You can easily make a simple bird call using items found around the house or in the woods. This toy has been around since the 1800s at the latest, and can be used to imitate the call of many different types of birds.
Take two small rectangular blocks of wood and cut a wide, shallow notch on one side of both of them. Lay the two blocks on top of each other with the notches facing inward, so as to leave a gap in the middle---it ought to look like a long, narrow hole passing horizontally between the two blocks. You should be able to completely cover the hole on one side with your mouth.
Cut a wide rubber band so that it forms a long strip, and stretch it across the hollowed side of one of the blocks. Lay the other block on top the same way as before, so that the two blocks hold the rubber tightly between them. Secure the two blocks together with another, smaller rubber band.
If you happen to be on a camping trip or long hike, you can still make your own bird call, using natural materials. The thick stalks of some plants can be substituted for the wooden blocks---cut a length of stalk four or five inches long, split it lengthwise, and hollow out the middle part. Any sort of wood will do, though, and it should be easy to find bits of wood on a hike. The wide rubber band can be substituted with a blade of grass, and the smaller fastening rubber band can be replaced with string or twine.
To play the bird call, hold it up to your mouth and blow into the gap on one side. This will make the rubber band or blade of grass vibrate, making a shrill singing sound. By blowing harder and softer, in different patterns of notes, you can imitate nearly any bird's call---all it takes is a little practice.