Quebec, 6 August 1982

Scotland, September 1982

American Wigeon Anas americana

Green-winged Teal Anas crecca

American Black Duck Anas rubripes

Mallard Anas platyrhynchos Ring-necked Duck Aythya collaris Ring-billed Gull Larus delawarensis

Lapwing Vanellus vanellus

Semi-palmated Plover Charadrius semipalmatus

New Brunswick, 5 August 1966 New Brunswick, 29 August 1968 Prince Edward Island, 30 August 1977 New Brunswick, 13 August 1986 New Brunswick, 8 August 1982 New Brunswick, 22 August 1970 Prince Edward Island, 16 August 1970 Newfoundland, 22 September 1985 New Brunswick, 27 July 1976 New Brunswick, 5 September 1970 Michigan, 19 August 1975 New Brunswick, 7 September 1967 Michigan, 14 June 1950 Ontario, 10 June 1945 Ontario, 27 June 1964 New York, 21 June 1980 Ontario, 8 July 1980 Cumbria, England, May 1926 Quebec, 24 July 1972

Scotland, 7 October 1966 Ireland, 12 October 1968 Ireland, 8 October 1977 Scotland, 21 September 1986a France, 9 December 1982 England, 2 January 1971 Iceland, 17 April 1979 Ireland, 31 October 1985 France, 12 October 1976 Germany, 10 February 1988 Iceland, 3 January 1978 Wales, 26 December 1967 Spain, 18 January 1951 Azores, 4 November 1945 Spain, 20 January 1965 Ireland, 28 December 1981 Spain, 21 January 1981 Newfoundland, December 1927 Azores, 23 September 1972

a Later shot County Wexford, Ireland, 30 November 1986.

often turn up together, nor why the vast majority of vagrants are inexperienced youngsters. At the least, vagrancy is a vivid tribute to the dispersive powers of some migratory birds.

The preponderance of juveniles among vagrants would be expected for several reasons, such as their inexperience, and their greater dependence on compass orientation as opposed to the bi-coordinate navigation (=homing behaviour) used by adults. Also, inherent errors in orientation and migratory timing would be expected more frequently in juveniles, because at that age deviants from the norm would not have been exposed to natural selection. For the most part, only successful individuals, behaving normally, are likely to reach adulthood.

While the arrival of birds in unexpected places has led to speculation on how they might have got there, only limited testing of ideas has occurred. The roles of different proposed mechanisms of vagrancy clearly vary between the species occurring at any one site, and some species may be affected through more than one mechanism. Off-route displacement by inclement weather is usually little more than a temporary phenomenon for which birds are normally able to correct at a later date. Other types of vagrancy probably result from deliberate behaviour on the part of the birds, but acting on the basis of flawed instructions -some genetic or other change in the orientation or time-keeping mechanism which leads them to migrate in an atypical direction, or fail to switch off their migratory behaviour at an appropriate time. In theory, more than one kind of genetically-based flaw could occur simultaneously in the same population, with some individuals showing behaviour consistent with mirror-image migration (say) and others reversed-direction migration. Although some observers have tested directional preferences of vagrants in orientation cages, and the location may eliminate some possible explanations of their presence, it is seldom possible to say which particular theoretical mechanism might have brought most vagrants to a given site.

Several authors have suggested that vagrancy is more common in nocturnal than in diurnal migrants. However, many more bird species migrate by night than by day, and nocturnal migration is particularly common among longdistance migrants. It is therefore hard to tell without more detailed analysis whether vagrancy is associated with nocturnal migration as such, or with the longer and more difficult journeys that nocturnal migrants often make. What is clear, however, is that vagrancy cannot be explained in terms of only one phenomenon, even though some of the proposed mechanisms are no more than working hypotheses.

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