Answer: Birch trees are notorious for attracting aphids. As these pests feed they drip a sticky liquid called honeydew, which can sometimes look like sap. Aphids prefer tender new leaves which might be why the problem is concentrated on the tips of the youngest branches. Closely inspect an affected branch to see if you can find aphids feeding. If not, and the fluid is really sap, I suspect your tree is coming out of dormancy and fluids are running at high volumes. When they have no where else to go, they drip out of the ends of the tenderest branches. If this is the case, it will stop soon, all on its own, and you don't need to do a thing.
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