B757 Captain experienced sluggish climb performance caused by abnormally high indicated OAT. A decision was made to return where Maintenance found a bumblebee wedged in the total air temperature probe.

Narrative

After takeoff initial auto throttle power reduction from takeoff to climb setting was noted as larger than normal. During remainder of climbout aircraft performance was monitored and demonstrated sluggish climb values; normal airspeed but 1;000 FPM maximum rate. All engine indications were normal except slightly lower EPR than standard climb values. TAT and SAT however were clearly abnormal. Upon reaching FL340 and; after comparing TAT/SAT with other aircraft in our vicinity and altitude; we determined that our indicated TAT/SAT on EICAS and FMC's were showing 29-35 degrees hotter than actual outside temperatures. Switching ADC's had no effect. There is no QRH or other procedure that directs flight crews how to handle this anomaly but clearly the erroneous TAT/SAT data was affecting aircraft FMC power computations/commands. We established HF phone patch to Dispatch and Maintenance Control and; after conferring; elected not to continue and instead decided to return to departure airport for evaluation/repair. We declared an emergency and were given a clearance to return; where we executed an uneventful visual approach and overweight landing (within specifications) on the runway. Subsequent removal of TAT probe by AMT's revealed a large bumblebee that had wedged itself deep into the scoop of the probe; blocking airflow and causing TAT/SAT deviations once airborne. TAT/SAT temperatures were normal on taxi out and also during taxi in after the event.