|Title||The role of environmental factors in Ring-billed and Herring Gull orientation|
|Year of Publication||1968|
|Degree||Doctor of Philosophy|
|Number of Pages||281 pp.|
During the summers of 1963 and 1964, a total of 723 Ring-billed and Herring Gulls from the Rogers City, Michigan colony were used in homing trials, of which 373 were adults or subadults, 56 juveniles and 294 chicks. An additional 1105 adult and subadult Ring-billed Gulls were captured at the same colony during the summers of 1965 and 1966 in an attempt to evaluate the rates of inter-colony dispersal and homing to natal colonies. Adult gulls were captured by means of cannon net traps and nylon snares. The classical type of homing trials were used over short distances of 2 to 150 miles from the colony. In addition, a series of gulls were released in or near the colony as controls. Forty-one Herring Gulls were radio-tracked during part or all of their homing flights. This technique significantly aided the project but various complications prevented its fullest potential from being realized. ... Analysis of current data and interpretation of the available literature resulted in a conclusion that there is no one answer to the way by which birds orientate. There have evolved different modes of orientation which are dependent upon the requirements of the species. Speculation regarding origin and evolution of this behavior in conjunction with migration have been presented. The conclusions resulting from this study are considered as being primarily applicable to the species studied. True navigation is not performed by these species. Orientation involves what appears to be an inherited preference for particular migrational headings by chicks and fledged juveniles. Selection of these gross headings is later modifed as a result of experiences acquired during long flights. Goals are learned, as are particular landmarks and possibly other environmental clues associated with routes. Thus adults have a variety of possible responses during migration or homing trials and are not rigidly bound to a particular response pattern, i.e., the innate behavior guiding juveniles. Searching for landmarks is apparently essential to successful homing by adult gulls and landmarks similar to ones at home can provide false clues. Direction selection by young gulls may be associated with magnetic clues although as yet there is no evidence to support an ability to interpret such weak stimuli. If magnetic clues are not associated with the innate machanism for direction selection, then some as yet unidentified factor is responsible.