Bird Key is at the center of a potentially controversial sale of the private island.

After being put on the market, it is sparking increasing concern in Miami's environmental circles because of the island's reputation as a bird sanctuary.

The 37.5-acre private island was listed for $31.5 million.

Finlay Matheson, the island's owner, said he bought the land in 1985 for $36,000.

“It is unique because it is the only undeveloped island in North Biscayne Bay and the only natural island in Biscayne Bay,” Matheson said. “All the other islands were created and filled in by human hands over the years.”

After owning the land for 40 years, Matheson told NBC6 that now is the perfect time to sell.

“I thought, well, the real estate market is hot. I have a great agent,” Matheson said. “So why not?”

Located on the East 79th Street Viaduct, Bird Key is listed on the county's Register of Environmental Endangered Areas.

For this reason, local environmentalists are concerned about the possible sale of the island, fearing that it could harm the natural habitat for birds.

Jose Francisco Barros, president of the Tropical Audubon Society, said construction on the island would most likely involve logging and filling existing wetlands.

“Much habitat has been lost from Miami Beach and along the Miami-Dade coast. This island in particular serves as a refuge and resting place for these birds,” Barros said. “These islands lie in the center of the Biscayne Bay Water Conservation Area. They are subject to many state laws and federal laws.”

Still, Matheson said he wants to sell the island as soon as the right buyer makes an offer.

“Hurry up… make an offer. Get it done,” Matheson said.

Miami-Dade County attempted to purchase the island, but the office of county Mayor Daniella Levine Cava told NBC6 the offer was rejected.

“We are committed to finding alternatives, including state or federal assistance, to purchase this property and expand the reach of our Biscayne Bay conservation programs,” her office said in a statement.