MIGRATION IN SPRING RECORDED BY RADAR AND FIELD OBSERVATIONS IN SWEDEN

SUMMARY In April 1960, the volume of migration over an inland area in Uppland, Middle‐Sweden, was simultaneously recorded by radar and field observations. From the data obtained frequency curves were plotted for each day and compared. Correlation between the radar and field observation curves (considering the total field material as well as the separate species) was poor, and it is concluded that most of the radar echoes must have originated from birds flying at heights invisible from the ground, while most of the visible migration was too low (mostly below 650 feet) to be caught by the radar. The directions of movement were much the same on all days, heading mainly E.N.E.‐N.E., both on radar and in the field, but on the last day visible migration headed distinctly more to the north. The mean speeds of the echoes varied between 43 and 70 m.p.h. (maximum 75 m.p.h.). Allowing for wind the still‐air flight speeds were 35–52 m.p.h. on all days except the last day, when few were above 45 m.p.h. This suggests that mainly the same groups of species were participating in the invisible movements recorded by radar every day. The high‐flying visible migrants (650–1,650 feet) were mainly water‐birds, which presumably formed at least part of the radar migration. This assumption is well supported by the recorded still‐air speeds of the echoes. On each day an appreciable return migration was noted in the field but scarcely at all by radar, the returning birds generally moving low against the wind. On a day of cold northerly wind later in the spring, however, a distinct return migration was seen on the radar during the first hours of darkness. The commonest genera observed in field were Fringilla, Turdus, Corvus, Columba, Vanellus, Larus, and Numenius, in that order. On two days larger numbers of Lapwings and crows were recorded, but the total number of flocks recorded was unaltered. In such cases the increased numbers of birds will go unrecorded by radar.