Flight speeds of birds observed with radar: Evidence for two phases of migratory flight

Summary

Horizontal flight speeds relative to the air (air speeds) of about 3,500 birds flying at night over southeastern New York State were measured with a tracking radar, taking special care to obtain accurate local wind measurements. Results were in agreement with previous data (Larkin et al. 1979) indicating that migrating birds sometimes fly at slower speeds than seem consistent with energetically efficient flight.

In both spring and fall, about 30% of the birds flew at air speeds of less than 5 m/s (Fig. 5). Two phases of flight were observed, often occurring together on the same night (Figs. 7 and 9). Faster birds generally flew at headings closer to the seasonally appropriate direction and sometimes tended to fly at higher altitudes. Slower birds' headings tended toward the expected direction only part of the time and were more scattered, both between and within nights of observation.

The observed relationship between heading, flight speed, and wind direction confirms the ‘following component correlation’ reported by previous investigators. But the strongest correlation was with relative wind direction rather than the speed of tail winds.

Direct visual observations of a representative subsample of the targets confirmed other indications that they were small birds engaged in flapping flight.

Nights when a large proportion of birds had air speeds below 5 m/s were often characterized by seasonally unfavorable winds. Nights with a single air speed mode below 8 m/s occurred when winds were unfavorable or when a high-pressure system was centered near the site of observation.

A small but appreciable proportion of birds, here called ‘retros’, had such low air speeds that, even though heading more or less upwind, they were being carried backward relative to the ground.

A number of other variables, including altitude, time of night, time of year, and rate of ascent/descent, sometimes correlated with speed of flight (Table 3).

Generally accepted methods of estimating such parameters as birds' headings and wind drift may be seriously in error if the relationships found in the present investigation are characteristic of other geographical areas.