The effect as the instructions state:

While your back is turned, a spectator arranges several pictures of birds into a row on the table top. The total number of birds is known only to your spectator. As soon as you turn around you are able to accurately state the number of birds. In the final stage, you are able to state exactly how many birds were chosen even while the cards are out of view and inside a sealed case! You explain that your new hobby is bird watching. You then demonstrate your ability to count how many birds are showing. Includes illustrated instructions. No cover ribbon supplied. Condition very fine.