Laboratory Research On Physiology Migratory Restlessness And Directional Preferences

Laboratory research on bird migration began in the mid-1920s and rapidly gained ground. Hundreds of experiments on migratory physiology, orientation and other aspects have now revealed most of the relevant physiological processes and controlling mechanisms, at least in broad terms (Chapters 11 and 12). An important discovery was that, at migration times, migratory birds in captivity developed migratory restlessness (Zugunruhe in German), in which they hop and flutter around their cages, an activity that can be registered automatically by use of electronic trips under perches (Chapter 12). Migratory restlessness in captive birds occurs either by day (in diurnal migrants) or at night (in nocturnal ones), and has been regarded by some as the laboratory equivalent of migration itself (see Box 12.1). It appears chiefly in birds from migratory populations and much less so, or not at all, in birds from resident populations. The number of days on which migratory restlessness is shown has been found to correlate with the natural duration of migration (and hence distance travelled) in the population concerned. Migratory restlessness therefore provides a useful means of comparing the migration seasons of captive birds from different populations, and of testing the influence of various factors on migration timing (Chapter 12).

In particular, the role of daylength in influencing migration timing has been examined by manipulating the artificial daylengths (photoperiods) to which captive birds are exposed, and then recording their condition and behaviour. Metabolic rates, food consumption, fat deposition, body weights and migratory activity can all be studied at the same time.

Figure 2.4 Equipment for measuring directional preferences of birds in field conditions: protective non-transparent wall around, and test cage within. The cage is shaped like a round cake. It is made of two circles of wire, connected by eight vertical wires. The top is covered with wire-netting through which the test bird can see the sky. The sidewall is covered by transparent foil (kitchen wrap or cling film) on which pecks and scratches are made by the bird in its attempts to escape the cage. The cage is placed in the centre of a circular fence of uniformly coloured solid plastic that prevents the bird from seeing any landmarks other than the sky. After a standard time (say 10 minutes) in the cage, the bird is removed, as is the transparent foil, and the pecks and scratches are counted in each sector of the foil. A new piece of foil is then attached in preparation for the next bird. With two cages available, up to six birds can be tested each hour by a single observer. From Busse (1995, 2000).

Figure 2.4 Equipment for measuring directional preferences of birds in field conditions: protective non-transparent wall around, and test cage within. The cage is shaped like a round cake. It is made of two circles of wire, connected by eight vertical wires. The top is covered with wire-netting through which the test bird can see the sky. The sidewall is covered by transparent foil (kitchen wrap or cling film) on which pecks and scratches are made by the bird in its attempts to escape the cage. The cage is placed in the centre of a circular fence of uniformly coloured solid plastic that prevents the bird from seeing any landmarks other than the sky. After a standard time (say 10 minutes) in the cage, the bird is removed, as is the transparent foil, and the pecks and scratches are counted in each sector of the foil. A new piece of foil is then attached in preparation for the next bird. With two cages available, up to six birds can be tested each hour by a single observer. From Busse (1995, 2000).

Another discovery was that captive birds also developed strong directional preferences at migration times. Such preferences could be measured in individuals using circular 'orientation cages', which typically have solid sides and wire tops affording a view of the sky. In one early type, the cage was shaped like a vertical funnel, with an inkpad on the floor. The pattern of footprints up the sides of the funnel (lined with white filter paper) indicated the directional preferences of the occupant (Emlen & Emlen 1966). Automatic registration was achieved in circular cages equipped with radially-arranged perches fitted with micro-switches or other devices to record directional activity (e.g. Wiltschko 1968). Such apparatus has been used to study the orientation and navigation behaviour of laboratory birds, or of wild ones trapped and tested in field conditions during the migration season (Busse 2000). The simplest of all such cages was designed by Busse (1995) specifically for use in the field on birds trapped on migration (Figure 2.4). Using only two cages, up to six birds can be tested each hour by a single observer.

Orientation cages can provide unbiased information on the directional preferences of birds caught at migration times at particular localities (Ozarowska et al. 2004). Most methods of analysing directional preferences (so-called circular statistics) start with the assumption that the birds sampled show only one main directional preference. Yet many individual birds that have been tested have shown more than one migratory axis which is best expressed as a bi-vector individual pattern (see Busse & TroCinska 1999 for analytical method). In addition, when samples of migrants are caught and tested at particular localities, two or more migration axes frequently emerge, as birds migrating through the same locality but from different areas take somewhat different directions (Busse 2000). This is another reason for using a statistical method that can pick out the different directional preferences, rather than calculate a single amalgamated mean from all birds tested. While in theory directional information could be obtained from ring recoveries, in practice many years of data are needed, and geographical bias in ring reporting can distort the picture.

Another major benefit of studying migratory orientation in caged birds is that the external information received by the bird can be manipulated. For example, the perceived position of the sun can be altered by use of mirrors, star patterns can be modified in a planetarium, or the geomagnetic field can be altered using large magnetic coils (Wiltschko & Wiltschko 1995). These procedures facilitate study of the external cues that might be used by birds to determine their migratory direction (Chapter 9).

Wind tunnels

Recently developed wind tunnels have revealed much about the mechanics and energy needs of bird flight (Pennycuick et al. 1997). A wind tunnel creates a smooth (laminar) air flow in a test section where birds are trained to fly. The artificial wind speed can be adjusted so that, when the bird flies against the wind, it maintains a constant position in an observation section. Low turbulence is important in order to generate a natural situation reflecting flight through nonturbulent air but, if desirable, turbulence can be created by inserting nets or other objects upstream from the test section. Wind tunnels have been used to test flight mechanical theory (Chapter 3), to measure the metabolic costs of flight and to study flight style using high-speed video cameras. To yield meaningful results, especially on energy consumption, birds must be trained in the wind tunnel beforehand, so that they 'feel at home' there, and fly steadily, maintaining constant position against the wind for long periods.

In consequence of work with captive birds, we now have some understanding of how birds orientate on migration, of the energy costs of flight, and of the physiological preparation for migration that occurs at appropriate times of year, including the deposition of internal body reserves to fuel the flights.

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