First it was the hawklet adopted by bald eagles. Then it was the lioness nursing the leopard cub. And now in the latest example of interspecies care, there’s video of a cardinal feeding goldfish.

Originally posted on YouTube in 2010, the footage shows the red bird hopping alongside a goldfish pond, then dropping what appears to be seeds into their waiting mouths.

According to the caption with the video, the cardinal would come back to the pond as many as six times a day to feed the fish. (Explore National Geographic's backyard bird identifier.)

Why would a bird feed an entirely different species? Princeton biologist Christina Riehl has a few ideas.

“My best guess is that the appearance of the goldfish’s open mouth at the surface of the water is just similar enough in size and shape to the open mouth of a baby bird that it triggers the instinct in the adult bird to provide food to it,” says Riehl.

Nestlings tend to have vibrantly colored mouths, often bright red and yellow. This acts like a bull’s-eye for the parents—a visual cue that says “Feed me here!”

“It’s an amazing demonstration of how simple stimuli can trigger very hardwired behaviors, even in situations that seem obviously wrong to us,” she says.

Happy for the Handouts

While the confused cardinal may make for an amusing video, “a cardinal feeding goldfish is certainly wasting its time, biologically speaking,” says Riehl. “Especially if the cardinal is feeding goldfish instead of feeding its own young.”

But from the fish's perspective, it's probably happy to accept a free meal, says Kevin Roche, a biologist at the Czech Academy of Sciences.

Roche says that carp—the fish family to which goldfish belong—are intelligent, and can remember areas where food is abundant or regularly provided. (Read about a three-pound goldfish caught in Detroit.)

While they may look dopey gulping at the surface of the water, it has a purpose: To suck down insects and other prey as well as acquire more oxygen.

The cardinal-feeding-goldfish video is not the first record of this kind of behavior.

Robert Mulvihill, an ornithologist at the National Aviary in Pittsburgh, Pennsylvania, says that the LIFE Nature Library books from the 1960s ran a black-and-white photograph of a cardinal feeding a goldfish.

As in the 2010 video, “the explanation, as best I can recall, was that it represented redirected parental feeding behavior, perhaps on the part of a bird that recently had lost its own brood,” says Mulvihill.

Duck, Duck, Goldfish

Interestingly, there are also clips online that seem to show black swans and a baby duckling “feeding” goldfish. But is it the same behavior? (See a picture of a see-through goldfish.)

Riehl isn’t so sure. She says that newborn cardinal chicks are altricial, which means they're naked, blind, and reliant on their parents for everything.

But most waterfowl young are precocial, which means they're fluffy, open-eyed, and capable of leaving the nest soon after hatching.

So, in theory, waterfowl shouldn’t react to a begging mouth the same way the cardinal does.

J. Dale James, director of conservation science and planning for Ducks Unlimited, seems to agree.

“Both the swans and the duckling look to me to be in a captive type of environment,” says James.

“As such, they are probably eating pellet type foods and it’s not unusual that they dip those in water while eating.”