Avian Migration: The Ultimate Red-Eye Flight
Birds that migrate at night enter a state of sleepless mania and gorge on foods by day, behaviors mediated by their biological clocks
Navigation and Orientation
Recently published research led by Heiko Schmaljohann at the Institute of Avian Research of Vogelwarte Helgoland showed that the northern wheatear (Oenanthe oenanthe), a small nocturnal migrant, travels 9,300 miles from Alaska, across Siberia and central Asia, to wintering grounds in eastern Africa, a phenomenal distance fraught with challenges such as traversing the Arabian desert. How do the birds find their way? The recent development of lightweight geolocators has made tracking routes of small, migrating birds possible, revealing details such as course, distance, speed and number of rest stops. Geolocator technology is still very new, but migration scientists have already noted its enormous potential for studying navigation in migrating birds, including sensory mechanisms, spatiotemporal memory, evolutionary adaptation, learning and plasticity.
Before modern tracking devices, navigation and orientation were studied in the laboratory using enclosures known as Emlen funnels, named after the researchers who developed them in 1966. Birds were placed individually into funnel-shaped, paper enclosures. An ink pad formed the base of the enclosure, and a wire screen placed on top prevented escape. The pattern of footprints in the funnel indicated the direction of attempted flight. Experimental manipulation revealed stimuli used for orientation, for example, celestial projections onto the ceiling or magnetic fields. When birds express Zugunruhe in captivity, they orient themselves in a seasonally appropriate direction (for North American birds, south in autumn and north in spring). Emlen funnels are still used for studying the neurobiology of orientation and navigation, although recording techniques and data analyses have been modernized (see Figure 6). Behavioral experiments using Emlen funnels show that birds in the Northern Hemisphere know to fly south during the fall and north during the spring based on perception of seasonal changes in photoperiod, with the aid of their internal clock. As it turns out, migratory birds use a combination of navigational tools, including a genetically hard-wired directional sense, a magnetic compass, celestial cues and, in nocturnal migrants, patterns of light polarization at sunset.
The role of a circadian clock in regulating navigation is controversial. One school of thought among biologists and physicists alike is that the magnetoreceptor birds use to navigate is cryptochrome, a circadian clock protein, and that this magnetic compass may be calibrated each sunset to help birds fly in the proper direction. Other researchers, using a clock-shifting or “jet lag” experimental paradigm, have failed to find a direct role of circadian clocks in migration. Additional experimentation directly testing the role of circadian clocks in navigation is warranted.