Why do both male and female nuthatches smear spruce or pine pitch (resin or sap) on the cavity entrance during nest building?

Spruce trees are sometimes confused with other evergreens like pine and fir trees. Pine needles are longer, even those of the Jack pine, and both pine and fir needles tend to be gentler on your fingers than spruce needles when you approach from the tips. The needles of a spruce tree are arranged all around the twig. Spruce needles are four-sided except for the flattened Sitka needles, which have a little dip at the bottom, like the keel of a boat. Another clue that you’re looking at a spruce is if there are little pegs protruding from the twig at the point where the needle attaches.