Ruby-throated hummingbirds are jewels of local summer landscapes. Hummingbirds arrive in Ohio from their southern wintering sites around the beginning of May and spend the summer feeding and raising young. By September, daylight, temperature, and nectar sources all are declining. It’s time to prepare for the return journey south! According to eBird, the online public database of bird sightings, early September is when reports of hummingbirds in Greene County are most common. By the end of the month, the birds are nearly gone.
Like all birds, hummingbirds are endothermic, meaning that they use their high rate of metabolism to generate heat that warms their body. It turns out that the smaller the animal, the relatively higher its rate of metabolism. And so ruby-throats, with an adult body mass of just a few grams (a penny or two), have exceptionally high relative metabolic rates. What that means is that a hummingbird’s energy reserves don’t last long. A hummingbird going about its daily business—mostly sitting around, interrupted by occasional bouts of flight–burns about 7 Calories per day. That might not seem like a lot (a human burns about 2000 Cal/d), but it’s equivalent to metabolizing nearly a gram of fat, about 25% of the bird’s body mass, each day.
With reserves that deplete so quickly, migration season poses a particular challenge. Ruby-throats mostly spend the winter in southern Mexico and Central America, about 2500 km (1500 miles) from Ohio as the crow (hummingbird?) flies. How do they manage the energy demands of that journey?
Not surprisingly, one strategy is to store energy before leaving. Despite eating a carbohydrate-rich diet (nectar is mostly sugar water), hummingbirds store most of their energy reserves as fat. Naturally, a hummingbird’s small size limits how much fat it can carry; 2 or 3 grams—about half a teaspoon’s worth—is about it! Each gram of fat provides about 9 Calories of energy, so a ruby-throat can carry something like 25 Calories of fuel reserves.
Interestingly, though, only some hummingbirds, mostly adults, fatten up before they begin their journey south. To do so, they add bouts of feeding during midday rather than resting at those hours, and they lengthen the duration of individual feeding bouts. In contrast, many juveniles forego increased feeding before they depart and instead feed en route. Hummingbird southward migration tends to track the peak blooming of orange jewelweed, a favorite source of sugar-rich nectar. The result is that hummingbirds reach the Gulf Coast in varying states of energy balance; some have well-stocked fat reserves upon arrival, and others are quite lean.
The big decision the birds then need to make is what path to follow from the southern US to their final wintering grounds. The most direct route to Central America is across the Gulf of Mexico, a journey of about 850 km (500+ miles). But that path does not provide opportunities to feed. Alternatively, a bird could follow the coast around the Gulf, a much longer flight but with food along the way. An intriguing question emerges from that decision: Is it really feasible for a hummingbird to fly non-stop across the Gulf of Mexico?
To answer that question, one needs three main pieces of information. First, how much energy can a hummingbird carry? (We have already answered that: about 25 Calories worth of fat.) Second, how much energy does it take for a hummingbird to fly? And third, how fast does a hummingbird fly while migrating?
A number of approaches have been used to estimate the energy cost of hummingbird flight. Those techniques include aerodynamic analyses; measuring rates of respiration (oxygen use or carbon dioxide production) of hummingbirds hovering inside of closed chambers; and tracking of chemical tracers in free-living birds (the so-called doubly labeled water method). One innovative study modified an outdoor hummingbird feeder to capture air as the bird hovered to feed, much like a human wearing a mask while testing on an exercise bike. Although the energy cost of migratory flight might differ somewhat from values measured in those other circumstances, the research indicates that hummingbirds use about 1 Calorie per hour while flying—and so each Calorie of stored fat can fuel about an hour of flight; 25 stored Calories can support 25 hours of flight.
How far will that get them? Hummingbird flight speed no doubt depends to some extent on environmental conditions like wind and temperature. However, 40 km/hr (25 miles/hr) seems to be a reasonable estimate. At that speed, 25 Calories of stored fat, lasting 25 hours of flight time, will carry the birds for about 1000 km (625 miles). That’s more than enough range for these tiny birds to cross the Gulf of Mexico non-stop without feeding. Wow!
Surprisingly, given the appeal of ruby-throated hummingbirds, there remain a lot of unanswered questions about their lives. Some of the hummingbirds that reach the southern US each autumn remain there through the winter, but most depart for destinations further south. What route do they follow: across the Gulf, or along the coast of Texas and Mexico? And how does an individual bird decide which path to take? While we are pretty sure that a hummingbird could cross the Gulf of Mexico non-stop, at least with decent weather, how many actually do that? We still don’t know.
Regardless of the answers to those questions, the life of a ruby-throated hummingbird is remarkable. The birds avidly feeding at jewelweed and backyard feeders in Greene County, Ohio in September will make a 3,000-mile journey to Central America and back, powered by about 21 million (!) wing beats during 120 hours of flight. And they do that year after year; the oldest recorded wild ruby-throat lived for more than 6 years. Hummingbirds pack a lot of awesome into those tiny bodies!
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.