One of our most compelling summer visitors keeps itself aloft in air that, to it, must feel more like maple syrup. The reason this bird does not soar through the air like a Bald Eagle is because it is, well, very, very small. It is in fact one of the smallest birds in North-America: I am speaking of course of the Ruby-throated Hummingbird.
Being small poses special problems to these birds as they try to stay aloft. This is because flight, whether it be by an airplane, bird or insect, requires a certain speed to prevent the air from sticking to the tips of the wing. The smaller the wings, the harder it is to achieve this speed. You can see this effect when you blow out a candle: the hot smoke molecules rise up in a straight column because they move too fast to mingle with the surrounding air. As the smoke cools higher up, it slows down into a turbulent viscous mess that does not support flight.
The hummingbird has made a few remarkable adaptations to combat this problem. Like an insect, it rotates its wings in a figure eight at the incredible pace of 80 times per second, producing lift on both upstrokes and downstrokes at sufficient velocities. The rough feathery surface of its wings helps it further reduce the turbulence at its wingtips. Beating its wings at this pace allows it not just to fly, but also to hover when still, like a little helicopter.
Just like a helicopter, the hummingbird needs a high-octane fuel to maintain this activity: the sugary nectars produced by our flowers in spring. To obtain it, it manages to migrate over 3000 kms twice a year. By January, it takes off from its overwintering spot in Central America to travel northward in search of nectar. According to a recent study, special adaptations in the retina of the hummingbird’s eye may provide it with a compass that allows it to "see" the earth’s magnetic field as it finds its way back to where it was born. It uses this and the elevation of the sun to time its arrival, usually within a week from the beginning of May. This is when the weather is warm enough to spawn the first spring flowers, and when the risk of a night frost has become low. For when it becomes too cold at night, their small bodies cool more than those of other birds. On a cold spring night, hummingbirds must therefore engage in another remarkable adaptation: they briefly hibernate (torpor), shutting down their bodily processes for just one evening.
Although its ability to hover is a perfect adaptation for drinking nectar from a flower, it is not just the nectar that the hummingbird is after in spring. Like many birds, the real reason it times its arrival so precisely is because of the abundance of airborne insects at this time of year. It makes for a perfect time to lay a tiny clutch of eggs and raise offspring here, where there are fewer competitors for this source of food. For hummingbird chicks need to power the growth of their muscles, and this can only be done with the protein provided by an abundance of insects caught by their parents. It was with great surprise that I witnessed a hummingbird catching an insect this summer, and with great agility, too.
Indeed, when hummingbirds were first dissected in the mid-19th century, they found no nectar in their stomachs, but flies, gnats, wasps, aphids, beetles and other insects, often 50 individuals or more, propelling the belief at the time that this was their sole source of food. This insight changed only when a special bypass at the beginning of the hummingbird's stomach was discovered. It allows nectar to flow directly into the gut for faster processing.
Today, the thought that hummingbirds only rely on nectar is common. It led me to engage in one of my great summer pleasures: to feed hummingbirds with a mixture of 4 parts water and 1 part refined sugar in a glass feeder off the window of my cottage. I boil a new mix weekly, cleaning the feeder with vinegar to remove any moulds that might upset the bird's fragile stomach. It is a delight seeing hummingbirds hover near this artificial flower right in front of me. At times they even fight over it, performing all sorts of aerial acrobatics to ward off unexpected visitors.
Feeding hummingbirds in this way has, however, always felt like a bit of a guilty pleasure. The feeder does not resemble a flower at all, nor does the sugar water resemble nectar. Am I doing the right thing? Should I allow the hummingbirds to sort out their own food at the risk of them leaving my yard? It was only recently that I learned I need not have worried. Not only do hummingbirds eat insects, the nectar from flowers is not their only source of sugar. They also drink the sap of our very maple tree. Hummingbirds, in fact, track Yellow-bellied Sapsuckers (not to be confused with Hairy or Downy Woodpeckers) as they bore holes in maples in search of sugar water. It is this behaviour that allows hummingbirds to safely negotiate the woods of North America on their way back to Central America in fall without running out of high-octane fuel. It made me feel a little better about feeding them, as the ability to drink tree sap is, perhaps, a more important survival tool for this amazing species of bird than any of its other remarkable adaptations.
The co-evolution of insects, hummingbirds, sapsuckers and maples shows that ecologies are always more complicated than we think. We need to not just take care of the species we love, but also of their support networks. To have these amazing birds return for our own offspring to witness we must therefore stop using insecticides in our yard, stop cutting down maples for real estate developments, and treat our woodpeckers well by leaving rotting trees standing. In fact, we need to start admiring the very insects that bug us. For the flight dynamics of their tiny wings are considerably worse than those of the hummingbird's.