Mysteries of Migration:
Every year I am cyclically drawn into the mysteries of migration by some particular bird, and this year it was the Ruby throated hummingbirds.
I habitually document the arrival and leave-taking of my hummingbirds observing and recording relevant contextual weather information. This year the first male arrived the morning of May 7th alerting me to his presence by landing on my head! Within a week other males and then the females began to arrive and over the summer I had, as usual, hoards of hummingbirds (probably more than 50). Beginning in mid – August the hummingbirds drained my two quart feeders every single day (the most ever). Because of the spring drought I had let my flower garden go, noting that my perennials – many of which were hummingbird plants – had gone by, so I assumed this frantic draining of sugar water might have something to do with the fact that I had so few flowers left overall. As the days shorten these birds also undergo hyperphagia, eating excessive amounts of food for two weeks or more to put on sufficient weight and to store it for migration, so no doubt hyperphagia was involved as well.
The night of September 4th – 5th I was watching masses of hummingbirds dive-bomb the feeders as dusk set in. The next morning (I awakened before dawn) I was outside with my dogs when it registered that the usual hummingbird twittering was absent. It hadn’t been a cold night so they hadn’t gone into torpor, although it was the dark of moon. It wasn’t until later that I realized that all but two hummingbirds were gone! I couldn’t believe it. Had they all begun to migrate in one night? When I checked my records they confirmed that nothing like this had ever happened to me before in almost 40 years. Naturally this occurrence sent me to the computer to check on hummingbird migration.
As usual the research is contradictory. A few sites suggest that hummingbirds did migrate at night; others like the Cornell site said they migrated during the day. All sites I consulted said each bird traveled independently. Because hummingbird migration is not well studied I can’t help wonder if these birds don’t actually migrate in small groups. Every year around here the bulk of them leave within a few days of each other, although I have never experienced a mass exodus before. I will probably never know what happened to all my hummingbirds this year, but I do know that they were all around as darkness set in and gone by dawn of the next day, so these hummingbirds left here at night.
Audubon informs us that Ruby-throated Hummingbirds double their body weight in fat, or more, before embarking on migrations. Some even gain close to half that in just four days. They need it, since their metabolism is one of the highest of any animal on Earth. They require the human equivalent of over 150,000 calories every day to power their fast-moving heart and wings, which can beat 1,000 and 3,000 times per minute, respectively. That fat accumulated before migration is burned in a steady release of energy, ideal for the 2,000-mile journey many Ruby-throated Hummingbirds make twice a year.
Banding studies suggest that individual birds may follow a set route year after year, often arriving at the same feeder on the same day. I keep one feeder outside my bedroom window and every year a female comes to claim that particular feeder. I am convinced this is the same bird or more likely (because hummingbirds don’t live more than 6 -7 years) a daughter who has taken her mother’s place. In all these years I have never seen a male at this feeder. We do not know if any individual bird follows the same route in both directions, and there are some indications that they do not.
Hummingbirds apparently evolved to their present forms during the last ice age. They were (and largely still are) tropical birds, but as the great ice sheets retreated from North America, they gradually expanded their ranges to exploit rich temperate food resources and nesting space, filling unoccupied niches in the U.S and southern Canada while evading intense competition in the tropics.
Most Ruby-throated Hummingbirds winter between southern Mexico and northern Panama. Since hummingbirds apparently lead solitary lives and neither live nor migrate in flocks, individual birds may spend the winter anywhere in this range where the habitat is favorable, but probably return to the same location each winter. Ruby-throats begin moving north as early as January, and by the end of February they are at the northern coast of Yucatan, gorging on insects and spiders to add a thick layer of fat in preparation for flying to the U.S.
Some will skirt the Gulf of Mexico and follow the Texas coast north, while most apparently cross the Gulf, typically leaving at dusk (hummingbird.net) for a nonstop flight of up to 500 miles, which takes 18-22 hours depending on the weather. Although some hummingbirds may fly over water in company of mixed flocks of other bird species, individual birds may make landfall anywhere between southern Texas and central Florida. Before departing, each bird will have nearly doubled its weight, from about 3.25 grams to over 6 grams; when it reaches the U.S. Gulf coast, it may weigh only 2.5 grams. It’s also possible that a few Ruby-throats island-hop across the Caribbean and enter the U.S. through the Florida Keys.
Males depart Yucatan first, followed about 10 days later by the first females. But the migration is spread over a three-month period, which prevents a catastrophic weather event from wiping out the entire species. This means that a few birds will arrive at a location early, but the bulk of the population will follow later. Around here it takes about two weeks for all the hummingbirds to arrive for the season. Each individual apparently has its own internal map and schedule.
Once in North America, migration proceeds at an average rate of about 20 miles per day, generally following the earliest blooming of flowers hummingbirds prefer. The northern limit of this species occurs after the arrival of the Yellow-bellied Sapsucker; if the earliest males arrive in Canada before sufficient flowers are blooming, they raid sapsucker wells for sugar, as well as eating any insects that might be caught in the sap. Every spring I also document the arrival of the sapsuckers to help me predict when I will see the first hummingbirds. The northward migration into Canada is complete by late May.
Because ruby throated hummingbird migration occurs over a relatively long period of time I am hoping that even with climate change bearing down on us that our hummingbirds will be able to adapt to a rapidly changing climate, although there is nothing we can do now to stop the extreme weather shifts that can’t help but have a negative impact on the lives of these heroic and engaging little characters.
Migrating birds can cover thousands of miles in their annual travels, often traveling the same course year after year with little deviation. First-year birds often make their very first migration on their own. Amazingly they can find their winter home despite never having seen it before, and return the following spring to the place where they were born.
We don’t know just how birds have developed such complex navigation skills but we have many theories/ studies and project that birds combine several different types of senses when they navigate. Birds may get compass information from the sun, the stars, and by sensing the earth’s magnetic field. They also may get information from the position of the setting sun and from landmarks seen during the day. There’s even evidence that sense of smell plays a role.
Some species, particularly waterfowl and cranes, follow preferred pathways on their annual migrations. These pathways are often related to important stopover locations that provide food supplies critical to the birds’ survival. Smaller birds tend to migrate in broad fronts across the landscape. Studies reveal that many small birds take different routes in the spring and fall to take advantage of seasonal patterns in weather and food.
Taking a journey that can stretch to a round-trip distance of several thousand miles is a dangerous and arduous undertaking. The physical stress of the trip, lack of adequate food supplies along the way, bad weather, and increased exposure to predators all add to the hazards of the journey.
In recent decades long-distant migrants have been facing a growing threat from communication towers and tall buildings. Many species are attracted to the lights of tall buildings and millions are killed each year in collisions with the structures. Birds have a remarkable homing instinct, allowing them to return to the same area year after year, even when their migration takes them halfway around the world.
Seasonal change, up until recently, has been one of the most dependable features of our planet, providing predictable resources such as spring leaf-out, monsoon rains, insect hatches, and fruiting seasons.
Each of the world’s bird species has adapted in some way to this seasonality—many by making long, precisely timed annual movements.
Birds that nest in the Northern Hemisphere tend to migrate northward in the spring to take advantage of burgeoning insect populations, budding plants and an abundance of nesting locations. As winter approaches and the availability of insects and other food drops, the birds move south again. Escaping the cold is a motivating factor but many species, including hummingbirds, can withstand freezing temperatures as long as an adequate supply of food is available.