Aural
An adjunct to the previously described nocturnal observation methods, which has potential for species identification, is the use of a parabolic reflector with attached microphone to amplify call (chip) notes (Ball 1952; Graber and Cochrane 1959). This device, when equipped with a tape recorder, can record night migrants up to 11,000 feet on nights with or without a full moon. A primary disadvantage is that one cannot tell the direction a bird is traveling and there is considerable difficulty in identifying the chip notes made by night migrants. In addition, the bird may not call when it is directly over the reflector and consequently it would not be recorded. These calls are quite different from the notes we hear given by familiar birds during the daytime while they are scolding an intruder or advertising their territory.
Preserved Specimens
Reference material consisting of preserved bird skins with data on time and place of collection exist in many natural history museums. The essential ingredient in studying migration by this method is to have an adequate series of specimens taken during the breeding season so differences in appearance between geographically separated breeding populations of the same species can be determined. Such properly identified breeding specimens may be used for comparison with individuals collected during migration to associate them with their breeding areas (Aldrich 1952; Aldrich, Duvall, and Geis 1958). This supplies a convenient way of recognizing and referring to individuals representative of known populations wherever they may be encountered.
Marking
If birds can be captured, marked, and released unharmed, a great deal of information can be learned about their movements. Many different marking methods have been developed to identify particular individuals when they are observed or recaptured at a later date. A few of the general methods are summarized in this section.
Bands, Collars, Streamers
Since 1920, the marking of birds with numbered leg bands in North America has been under the direction of the U.S. Fish and Wildlife Service in cooperation with the Canadian Wildlife Service. Every year professional biologists and voluntary cooperators, working under permit, place bands on thousands of birds, game and nongame, large and small, migratory and nonmigratory, with each band carrying a serial number and the legend, NOTIFY FISH AND WILDLIFE SERVICE, WASHINGTON, D.C., or on the smaller sizes, an abbreviation. When a banded bird is reported from a second locality, a definite fact relative to its movements becomes known, and a study of many such cases develops more and more complete knowledge of the details of migration.
The records of banded birds are also yielding other pertinent information relative to their migrations such as arrival and departure dates, the length of time different birds pause on their migratory journeys to feed and rest, the relation between weather conditions and starting times for migration, the rates of travel for individual birds, the degree of regularity with which individual birds return to the summer or winter quarters used in former years, and many other details. Many banding stations are operated systematically throughout the year and supply much information concerning the movements of migratory birds that heretofore could only be surmised. The most informative banding studies are those where particular populations of birds are purposely banded to produce certain types of information when they are recovered. Examples of such planned banding are the extensive marking of specific populations of ducks and geese on their breeding grounds by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service, as well as in "Operation Recovery," the cooperative program of banding small landbirds along the Atlantic Coast (Baird et al. 1958). When these banded birds are recovered, information concerning movements of specific populations or the vulnerability to hunting is gained. Colored leg bands, neck collars, or streamers can be used to identify populations or specific individuals, and birds marked with easily observed tags can be studied without having to kill or recapture individuals, thus making it a particularly useful technique.
We have learned about the migratory habits of some species through banding, but the method does have shortcomings. If one wishes to study the migration of a particular species through banding, the band must be encountered again at some later date. If the species is hunted, such as ducks or geese, the number of returns per 100 birds banded is considerably greater than if one must rely on a bird being retrapped, found dead, etc. For example, in mallards banded throughout North America the average number of bands returned the first year is about 12 percent. In most species that are not hunted, less than 1 percent of the bands are ever seen again.