Pigeon homing: Does initial orientation include a ?preferred compass direction??

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Abstract
To find out whether the initial orientation of pigeons is affected by a spontaneous directional tendency, as postulated by Wallraff's (1974, 1978, 1982) hypothesis, experienced birds were released at test sites distributed symmetrically around their loft. — The length of the mean vectors of the single releases, the deviations from the home direction, the homeward components as well as the homing speed did not show a correlation with the geographic position of the home direction. Summarizing four sites each on 21 experimental circles, we frequently obtained significant compass vectors, but they varied in direction between 115° ESE and 351° N, depending on what sites had been used, and did not indicate a uniform trend. A ‘preferred compass direction’ as an integrated part of pigeon navigation, being the reason for the frequently observed deviations from the home direction, could not be confirmed. — The problematic nature of simply pooling the data of several symmetrically distributed test sites and calling any resulting significant vector a ‘preferred compass direction’ is discussed, together with other possible reasons for asymmetrical distributions of release site biases.