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QUANTITATIVE STUDY OF MIGRATION WITH 23‐CENTIMETRE RADAR*
Author(s) -
Nisbet I. C. T.
Publication year - 1963
Publication title -
ibis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.933
H-Index - 80
eISSN - 1474-919X
pISSN - 0019-1019
DOI - 10.1111/j.1474-919x.1963.tb01586.x
Subject(s) - passerine , radar , bird migration , geography , cape , nocturnal , emberizidae , remote sensing , physical geography , geodesy , ecology , biology , telecommunications , computer science , habitat , archaeology
Summary A quantitative study was made of the displays of “angels” on the screen of a 23‐centimetre radar installation on Cape Cod, Massachusetts. The birds responsible for the displays were nocturnal migrants, mainly passerines. Other things being equal, the density of angels decreased roughly exponentially with distance from the centre of the radar screen, being halved about every eight miles. The rate of decrease varied only slightly with total migration density, and could be used to estimate the total number of bird targets in the air. Estimates obtained by this means were compared with estimates of migration density obtained from “moon‐watching” observations, which were reduced by a new method of calculation which incorporates radar measurements of the height of flight. Each angel was found to correspond to between two and twelve birds. After considering and rejecting other explanations, it is concluded that many birds migrate in groups at night. There is evidence that many species (including Parulidae, Turdidae and Emberizidae) migrate in small groups, rather than a few species in large groups. Groups of passerine night‐migrants are probably spread over a wide area, since it is rare to see two birds cross the moon together. Once the average group‐size has been determined in this way, radar can be used to measure the density of migration over a large area. Estimates of low migration density have a standard error about 25%, but estimates of high migration density are less accurate, and very high migration densities cannot be measured at all, although they can be accurately identified as such. The results in this paper cannot be applied directly to other radar stations, nor to other species of birds.