Seasonal altitudinal migration of White-chested Alethe Chamaetylas fuelleborni in Tanzania documented with a barometric pressure logger

Abstract

Seasonal altitudinal movements from breeding areas in montane forests to non-breeding or wintering areas at lower elevations, during the cool dry season from April–June to September–October, has long been suspected for several species of birds breeding in Tanzania’s Eastern Arc Mountains. This study provides new insight into the altitudinal migration of a White-chested Alethe Chamaetylas fuelleborni, which was deployed with a barometric pressure logger during the cool dry season at 350 m altitude in Kimboza Forest, a lowland forest at the base of Uluguru Mountains in eastern Tanzania. The bird was found to move up to 1 500 m altitude in mid-October, most likely to montane forest in the Uluguru Mountains about 15 km away, and remained at this elevation for 3½ months, a period corresponding to the presumed breeding season, before moving down to the lowlands by the end of January. Unexpectedly, it returned to 1 500 m in mid-April where it stayed for five weeks before moving down to Kimboza Forest again where it was recaptured in July. The altitudinal migration movements were mostly started at first light about half an hour before sunrise, lasted, with one exception, no more than an hour each day and were completed in two or three days. Food is often believed to be the primary driver of altitudinal migration. The timing of the two upward movements we recorded support this hypothesis, as both coincided with the beginning of the two rainy seasons in eastern Tanzania, when army ant swarms, with which the Alethe is closely associated when foraging, become more active.