2 160 mW/cm had considerable elevated cloacal temperatures, i.e. 42.3°C. Althou^i additional exposures to these high power densities will be carried out, preliminary estimates of the median lethal dose for microwaves for resting birds in the field£during winter conditions (e.g. in Massachusetts) appears to be about 150 mW/cm . It is reasonable to expect that when birds are exposed in flight, or in still air, or at higher humidities and air temperatures they will be incapacitated and die at lower power densities than the above. If gaping occurs in birds under normal heating conditions only after core tempteratures exceed a fixed temperature above which heat stress occurs, e.g. >39°C, then the rapid onset of gaping under microwaves, e.g. 30 seconds, while birds core temperatures, presumably still low, may be a valuable and sensitive indicator for microwave effects. The effect of microwave irradiation on orientation will be studied in Emlen cages at Manomet Bird Observatory during the fall of 1980-81 if the level of sky glow does not interfere with orientation. White-throated sparrows will be irradiated by 10 or 25 mW/cm^ while orienting in Emlen cages. Boston University, Boston, MA. 2.Manomet Bird Observatory, Plymouth, MA. 3,Harvard University, Cambridge, MA.
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