Wildlife Matters

 

In my column on migration, I pointed out that birds (including young of the year) were innately equipped with bearing and distance navigation capability as well as knowing when to migrate. They were able to maintain a straight course by sun-compass orientation/navigation by day and star orientation/ navigation at night.

However, non-migratory flights often require additional and sometimes greater navigational ability. The homing ability of birds has been utilized by man for centuries to rapidly transport messages or other information. It has also led to a competitive sport — pigeon racing.

Much of the navigational research has involved pigeons rather than other birds. Pigeons are easy to breed in captivity and are abundant as there are many people breeding them in many geographic areas. This is important as not only are some individual birds better “homers” than others; some stock (flocks) can be better “homers” than are other stock raised nearby. It has been estimated that only 5 to 10% them are capable of long-distance returns at high speeds. Good stock is needed for research studies to produce more consistent results of navigational ability.

Pigeons home only to the place they were raised, not to where their stock originated. Also, “home” does not appear to be permanent until they have bred there. Therefore, all really important experiments have been done with actively breeding adults.

Homing from release distances of only 20 to 25 miles probably is based on recognition of landmarks and then the direction taken likely involves their sun-compass. If there are no learned landmarks and they cannot see the horizon, there can be a breakdown of homing ability. Pigeons have homeward directed orientation below about 12 miles and over about 60 miles. In between is what is called a zone of disorientation. It is widely accepted that here some form of sun navigation is used involving comparison of observed (stimuli) coordinates vs remembered (home) coordinates.

This bi-coordinate navigation would account for the ability of adult birds displaced to the side of their migration route (mentioned in the column on migration) being able to compensate for the displacement. They compared the observed coordinates of their displaced location with the remembered coordinates of their wintering area and navigated a new course which would take them directly to the wintering area. There are records of long homing flights by Manx Shearwater (3,000 miles) and Laysan Albatrosses (3,200 miles in one case and 4,000 miles in another).

Bi-coordinate sun navigation requires birds to be able to detect and measure the sun’s movement along its arc through the day. Opponents of this concept felt this was impossible as this movement is slower than the hour hand of a watch. Experiments found that birds were able to distinguish between movement and non-movement even at that of the sun along its arc. Another example of movement detection capability of the pigeon eye involved flicker-fusion. Pigeons could distinguish between flashes at the rate of 150 per second, compared to 60 per second for humans. Therefore, the bird eye is better than the human eye in detection of movement.

Birds appear to be capable of some navigational ability by the stars and moon. A nocturnal eye must be capable of functioning with little light available. A large pupil is needed, which necessitates a large lens. The retina must contain many rods; these darks adapt slowly but become extremely sensitive to even very little light. Birds that often fly at night, as well during the day, have far more rods than cones.

Diurnal bird’s eyes have a small pupil and lens, and also have far more cones than rods. Cones dark-adapt in a few minutes, but light sensitivity increases far less than that of rods. Regarding night-time navigation, diurnal birds probably see fewer stars than do humans. This is not necessarily a disadvantage as it is easier for humans to identify the patterns of constellations when the lesser stars are dimmed a bit by haze. Perhaps not seeing the lesser stars would be beneficial for night-time navigation by diurnal birds.

Another feature of the avian eye is the shape of the central area of high cone density. In many birds of open spaces (gulls, wading birds, geese etc.), but not pigeons, this area is elongated horizontally and sensitive to vertical movement of objects in relation to the horizon. Some researchers feel this area to be similar to an aircraft’s “artificial horizon” in helping a bird establish vertical, if the bird can tell the effects of acceleration, rotation, and other changes in posture from the effects of gravity. Others say that is impossible for birds to do because humans have difficulty when flying blind without instruments.

The counter arguments include that a pilot is enclosed in a space with no sensory contact with the surroundings other than that of gravity; whereas a bird is the aircraft and is in full contact with the environment. Also, birds fly at slower speeds than do aircraft which reduces forces involved in turns; and that a bird’s head is very stable in most flight.

An experiment in the plains of Manitoba involving 18 species of birds and 249 individual flights of birds thrown up blindfolded, found that 95 percent quickly obtained a normal flying position. These birds flew for a variety of times; two-fifths flew for more than a minute, and some flew out of sight. Ducks and some small birds (blackbirds and sparrows) flew in a normal manner, but some of the small birds hovered and some flew in circles. Only 20 percent of those that flew, crash landed; most spiraled down gently or parachuted with hovering wings and outstretched legs. These were opaque hoods the birds could easily scratch off when they landed.

Therefore, birds can maintain stable, level flight without depending on visual stimuli. What is not known is how much of this is due to inherent stability of bird flight structure and how much may be due to correction of displacement by some “feed-back” mechanism.

It has also been pointed out that humans have been piloting and navigating flight for little more than 100 years, whereas birds have been depending on flight for their existence for 140 million years.