Temporal and spatial distribution, and flight directions of migratory birds in Tsavo West National Park, Kenya: a comparison of radar and ringing data

Abstract

From 1 November 2013 to 30 April 2014 an avian radar system was operated in Tsavo West National Park, Kenya. The aim of this research was to study the temporal and spatial distribution of migrating birds over Ngulia Safari Lodge and to compare radar data with catching results of grounded birds. Additionally, the general pattern of flight directions during the six-month season is shown. From 25 November to 12 December 2013 more than 21 000 birds of 29 species were caught and ringed under misty conditions, supported by floodlights and sound luring (Pearson 2013). A total of 8564 individuals (41%) were caught during the night. The majority of the birds were Marsh Warblers Acrocephalus palustris (4442 ringed) and Thrush Nightingales Luscinia luscinia (2719). Radar data of bird migration intensities (migration traffic rates, MTR = birds/km/h) show an increase in late November to December, decreasing towards February and rising again in March and April when birds are migrating north to their breeding grounds in Eurasia. These MTRs correlate very well with the numbers of birds on the ground. They also show that birds are still migrating under clear, mist-free conditions, when no attempts were undertaken to catch birds. As expected, the flight directions changed from south in the autumn, to north in March and April. The support of the wind is optimal for migrating birds. At the lower altitudes the wind direction changes from northeast (November – January) to southeast (March, April), thus supporting migrating birds with optimal tailwinds. This means that birds are not obliged to change their flight altitude between seasons. The wind support originated in the calm Intertropical Convergence Zone and is optimal throughout the season from November to April. To our knowledge this is the first time that a study has shown the magnitude of bird migration in eastern Africa and the temporal and spatial distribution for half a year.

Keywords: Palaearctic migration, radar, Kenya