Vertical distribution of bird migration between the Baltic Sea and the Sahara

This review covers 40 years of migration studies of the Swiss Ornithological Institute with the tracking radar “Superfledermaus”. Since 1968, this pencil-beam radar was—according to its (military) capacities of surveillance and tracking—applied for two different tasks: (I) To record the intensity of migration at various heights (either based on densities recorded by conical scanning, or frequencies of passage recorded with fixed beams at various elevation angles); densities (birds km−3) and frequencies (birds km−1 h−1) are averages per height intervals of 200 m and do not refer to the position of single targets; they are suited to show average height distributions. (II) Tracking individual birds or flocks provides information on the exact position and flight behaviour of single targets over time. Part I: 16 (among 22) study sites with data from 1991 onwards are chosen to visualize and explain the vertical distribution of nocturnal bird migration according to regionally shaped environmental conditions in the Western Palaearctic and in the trade-wind zone. Average distributions at sites devoid of important orographic or persistent meteorological distortions usually show 20–30% of nocturnal migration in the lowest 200-m interval, 50% below 700 m above ground level (agl), and the 90% quantile reaching heights between 1400 and 2100 m agl. The remaining 10% of migrants are usually scattered up to about 4000 m above sea level (asl). The lower parts of migration are forced upwards when crossing mountain ridges and often remain high after such crossings; at subsequent observation points, migration is often high above ground. Particularly high altitudes prevail where wind conditions are improving with altitude, as e.g. during spring migration in the trade-wind zone. Part II deals with the seasonal and diurnal variation in the spatial distribution of particular bird targets. Birds were on average higher up at night compared to daytime and avoided flights close to the ground in hot desert areas. Highest flights were recorded during diurnal migration above desert areas. The highest 0.3% of the tracked birds were found between 3500 m and 4900 m asl at most sites, but at 5000–6870 m in spring migration of the Mediterranean and the trade-wind zone. The most extreme tracks were at altitudes between 4500 and 6600 m asl above the European mainland, and between 6000 and nearly 9000 m above the Balearic Islands and in the trade-wind zones. A concluding discussion deals with the reasons for generally low flight levels and the particular conditions and requirements of high-altitude migration. Flight conditions in the lower atmosphere deteriorate with altitude. Cost of climbing may impose additional restrictions (specifically for large non-passerines). Decreasing oxygen density imposes limits to high-altitude flight, and compensatory ventilation may induce increased water loss, if no physiological countermeasures are available. High flights are compulsory when crossing high mountain ranges and favoured when wind support increases with altitude. This is particularly true when long non-stop flights lay ahead, when turbulences and/or high temperatures at lower levels can be avoided, and possibly when time for homeward flights in spring can be minimized.