I would recommend not doing this. The reason a heat gun will make scratches seem to fade away is because the heat makes the paint swell, hiding the imperfections. This will eventually come back. If the marks from bird crap were very superficial, or not completely etched into the paint they will disappear with time alone, or a good wash. Another problem with heating the paint is that you can actually induce delamination of the paint from the metal. If a good etching primer was not used, paint relies on a mechanical bond to the metal on the car... making the paint swell can cause the little bit of primer imbedded into the metal to push out and separate. Back in the day they used to polish cars at the dealers and docks under full sun, the paint was overheated, car looked fantastic, then a few years down the road the paint started to flake out in a banding pattern.
Looked something like this. Most people thought it was just sun fading, and it mostly affected import Japanese cars (Mitsubishi was a big one with this problem), but some domestics had the same problem as well.
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