Ardea
Official journal of the Netherlands Ornithologists' Union

login


[close window] [previous abstract] [next abstract]

Evans P.R. (1966) An approach to the analysis of visible migration and a comparison with radar observations. ARDEA 54 (1-2): 14-44
The paper presents a mathematical model which relates the size of visible passage, seen by an observer sited on a 'leading-line' such as the coast, to the density of departure of migrants, the time of observation and the wind conditions. In theory, diurnal movements along a coastline could comprise both birds which departed inland (and reached the coast during the morning), and those which concentrated along the coast on previous days. The cumulative number of birds reaching and passing a coastal observation point from a uniform departure inland should be proportional to the square of the time after migration begins. Field observations of Skylarks in north Northumberland confirm this, and suggest that birds which have concentrated along the coast on one day do not usually coast on the following days. From a single simultaneous departure, the model explains the origin of a time of peak passage during diurnal migration and accounts for variation in this 'peak' time in different species and on different days. Differences in airspeed between species lead to different volumes of migration (counted in the same time) from the same departure densities. A method is given for calculating, from counts of coasting movements, the average number of birds leaving each square mile inland each morning. Coastal counts are corrected first for the time of observation, and then for the effects of the wind, assuming that the birds (1) are drifted by the wind (2) compensate for drift overland. For a given density of departure, the number of migrants seen at the coast may vary by a factor of ten or more with change of wind conditions. For a given size of passage (e.g. 'large', 'moderate'), the departure densities of migrant Skylarks in Northumberland were of the same order of magnitude as the echo densities seen on radar. In autumns of predominantly westerly weather, a large proportion of Skylark emigration from northern Britain may be visible to a ground observer, but not to radar. Records of visible migration are of greatest value when they start at 0 r before the time the migrants depart each morning. Departure densities cannot be estimated from counts which start more than two hours after the start of migration, and casual watches, even at the same hour on different days, are not comparable and of very little value in the study of the influence of weather on migration.


[close window] [previous abstract] [next abstract]