How a hummingbird carries a crossing in its body
Before migrating, a ruby-throated hummingbird can spend more time feeding and build up fat reserves. That extra weight is not merely baggage: fat is a compact fuel supply, holding far more energy per gram than the bird could carry as sugar-rich nectar alone. During flight, the bird breaks down that fat and uses oxygen to release energy for its muscles.
The strategy matters because hovering and fast, continuous wingbeats demand a great deal of energy. Over open water, there are no flowers where the bird can pause to refuel, so the fuel it loaded beforehand has to last until it reaches land.
A small margin for error
A bird beginning the crossing carries more fat—and therefore more weight—than it will have when it arrives. As that reserve is consumed, its load gets lighter. But the plan depends on having enough fuel to cover the distance under the conditions it encounters; wind can affect how much effort the crossing takes.
That makes feeding before departure part of the journey itself. Nectar gathered on land is converted into stored energy that can carry the hummingbird across a stretch where its usual food source disappears entirely.
