A bird book or internet, bird feeder (purchased or homemade), birdseed, paper or a journal
Put bird seed in the bird feeder. For the youngest children, simply watching for birds at the feeder will be exciting. For preschoolers and older, document your findings by taking pictures, writing the words they say, or making a chart. If using a chart, you can tally the kinds of birds that are observed. Older children might research birds in books or on the internet at www.nbr.nbs.gov (Bird identification Center -- you can even hear bird songs on this site). Make a bird journal with pictures and results. Have your child note if he sees any differences in varieties of birds in different seasons.
The more excitement you show about the birds that are in your yard, the more interest your child will show.