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hummingbird migration - ruby-throated

 

Migration of the ruby-throated hummingbird has been an inspiration and a wonder and a scientific puzzle for years. Shrimp fishermen and workmen on oil rigs reported that they found ruby-throated hummingbirds far out in the Gulf of Mexico (100 and 200 miles) where they did not belong. There were sightings along the coast of hummingbirds skimming fast over the water onto shore – annually – in step with migration cycles. Where did they come from?

Now we know that one of the common migration pathways of the ruby-throated hummingbird directly crosses the Gulf of Mexico. They fly across the water and travel north to breed – as far as Canada – and then south again to reach their winter homes in Mexico and Central America.8

The idea that a hummingbird could or would! fly across a minimum! 457 miles of water was dismissed as illogical and impossible. Hummingbirds have to eat and drink constantly – there would be no food or rest. Hummingbirds could not fly against winds – a headwind of only 20 miles per hour brings them to a complete stop.2 Yet the consistency of amateur sightings caused the scientific community to look for the improbable and they found it.

Migrating hummingbirds are fat! Just before they answer the call to travel north or south, they eat in excess and build a layer of rich fatty fuel just under their skin – along the back and belly and throat. A hummingbird will gain 25 – 40% extra body-weight in fat8 and that is enough weight to keep larger birds on the ground. The smaller the bird, the higher the proportion of body fat it can carry and still fly.2 Add the strength and efficient flight of a hummingbird and you are ready for a probable journey of 22 non-stop hours across the Gulf of Mexico.10

It is dangerous. A ruby-throated hummingbird that started out weighing 1/10th of one ounce might gain .07 ounces of fat and therefore have enough fuel to travel 1,400 miles – with no wind of any kind. A headwind of only 10 miles per hour will cut that distance down to 600 miles and more than 20 mph will push them backward.2 They can be blown sideways – Gulf winds are unpredictable.

It is bold because there is no guarantee that reaching land means reaching food. There may be no flowers in bloom and the migrating hummingbirds will have to search for nectar when they are exhausted.

The Gulf of Mexico is not the only barrier that imposes this migrational fasting. A researcher reported that a Rufous hummingbird fell out of the sky over the Mojave Desert – the bird did not make the crossing safely. It is reported that ½ to ¾ of the newborn ruby-throated hummingbirds do not survive their first year of migration.10

Migrating birds of all kinds have adapted to the weather and wait for favorable winds. The ruby-throated hummingbird takes advantage of tail winds constantly. Research in the Appalachian Mountains of Pennsylvania showed that migrating hummingbirds appeared in greater numbers when the winds blew favorably and even more when the winds were stronger.

Another study of ruby-throated hummingbirds has reported that when they gain 2 grams of fat, they have enough fuel to cross 600 miles of water without tailwinds.8 So they eat in excess and wait for cold fronts to carry them on their way.

The Eastern United States is host to the ruby-throated hummingbird for much of the spring and summer every year. They usually travel north and south along the Appalachian Mountains and the earliest birds to come in the spring seem to follow available insect populations rather than flowers (see Dan True's listing of arrivals and departures of all hummingbirds, per State – appendix – Species List.)11

Ruby-throated hummingbirds have an urgent drive to reach their breeding grounds and sometimes arrive before the flowers bloom - surviving on insects and sugar water provided by kindly humans.2 They will only stay in one place, during migration, for an average of 7 – 14 days. The first arrivals in spring are usually male and they may be seen as much as three weeks before the others. It can be that this protects the females and young because they follow the bright plumage of the male and find a ready food supply. If a northbound male hummingbird finds an ample supply of flowers and nectar he may be enticed to stay and establish a territory and therefore attract females and competition from other males. This is the principle behind hanging your hummingbird feeders strategically to “capture” their attention.11

The East Coast range of the ruby-throated hummingbird broadens out as far as all the deciduous forests. The forests are rich in flowers and insects and provide an unusual source of food for this bird – tree sap. The farthest north that the ruby-throated and Rufous hummingbirds will range is directly related to the Yellow-bellied Sap Sucker.8 The Sap Sucker bores holes through the tree bark and the sap that weeps out of the “wells” provide food for both of these hummingbirds. Tree sap is similar in nutrition to flower nectar.2

In the fall there is an instinctual clock that tells the hummingbirds when to head south. People still disagree over the precise mechanism within the bird that causes this. Most sources say that every expert agrees that food supply is not a factor and there is no reason to take down hummingbird feeders to stimulate migration.

Most experts believe that migration in hummingbirds is stimulated by changes in sunlight but hummers do not seem to have the same biological components that cause this in other migrating birds – hence the lack of proof.10

Then we have an expert biologist from Canada who says that in the northernmost climates, he has seen hummingbirds stay too long because of feeders and become out-of-sync with the supplies of food farther south.11

Another expert in hummingbird banding says that only underweight birds stay at feeders – and they are already out-of-sync with their migration because they did not or could not fuel up soon enough. Birds that are born late in the season are vulnerable. The time it takes to migrate far north into Canada may put high numbers of Canadian birds at a weight disadvantage. Leaving your feeders up may provide a critical opportunity for these hummingbirds to build reserves and “catch up.”10 Our northern biologist would say - fatten up and then travel south into deprivation. Our banding expert would say, this is a bird who is already in trouble – not a problem created by feeders.

These are the ethical choices for feeding wild hummingbirds. My own conclusions are to take the feeders down in the far north and leave them up in the middle and southern states. I am content that this is illogical. Truthfully, we just don't know enough and more recent research in science publications may shed new light on the subject – see - www.trochilids.com/links.html

As the ruby-throated hummingbird migrates southward, it averages about 23 miles per day.8 This sounds like a lot but is actually somewhat leisurely and not extreme for the bird. They are used to accumulating many miles every day while feeding on nearly 2,000 flowers.

Southbound ruby-throats rebuild their reserves in the early morning, travel midday and forage again in the late afternoon to keep up their body weight. There is a convenient refueling system provided by flowering spotted jewelweed. It consistently blossoms a few days ahead of the migration – providing abundant nectar for the hummingbirds. There is thoughtful speculation that the blossoming of jewelweed is an integral part of the timing that paces the hummingbirds as they travel south. Without jewelweed, there would not be enough to eat. A bird that misses the “wave” of flowers opening southward, may not survive.8

Not every ruby-throated hummingbird crosses the Gulf of Mexico to get “home”. There are three other common routes. Many come down from Canada on a western route through Texas and cross into Mexico directly. Others reach the coast and turn west - traveling around the Gulf so they do not have to make the water crossing. Still others circle east and hop through the Caribbean Islands into Cuba so their journey across the ocean is not so long.8

It has been reported that hummingbirds that do fly across the sea will wait for favorable winds and then climb high out of sight – as much as 200 feet - for their southern journey. When they come north, they are often seen skimming close to the waves.2

Regardless of which migration route they take, hummingbirds will take the same path that was imprinted the first year that they flew. And they fly alone. Young hummingbirds do not follow their parents and they do not fly in flocks.10 You have to wonder what causes one to circle left and one to circle right and one to rise higher and higher and fly out over that nothingness of water. It is an inspiration and a wonder and remains a scientific puzzle.

       
 
 
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