The hummingbirds have taken ownership not only of the sugar water feeders but also of the lilac bush where the feeder hangs. The male gives me noisy scoldings when I am out in the morning filling the nearby sunflower seed feeders. He and the female also buzz the house finches and titmice and chickadees if they try to perch on the lilac while waiting for a turn at the seed feeders. They also chase each other although I haven’t been able to determine who is dominant or who is the more aggressive.
We have had a great summer watching this ongoing soap opera of our “Hummingbird Manor.”
Soon, though, the hummingbirds will leave and head south on their ways to southern Mexico and Central America. Most papers I have read about hummingbird migration agree that the males leave first (sometimes as early as late July) and that the females, possibly because of their need to recover metabolic reserves that were depleted from the intense and unassisted work of egg laying, incubation, and nestling and fledgling rearing, linger even to the end of September especially if food supplies are abundant. Both genders time their treks with the blooming of fall plants along their migratory paths. They rely on these plants for the nectar to power their long, sustained flights.
I used to worry that leaving the sugar water feeders out too late in the fall would delay the departure of the hummingbirds. They might not notice the building fall season if their supply of food was still rich and plentiful. Hummingbird experts assure us, though, that food supply will not interfere with the timing of the migration, and, in fact, leaving the feeders out into late September or even early October might be of benefit to other hummingbirds passing through our area on their own migratory treks.
Food supply, then, is not the critical determinant that triggers migration. How does a migratory bird know that it is time to head south for the winter?
The research of Paul Bartell, an assistant professor of biology at Penn State, can help us better understand this aspect of bird behavior.
Bartell describes four biological clocks that operate inside a migratory bird. One of these clocks is connected to the bird’s retina and is exquisitely sensitive to changes in day length. Another clock is centered in the pituitary gland, the “Master Gland” of the endocrine system. The pituitary controls the thyroid, the adrenals, and the reproductive glands and can alter the fundamental nature of a bird’s metabolism. The third clock is centered in the pineal gland. The pineal produces the hormone that triggers the onset of sleep (“melatonin”) and also may be the source of numerous micro-hormones that further regulate brain and endocrine functions. And finally, all three of these clocks interact with what Bartell calls the “migratory clock” which triggers the changes in behavior and physiology that both compels and allows the bird to accomplish migration.
One of the culminations of these biological clocks synching into migration physiology is a change in the day/night timing of a bird’s activity. Most birds in non-migratory periods are diurnal (i.e. are active during the day). As the biological clocks move into migration mode, though, the timing of a bird’s activity pattern drastically changes. The development of an agitated, nighttime behavior in caged birds has been described many times over the centuries. This behavior (called “Zugunruhe”) indicates that the bird is physiologically ready to begin its migratory flight. Migration, it turns out, even in our typically diurnal bird species, is primarily a nocturnal event, and the migrating birds quite amazingly seem to neither sleep nor to suffer any apparent stress from the absence of sleep during their migration periods. Their physiological state becomes so altered by the shifting of their clocks and hormones, that they become radically different organisms during the weeks of migratory activity.
Why is night migration preferable to day migration? It is cooler at night and thus the bird’s thermal stress is greatly reduced. The birds lose less water at night and thus their moisture stresses are reduced, too. There are also fewer predators. Since migration accounts for the majority of adult mortality in land birds (Sillett and Holmes 2002), any edge that increases survival is likely to be evolutionarily conserved.
There are ten thousand species of birds (or so) on Earth and slightly less than two thousand of these species are long distance migrators. Many other birds migrate between nearby habitats (“local migration”) or along altitudinal gradients in order to find climates and food sources and other resources that they need for reproduction and survival.
Migrating birds use many features of the environment to help them find their distant and nearby destinations. They use the sun as a guide for compass directions during the day, and the stars in the night sky for their night flights. They use the terrain and the landscape features of their breeding and wintering ranges and flight paths to guide them. Older, more experienced birds in particular are more efficient in following these landscape maps. Possibly it is a skill that is passed down through generations of individuals. William Keaton (whom Deborah and I had the great pleasure of meeting while we were in graduate school) also showed that the magnetic fields of the Earth form map lines and guides for migrating birds. And, finally, homing pigeons make “odor maps” of their home ranges and guide themselves very efficiently through many paths toward their home roost by sensing and interpreting olfactory information.
Migration is stressful and results in high mortality rates in almost all migratory bird populations. The benefits that each species realizes via finding safe habitats for nesting in the “summer” and then reliable food supplies and moderate weather conditions in the “winter” must outweight the shockingly high rate of loss and death.
I hope that we get to see our hummingbirds next summer. It wouldn’t be the same without “Hummingbird Manor.”