These giant, gorgeous birds may look pretty, but scientists say these mute swans are a growing threat to California's few remaining wetland habitats.

Biologists have seen an explosion in California's mute swan population in a very short time.

In 2022, there were around 1,500 swans.

This spring, biologists estimated there were over 12,000, mostly in the Sacramento-San Joaquin Delta.

Native to Europe and Asia, the swans were likely brought here to adorn private ponds and parks or released to chase away nuisance Canada geese.

Mute swans can weigh up to 30 pounds.

They're aggressive defenders of their territory, and they can kill or displace native species such as ducks or shorebirds.

They also eat massive amounts of wetland plants on which native wildlife depend.

Because of the threats to the Delta ecosystem, wetland managers would like to see them gone.

A pending bill in the California legislature would allow hunters and landowners to shoot mute swans to bring down their numbers.

As was referenced by the assembly member, these swans are very aggressive, very large.

That's Mark Hennelly, a lobbyist for the California Water Fowl Association one of the bill's supporters.

If the population gets too large and out of control, it may be beyond our ability then to really effectively manage them, so we want to get ahead of the problem.

On the East Coast, some states faced similar surges in mute swan numbers in the .. and some aggressively wiped them out.

Maryland, for instance, reduced its population from thousands to just a handful and targeted swan killing programs.

So far, California's swan killing proposal easily passed the state assembly and is pending in the Senate, but it may face controversy in the coming weeks.

Animal welfare advocates fought similar efforts on the East Coast The anti-swan hunting activists say the birds don't deserve to be shot, and they plan to try to persuade lawmakers to kill the bill.

Reporting for CalMatters, I'm Ryan Sabalow.