Last year (July 26, 2012 ) I wrote about “Hubert” the female, ruby-throated hummingbird that fed, reproduced, and guarded our yard and nectar feeder all summer. Well, this year we had another female ruby-throat, and, based on the scientific literature that documents the species’ preferential return to both their summer and winter locations, we assumed that she was Hubert.
Last year it was our bee-balm that initially attracted Hubert to our yard, but this year due to some very heavy deer predation on the bee-balm I believe that it was mainly the nectar feeder that kept her close by all summer.
Last year I wrote about the importance of hummingbirds as pollinators (over 150 N. American plants are pollinated by hummingbirds!). I also wrote about their mating, nesting, and feeding behaviors and territories. This year I want to talk about their migrations.
Migration is an absolute requirement for most hummingbirds. They are not able to eat seeds or fruit or other food that might persist through a cold, temperate zone winter. They are nectar fueled carnivores that must have abundant sources of flowers and insects throughout the year. The ruby-throats that summer around here spend their winters in the dry forests, citrus plantations, and scrub forests of southern Mexico, Guatemala, Honduras, Nicaragua, Costa Rica, and northern Panama.
Hummingbirds are solitary birds. They do not form migratory flocks for a number of very logical reasons. A single hummingbird is very inconspicuous where a flock might attract the attention of predators. Also, hummingbirds have to regularly stop and feed along their migration routes and traveling alone means shorter lines at potential nectar sources and more insects and spiders in whatever habitat they happen to visit.
Hummingbirds fly very low on these long migrations. Typically they are seen just above the tree tops or just over the surface of any body of water (like the Gulf of Mexico!) that they might have to cross. Often they wait for favorable winds and weather fronts to assist them along, but frequently they have been observed flying into very severe headwinds. Migration takes one to four weeks, and they fly an average of twenty to twenty-five miles each day. The five hundred mile span of the Gulf of Mexico, though, has to be done in one stretch unless the birds happen to pause on an off-shore oil derrick or passing shrimp trawler. Most flying is done during daylight and the nights are used for resting. Males and females and mature and immature birds all have their own general times for migration. Males typically migrate several weeks before females, and older, mature birds, probably because they build up critical body fat reserves more rapidly than younger birds, tend to migrate before the younger individuals.
This hummingbird migration is a very early occurring Sign of Fall here in Western Pennsylvania. The local, male ruby-throats left the area by the end of July and the females are starting to leave right now. This year’s fledges, who are busy fattening themselves up, will be going in the next few weeks. We will continue to see ruby-throats at our nectar feeders well to the end of September, but most of these individuals are summer residents of more northern habitats and are just passing through and “nectaring up.” It is very important to keep your nectar feeders out through the month of September to provide these migrants a critical food source. It is not true that leaving the feeders out cause the ruby-throats to delay their southerly migrations. Changing day lengths rather than dwindling food supplies seems to be the critical factor that triggers their flights south.
About twice as many ruby-throats start the Fall migration south as finished the Spring migration north. Migration takes a heavy toll on these birds, but their robust reproductive potential continuously compensates for the yearly declines.
Mature ruby-throats throughout their lives seem to follow the migration path that they initially followed on their first flight south. They return to both their wintering sites and summer breeding sites with great consistency and accuracy. On the southward migration the birds fly over land in greater numbers (down the Appalachians, across the Gulf Coast, down over Texas, to Mexico and Central America). This is possibly due to the potential occurrence of fall hurricanes over the Gulf of Mexico. It has been speculated that individual birds whose behavior emphasized flying over the Gulf in the fall were selected out of the population by encounters with these catastrophic storms. On the spring northward migration, though, hurricanes are unlikely over the Gulf and the “short-cut” from the Yucatan to the U.S Gulf Coast is the preferred pathway for these birds.
The ruby-throats begin their northward migrations often as early as January. Many of them have gathered in the Yucatan Peninsula by late February where they feed avidly on abundant insects and spiders. Each ruby-throat doubles its weight (going from just over 3 grams to just over 6 grams) during this period of feasting. Then, over a period of three months, wave after wave of ruby-throats cross the Gulf (about 500 miles taking 18 to 22 hours) ending up scattered across the U.S. Gulf Coast from Texas to Florida. When they get to land they weigh about 2.5 grams!
These ruby-throats must then feed and get back to their migrations weights. They then work their ways steadily northward at a pace of about twenty or twenty-five miles per day. The Western Pennsylvania birds will arrive here in late May just in time for the riot of spring flowers and the emergence of the early summer insects.
On average a wild ruby-throated hummingbird will live for three or four years. One banded, wild ruby-throat, though, lived for just over nine years! That means that that individual made seventeen or eighteen migrations during its lifetime! If that bird flew from Pittsburgh to Panama its migration would be two thousand miles each way! For a bird that only weighs as much as a penny, that’s a lot of miles to fly!