The royal palm (Roystonea regia) is a species of palm tree indigenous to the Caribbean, generally growing 15-20 meters (approximately 50-65 feet) in height.
On the other hand, "in Spanish-speaking countries, guano is a common name applied to Coccothrinax species" of palm tree, which is characterized by fan-shaped leaves that are often used for thatch in buildings.
Hemingway mistakenly conflates the two types of palm trees in this sentence. Apparently he wasn't sure who his fronds were.
Regarding "budshields," according to the EDIS, University of Florida IFAS Extension, "the growing point of the palm [from which all leaves originate] . . . is often called the bud or heart." The leaves or fronds that grow from it are called "budshields" and are used as building material in poverty-stricken areas in the Caribbean.
For a helpful discussion of Hemingway's obscure references in this sentence, see WordReference.com.