Altitude choice by night migrants in a desert area predicted by meteorological factors

Abstract

The height distribution of nocturnal migrants in southern Israel was determined by con‐ically scanning the sky with the pencil‐beam of an X‐band radar at different elevation angles. Altitudinal profiles of meteorological parameters were derived from radio sondes launched at midnight and from pilot balloons launched every 4 h. A model to predict the height distribution of birds by means of meteorological variables was developed by assuming that the observed proportions of birds within a height zone, compared with the neighbouring height zones, reflect the degree of the birds' preference for that height zone. Only one among the variables included in the multiple regression analysis proved to have a significant influence on the height distribution of migrants: the difference of tailwind speed between height zones. Simulations with 1000 birds choosing altitudes by means of the night's altitudinal profile of tailwind speed closely traced the observed distributions. The fact that all the other meteorological factors which were previously suggested to have an influence on the flight range in trans‐desert migration were not selected as relevant factors is discussed. The following basic information on nocturnal bird migration in the Negev is provided as a background for the statistical analysis: Directions of migration are within very narrow limits. During the first hour after take‐off, 60% of autumn migrants and 75% of spring migrants are climbing, with vertical speeds of 0.1–2 m per s and 0.1–4 m per s, respectively. During the rest of the night, climbing and descending birds are in nearly equal proportions. Thus, there is a high potential of sampling atmospheric conditions at different altitudes. Height distributions in spring and autumn show the influence of the trade wind situation, autumn migrants making use of the northerly winds at low levels in spite of high temperatures, while spring migrants tend to reach the southwesterly winds at higher levels.

Atmospheric trajectories and spring bird migration across the Gulf of Mexico

“…It is also possible that migrants flying over the Gulf are actively correcting for wind displacement. They can accomplish this by maintaining a preferred track either by changing their headings in response to different wind directions over the Gulf (sensu Alerstam and Hedenstro¨m 1998) or by changing their altitudes to locate more favorable winds aloft (Gauthreaux 1991;Bruderer et al 1995 Fig. 6 Regression analyses of the relationship of the azimuth direction of backward wind trajectories (direction from which wind was blowing) at 500 (a), 1,500 (b), and 2,500 (c)m above ground level (agl) to the longitudes of peak arrival of trans-Gulf migrants on the northern coast of the Gulf of Mexico.…”

Adjustments of wingbeat frequency and air speed to air density in free-flying migratory birds

“…(Liechti and Schaller, 1999;Schmaljohann et al, 2008b), which is higher than the maximum flight altitude considered in this study. Furthermore, bounding flyers did not prefer different flight altitudes due to differences in body size and mass but simply selected the altitude with the best meteorological conditions for their migratory flights (Bruderer et al, 1995b;Liechti et al, 2000;Schmaljohann et al, 2008c;Schmaljohann et al, 2009). Although we cannot categorically exclude any size effect on flight altitude, the substantial changes in wingbeat frequency and air speed are a result of behavioural adjustments of the birds to changing meteorological conditions, mainly air density.…”

Unexplained altitude changes in a migrating thrush: Long-flight altitude data from radio-telemetry

“…However, in most cases radar data are limited in duration to several minutes per bird, and nothing is known about the individuals tracked except their size. Thus, most radar studies have focused on large-scale timing and altitude of migration in response to changes in weather patterns (Richardson 1978, 1990, Kerlinger and Moore 1989, Bruderer et al 1995, Dokter et al 2010, Bridge et al 2011, La Sorte et al 2015. A few studies have tracked individuals up to several tens of minutes or even hours, but have not investigated altitude or changes in altitude per se (e.g., Gudmundsson 1994).…”

“…A few studies have tracked individuals up to several tens of minutes or even hours, but have not investigated altitude or changes in altitude per se (e.g., Gudmundsson 1994). From the former, we know that passerine migrants fly relatively low in the atmosphere (,1,000 m) most nights (Gauthreaux 1991), and that the flight altitudes of large groups of migrants can be affected by air temperature, location of boundary layers, turbulence, precipitation, and other atmospheric variables (Richardson 1978, 1990, Kerlinger and Moore 1989, Bruderer et al 1995. Cochran and Kjos (1985) used adjustable antennas to measure the altitudes of individual thrushes outfitted with pulsed radio-transmitters and estimated each thrush's altitude 2-5 times during its flight.…”