The Hughes H-4 Hercules (also known as the Spruce Goose; registration NX37602) is a prototype strategic airlift flying boat designed and built by the Hughes Aircraft Company. Intended as a transatlantic flight transport for use during World War II, it was not completed in time to be used in the war. The aircraft made only one brief flight on November 2, 1947, and the project never advanced beyond the single example produced. Built from wood because of wartime restrictions on the use of aluminum and concerns about weight, it was nicknamed by critics the Spruce Goose, although it was made almost entirely of birch. The Hercules is the largest flying boat ever built, and it has the largest wingspan of any aircraft that has ever flown. The aircraft remains in good condition. After being displayed to the public for almost 11 years in Long Beach, California from 1980 to 1991, it is now displayed at the Evergreen Aviation & Space Museum in McMinnville, Oregon, United States.

Based on this passage, what was the spruce goose made out of? Let's give stream of consciousness first:
To answer this question, we should know that: Built from wood because of wartime restrictions on the use of aluminum and concerns about weight, it was nicknamed by critics the Spruce Goose, although it was made almost entirely of birch.
So, the final answer is wood.