A PIPER PA31 PLT HAD SMOKE IN THE CABIN DURING TAXI FROM DCA.

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WHILE TAXIING AT DCA; THE CABIN QUICKLY FILLED WITH SMOKE. THE HEATER WAS OPERATING AND HAD BEEN USED PREVIOUSLY IN THE SEASON. I PULLED ONTO THE RAMP AND TOLD GND CTL THAT I THOUGHT I HAD A HEATER FIRE WHILE SHUTTING THE HEATER OFF AND THE AIRPLANE DOWN. THE FIRE DEPT RESPONDED AND ALSO DETERMINED THAT THE SOURCE OF THE PROB WAS THE HEATER. NO FIRE DAMAGE WAS EVIDENT AND THE SMOKE APPEARED TO BE THE WORST THAT OCCURRED. I ARRANGED FOR A CHARTER PLANE FOR MY 2 PAX AND AS AN A&P; PERFORMED A VISUAL INSPECTION AND OPS CHK OF THE HEATER. THERE WAS EVIDENCE OF A BIRD STRIKE JUST FORWARD OF THE HEATER AIR INLET ON THE LOWER R SIDE NOSE OF THE AIRPLANE. IT WAS CLEAR THAT THE HEATER HAD INGESTED SOME OF THE BIRD REMAINS AND HAD BURNED THEM WHEN THE HEATER WAS TURNED ON. I PERFORMED A LOGBOOK ENTRY REFLECTING THAT I HAD PERFORMED A VISUAL AND OPS CHK OF THE HEATER AND RETURNED THE ACFT TO SERVICE. I THEN FLEW THE AIRPLANE TO ZZZ ON AN IFR FLT PLAN. NO PROBS WERE ENCOUNTERED AND AS A PRECAUTIONARY MEASURE; THE HEATER WAS TORN DOWN AND INSPECTED BY OUR MAINT FAC. THEY ALSO AGREED THAT THE HEATER AIR INLET HAD INGESTED A PORTION OF A BIRD.