Listen to Nature!

Yesterday was the first truly warm day of spring so far; In the evening I took a short walk around the woods, chasing the fading orange light and taking mental notes on the activity of the fallow deer, when I heard a crackling, popping sound coming from the canopy. I looked up and around, expecting to see a grey squirrel munching on the pine cones but none could be found. No birds either. What could it be?