The following original classification of bottleneck sites has been used by us:

SG – Site for fly over only with gliding or flapping flight and without stopping for soaring, overnighting or staying for several days; usually such sites are situated at water bodies or forests;

SR – Site for fly over with mainly gliding and soaring flight; usually such sites are situated at arable or fallow land, stubble fields, rocky areas and other places with thermals;

SO - Site for overnighting (or ‘bird hotels’); usually such sites are situated around/in wetlands, fallow land, stubble fields, forests, single trees and other places without human disturbance.

SoS – Stop over Site for a longer stay for more than twenty-four hour period; such sites are situated in large wetland complexes with foraging resources and absence of human disturbance; these sites have great conservation importance and reputation.

This classification has been included in our manuscript, entitled 'LONG-TERM CHANGES IN AUTUMN MIGRATION OF SOARING BIRDS AT BURGAS BAY, BULGARIA'

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