Yesterday, I attended Science Cafe Cleveland, an international monthly science colloquium sponsored by Sigma Xi. The topic of burning interest this month was bird migration. Though a general overview of the vast and profound physiological processes required for migration were narrated by Dr. Sarah Nabey of Hiram College and Harvey Webster of the Cleveland Museum of Natural History, the audience was nonetheless concerned with an assumed looming issue: the effect of global climate change on the timing of bird migration.
As a chronobiologist, I would argue that global climate change does not affect the timing of bird migration. Emphasis on timing. Sarah agreed. Why? Migrating bird species have adapted to plastic weather, climate, and food availability by relying on the single, least plastic, most constant, reliable phenomenon on mother Earth; the hours of light (photoperiod) and dark (scotoperiod) in a given day. Of course, we all know that the lengths of photo and scotoperiods vary seasonally due to the Earth’s tilt. Unless global climate change alters the Earth’s tilt, then the timing of migration and return home will not change. Yes, the suprachiasmic nuclei’s abilities to integrate photic information and modulate behavior and physiology accordingly are that awesome.
Another intriguing question was whether the length of the photo or scotoperiod is a more important determinant of the timing of migration. From a chronobiological perspective, I would postulate that the scotoperiod is more influential. Why? The onset of all reproductive phenomena and related physiological stress, which is highly important for the mobilization of energy resources during the migratory process (on average, an 80 hour, non-stop flight, with no meals, drinks, and in-flight entertainment), are mediated by melatonin, the hormone of darkness. In mammalian species, the pineal gland, responsible for nocturnal melatonin secretion, is indirectly wired to the retina, the locus of photic integration, via the spinal cord. In avian species, the pineal gland is directly wired to the retina, minimizing interferences of transmitted photic information from other indirectly projecting brain areas. This suggests that melatonin secretion from the pineal gland in avian species is critically involved in the timing and onset of physiological phenomena influenced by the lengths of daylight and/or darkness.
For trivial purposes: The two prominent killers of birds, migratory and non-migratory, are felis catus (the domestic cat) and illuminating, ambient lights of skyscrapers.
Below is a picture from www.montegraphia.com of a migratory Copper’s Hawk, we saw in Letchworth State Park near Rochester, New York.