Overflying the Himalayas; the Northward Migration of Sri Lankan-wintering Brown-headed Gulls

Abstract

The Himalayan Mountain range, an average elevation of 6,100 m above mean sea level (AMSL), forms the most formidable geographic barrier to birds migrating along the Central Asian Flyway (CAF). In an attempt to understand the poorly studied migration routes of species occurring in the CAF, we are tracking several migratory waterbird species including Brown-headed Gull Larus brunnicephalus from Sri Lanka, which encounters the Himalayas during its northward migration to reach its breeding grounds in south-central Asia. In March- April of 2021 and 2022, two Brown-headed Gulls were caught at their non-breeding sites in Mannar Island of Sri Lanka, and were fitted with GPS-GSM transmitters, just before their northward migration, and their movements were tracked. The birds departed Mannar between 25th April and 14th May and arrived at their breeding grounds on the Tibetan plateau between 4th and 20th May. This northward journey of the gulls lasted 7.5 ± 2.1 (Mean ± SD) days during which they covered a distance of 3,173.2 ± 534.3 km. Both gulls stopped over in the Ganges River, India, for approximately two days. Accordingly, their overall migration speed (including stopover duration) was 451.2 ± 198.9 km/day while the travel speed on travel days was 24.2 ± 10.5 km/h. During migration, the gulls crossed the Himalayas through Nepal at an altitude of 5,744.1 ± 298.5 m AMSL. In the course of this crossing, they flew 888.0 ± 116.8 m above ground (where surface elevation is 4,856.1 ± 181.7 m AMSL), over the peaks of Panbari Himal & Cho Oyu. This is the first evidence of the Brown-headed Gull encountering such high altitudes during migration. Weighing only 372.5 ± 53.0 g, the Brown-headed Gull is likely to be the lightest gull species to be recorded to cross the Himalayas.