Last year, while out for a walk with his new owner Ed Gernon in California, Rex spotted a hummingbird on the ground, covered in ants. Gernon says he thought that the bird was dead at first, but the dog refused to walk away.
That's when they met Hummer, in poor health but still clinging to life. Rex's concern for the bird convinced Gernon to try to help her.
"It was this little creature," he told CBS Los Angeles. "This fragile creature that the whole world wanted to kill and he was trying to protect her so I thought I'd go the distance."
Over the following weeks under Gernon's constant care, little Hummer regained her strength and was soon flying - though seeing the remarkable trust she has in Rex suggests she remembers the role he played in saving her.
Here's video of the two of them interacting.