May 1951 – vol. 44, issue 5–157 of the observation work on the migration of the Swift (Apus apus) dor e in the years 1947-50 has been carried out at fixed vantage points. In general, this method has resulted in the acquisition of useful statistical data, but inherent in it are several possible sources of error. In the first place, a particular post may be outside a major migratory stream so that birds observed from it may repres... This article is only available to registered users.To read more, please log in or purchase a subscription.