Trees leaf out in mid-May, and birds become next to invisible, although you can certainly hear them. Here's a whole new world for your young birder to master. Once again start with the old stand-bys. Call attention to blue jay, crow, cardinal, chickadee and robin. Whose songs will already be familiar.
Acoustical aids help learn songs.
Some, like Birding by Ear in the Peterson series, provide guidance on what to listen for, especially when trying to tell apart species with similar songs. Now that you're in the woods, the list of species you're likely to encounter expands. Time of day, forest type and elevation, and age and size of wood lot all have a bearing on what species you're likely to find. It pays to have done your homework, to know the characters that distinguish one warbler from the rest of the ten or more species you might see. It's important to remember especially as you are starting out that not all sightings result in positive identifications.