Owls don’t weigh 16 pounds (except for fat owls). 300 kilowatts is a rate of energy, not a total quantity of energy. 300 kilowatt hours (which is possibly what they meant?) Is only around 260,000 kilocalories (which is called “calories” on food labels because units of measure were made up by humans). According to an extremely naive google search, that would only take an owl 5 years to consume, rather than 10. If the original number were correct, that would mean this owl eats 8,000 calories per day. Which is not typical.

Onto the broader point, the efficiency of birds in flight is not as simple as this image suggests. There is no (useful) formula that takes the weight of a bird and the distance it will fly and tells you how many calories that takes. Birds can fly at different elevations, at different speeds. They can fly with or against the wind. They can change many things about how they fly to be more efficient or less efficient.

If you really want to know how many calories it takes for an owl to cross the ocean, first get the owl to the point of starvation, then bring it on a boat to the middle of the ocean. Feed it a fixed number of Tootsie pops, then sink the boat. With nowhere else to land, the owl will be forced to fly to shore. Based on how far the owl makes it, you can determine how far each tootsie pop allowed it to fly, and derive calories per mile from that.