibit shallow regulated hypothermia, it has been proposed that their wing-spreading postures are energy-conserving behaviors; the birds may warm themselves or modify their thermal gradients by absorbing solar radiation (Heath 1962, Curry-Lindahl 1970, Ohmart and Lasiewski 1971, Kushlan 1973). Owing to their dark coloration, vultures would be expected to absorb much solar radiation (Hamilton and Heppner 1967, Lustick 1969, Marder 1973) at low wind speeds (Walsberg et al. 1978), even without spreading their wings. By adopting this posture, however, they may absorb much more sunlight, as suggested from the work of Lustick et al. (1980) with Herring Gulls (Larus argentatus). In Little Cormorants (Phalacrocorax niger), spreading the wings dries them efficiently; the wetter the bird, the longer it spreads its wings after swimming (Winkler 1983). Anhingas (Anhinga anhinga) also dry quickly when spreading their wings (Hennemann 1982, 1983). Their wings are often spread even when dry, enabling the birds to increase rates of heat gain by enhancing absorption of sunlight. Anhingas spread their wings most frequently when solar radiation is intense and when ambient temperatures are low (Hennemann 1982). Much is known about the wing-spreading behavior of aquatic birds, but the reasons why vultures spread their wings are far less certain ( .g., Cl rk 1969, Mueller 1972, Grier 1975). We therefore examined the behavior of Turkey Vultures in order to determine the significance of wing-spreading. We asked the following: (1) is wing-spreading independent of overnight low ambient temperature, or is wing-spreading by free-living vultures more frequent during cold w ather, as shown for Anhingas? (2) is wingspreading more common following rain than following dry conditions, owing to the heatdraining effects of wet plumage (Kennedy 1970, Mahoney 1984)? and, (3) is the occurrence of wing-spreading during the morning related to the intensity of solar radiation? Answers to hese questions would allow us to address other hypo heses advanced to explain the occurrence of wing-spreading by various birds (e.g., Kennedy 1969, Mueller 1972, Houston 1980).