Estimating the Effect of Wind on Avian Metabolic Rate with Standard Operative Temperature

-I develop a simplified procedure to estimate the effect of wind on avian energyuse rates from published and unpublished studies of 10 passerine and 7 nonpasserine species. Below the lower critical temperature, energy-use rates of passerines resting in the dark can be estimated as that of a bird of the same species in a metabolism chamber set to the standard operative temperature,T,, of the habitat, defined as Tes = Tb(1 + 0.26Vu/)(TbTe). Wind speed is u, and Tb is body temperature. Operative temperature (Te), is ordinarily close to air temperature for birds resting at night, but T. can include the effects of thermal and solar radiation. The 95% confidence interval for predictions of the average metabolic rate of a passerine is ?9.3% for air speed up to 4 m/s and temperatures below the thermal neutral zone. The procedure also appears valid for some, but not all, nonpasserines. Received 22 August 1989, accepted 25 February 1990. ENERGY flow is a significant factor in studies of community structure, of the distribution and abundance of organisms, and of behavior patterns and habitat selection. Maintenance (basal plus thermoregulatory) metabolism accounts for 40-60% of the total avian energy budget (Walsberg 1983, Weathers et al. 1984). A simulation model of avian communities (Wiens and Innis 1974) found population densities in various age classes and bioenergetic demands to be significantly sensitive to temperature via the thermoregulatory component of maintenance metabolism. The geographical distributions of the wintering ranges of 60% of passerine species have been reported to be associated with particular January isotherms (Root 1988, 1989; but see Castro 1989). These studies and the majority of similar reports do not include the effects of sun and wind, which may be regarded as effectively increasing or decreasing the temperature of the environment (Bakken 1980). However, it is often important to do so. The inclusion of the effects of sun and wind significantly improved time-budget estimates of energy expenditure (Weathers et al. 1984) and prediction of time allocated to social displays (Santee and Bakken 1987). Shelter from the wind is often a feature of roost-site selection by birds, and shelter can significantly affect total energy demands (Stalmaster and Gessaman 1984, Buttemer 1985, Walsberg 1986, Webb and Rogers 1988, but see Walsberg and King 1980). This reduction in thermoregulatory energy demand appears ecologically important. Winter survival is improved by supplemental feeding (Jansson et al. 1981, Brittingham and Temple 1988), although the mechanism is complex and may be mediated by predation (Jansson et al. 1981, Lima 1986, Rogers 1987). Some field studies have been based directly on laboratory measurement of the thermal effects of wind, radiation, or both (Goldstein 1984, Weathers et al. 1984, Buttemer 1985, Walsberg 1986, Santee and Bakken 1987). Considerable technical difficulty is inherent in the laboratory part of such studies. In many cases, the necessary level of effort is difficult to justify, and a simpler means of approximating the influence of sun and wind on heat loss would be valuable. My objective was to use published data on the effect of wind on the thermal conductance of various birds and the concept of standard operative temperature, Te% (Gagge 1940, Bakken 1976), to develop such an approach. The proposed model differs from an allometric model of conductance developed by Goldstein (1983). I use Tes, which leads to a relation independent of mass. Although the procedure could be generalized, few data are available on the conductance of birds exposed to wind, sun, or both, during their active phase, when the thermal environment, behavior, and physiological state of the animal differ significantly from those typical of rest phase. Thus, the reliable applicabil587 The Auk 107: 587-594. July 1990 This content downloaded from 157.55.39.215 on Wed, 31 Aug 2016 05:09:58 UTC All use subject to http://about.jstor.org/terms 588 GEORGE S. BAKKEN [Auk, Vol. 107 ity of this model is limited to the effect of wind on birds resting in the dark. Thermal radiation to the night sky with a radiation temperature typically 20-30?C below air temperature has also not been simulated in laboratory studies. However, the use of operative temperature rather than air temperature compensates for most thermal radiation effects, and rest-phase behavior and physiology are probably unaltered by thermal radiation.